diff git a/compiler/basicTypes/NameEnv.hs b/compiler/basicTypes/NameEnv.hs
index 0b1cf435bc6d3fa98c0a4e18500de6bf3fa91e3b..5f6bdc584644597dbd73d61a598d7affeb2cdfbd 100644
 a/compiler/basicTypes/NameEnv.hs
+++ b/compiler/basicTypes/NameEnv.hs
@@ 25,9 +25,9 @@ module NameEnv (
emptyDNameEnv,
lookupDNameEnv,
 delFromDNameEnv,
+ delFromDNameEnv, filterDNameEnv,
mapDNameEnv,
 alterDNameEnv,
+ adjustDNameEnv, alterDNameEnv, extendDNameEnv,
 ** Dependency analysis
depAnal
) where
@@ 151,8 +151,17 @@ lookupDNameEnv = lookupUDFM
delFromDNameEnv :: DNameEnv a > Name > DNameEnv a
delFromDNameEnv = delFromUDFM
+filterDNameEnv :: (a > Bool) > DNameEnv a > DNameEnv a
+filterDNameEnv = filterUDFM
+
mapDNameEnv :: (a > b) > DNameEnv a > DNameEnv b
mapDNameEnv = mapUDFM
+adjustDNameEnv :: (a > a) > DNameEnv a > Name > DNameEnv a
+adjustDNameEnv = adjustUDFM
+
alterDNameEnv :: (Maybe a > Maybe a) > DNameEnv a > Name > DNameEnv a
alterDNameEnv = alterUDFM
+
+extendDNameEnv :: DNameEnv a > Name > a > DNameEnv a
+extendDNameEnv = addToUDFM
diff git a/compiler/basicTypes/PatSyn.hs b/compiler/basicTypes/PatSyn.hs
index 008a0771fac9e64f51b0e672fd1e2153e8c137be..14a05b325836e83562c6eedee7652679bcb5419f 100644
 a/compiler/basicTypes/PatSyn.hs
+++ b/compiler/basicTypes/PatSyn.hs
@@ 184,6 +184,20 @@ to get the instantiation a := ty.
This is very unlike DataCons, where univ tyvars match 11 the
arguments of the TyCon.
+Side note: I (SG) get the impression that instantiated return types should
+generate a *required* constraint for pattern synonyms, rather than a *provided*
+constraint like it's the case for GADTs. For example, I'd expect these
+declarations to have identical semantics:
+
+ pattern Just42 :: Maybe Int
+ pattern Just42 = Just 42
+
+ pattern Just'42 :: (a ~ Int) => Maybe a
+ pattern Just'42 = Just 42
+
+The latter generates the proper required constraint, the former does not.
+Also rather different to GADTs is the fact that Just42 doesn't have any
+universally quantified type variables, whereas Just'42 or MkS above has.
Note [Pattern synonym representation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff git a/compiler/deSugar/Check.hs b/compiler/deSugar/Check.hs
index 602c125f67e681a2ea8983c3cc6be7938826a157..3a1515d0cc508f05d270a03c32ff0b5b84b92a19 100644
 a/compiler/deSugar/Check.hs
+++ b/compiler/deSugar/Check.hs
@@ 11,21 +11,28 @@ Pattern Matching Coverage Checking.
{# LANGUAGE TupleSections #}
{# LANGUAGE ViewPatterns #}
{# LANGUAGE MultiWayIf #}
+{# LANGUAGE LambdaCase #}
module Check (
 Checking and printing
 checkSingle, checkMatches, checkGuardMatches, isAnyPmCheckEnabled,
+ checkSingle, checkMatches, checkGuardMatches,
+ needToRunPmCheck, isMatchContextPmChecked,
 See Note [Type and Term Equality Propagation]
 genCaseTmCs1, genCaseTmCs2
+ addTyCsDs, addScrutTmCs, addPatTmCs
) where
#include "HsVersions.h"
import GhcPrelude
import TmOracle
+import PmExpr
+import PmOracle
import PmPpr
+import BasicTypes (Origin, isGenerated)
+import CoreSyn (CoreExpr, Expr(Var))
+import CoreUtils (exprType)
+import FastString (unpackFS)
import Unify( tcMatchTy )
import DynFlags
import HsSyn
@@ 33,42 +40,37 @@ import TcHsSyn
import Id
import ConLike
import Name
import FamInstEnv
import TysPrim (tYPETyCon)
+import FamInst
import TysWiredIn
import TyCon
import SrcLoc
import Util
import Outputable
import FastString
import DataCon
import PatSyn
import HscTypes (CompleteMatch(..))
import BasicTypes (Boxity(..))
+import Var (EvVar)
+import {# SOURCE #} DsExpr (dsExpr, dsLExpr)
+import MatchLit (dsLit, dsOverLit)
import DsMonad
import TcSimplify (tcCheckSatisfiability)
import TcType (isStringTy)
import Bag
import ErrUtils
import Var (EvVar)
import TyCoRep
import Type
import UniqSupply
import DsUtils (isTrueLHsExpr)
+import Maybes (isJust, expectJust)
import qualified GHC.LanguageExtensions as LangExt
import Data.List (find)
import Data.Maybe (catMaybes, isJust, fromMaybe)
import Control.Monad (forM, when, forM_, zipWithM, filterM)
+import Control.Monad (forM, when, forM_)
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.Maybe
import Coercion
import TcEvidence
import TcSimplify (tcNormalise)
import IOEnv
import qualified Data.Semigroup as Semi
import ListT (ListT(..), fold, select)

{
This module checks pattern matches for:
\begin{enumerate}
@@ 91,98 +93,44 @@ The algorithm is based on the paper:
%************************************************************************
}
 We use the nondeterminism monad to apply the algorithm to several
 possible sets of constructors. Users can specify complete sets of
 constructors by using COMPLETE pragmas.
 The algorithm only picks out constructor
 sets deep in the bowels which makes a simpler `mapM` more difficult to
 implement. The nondeterminism is only used in one place, see the ConVar
 case in `pmCheckHd`.
+data PmPat where
+   For the arguments' meaning see 'HsPat.ConPatOut'.
+ PmCon :: { pm_con_con :: PmAltCon
+ , pm_con_arg_tys :: [Type]
+ , pm_con_tvs :: [TyVar]
+ , pm_con_args :: [PmPat] } > PmPat
type PmM a = ListT DsM a
+ PmVar :: { pm_var_id :: Id } > PmPat
liftD :: DsM a > PmM a
liftD m = ListT $ \sk fk > m >>= \a > sk a fk
+ PmGrd :: { pm_grd_pv :: PatVec  ^ Always has 'patVecArity' 1.
+ , pm_grd_expr :: CoreExpr } > PmPat
+  (PmGrd pat expr) matches expr against pat, binding the variables in pat
 Pick the first match complete covered match or otherwise the "best" match.
 The best match is the one with the least uncovered clauses, ties broken
 by the number of inaccessible clauses followed by number of redundant
 clauses.

 This is specified in the
 "Disambiguating between multiple ``COMPLETE`` pragmas" section of the
 users' guide. If you update the implementation of this function, make sure
 to update that section of the users' guide as well.
getResult :: PmM PmResult > DsM PmResult
getResult ls
 = do { res < fold ls goM (pure Nothing)
 ; case res of
 Nothing > panic "getResult is empty"
 Just a > return a }
 where
 goM :: PmResult > DsM (Maybe PmResult) > DsM (Maybe PmResult)
 goM mpm dpm = do { pmr < dpm
 ; return $ Just $ go pmr mpm }

  Careful not to force unecessary results
 go :: Maybe PmResult > PmResult > PmResult
 go Nothing rs = rs
 go (Just old@(PmResult prov rs (UncoveredPatterns us) is)) new
  null us && null rs && null is = old
  otherwise =
 let PmResult prov' rs' (UncoveredPatterns us') is' = new
 in case compareLength us us'
 `mappend` (compareLength is is')
 `mappend` (compareLength rs rs')
 `mappend` (compare prov prov') of
 GT > new
 EQ > new
 LT > old
 go (Just (PmResult _ _ (TypeOfUncovered _) _)) _new
 = panic "getResult: No inhabitation candidates"

data PatTy = PAT  VA  Used only as a kind, to index PmPat

 The *arity* of a PatVec [p1,..,pn] is
 the number of p1..pn that are not Guards

data PmPat :: PatTy > * where
 PmCon :: { pm_con_con :: ConLike
 , pm_con_arg_tys :: [Type]
 , pm_con_tvs :: [TyVar]
 , pm_con_dicts :: [EvVar]
 , pm_con_args :: [PmPat t] } > PmPat t
  For PmCon arguments' meaning see @ConPatOut@ in hsSyn/HsPat.hs
 PmVar :: { pm_var_id :: Id } > PmPat t
 PmLit :: { pm_lit_lit :: PmLit } > PmPat t  See Note [Literals in PmPat]
 PmNLit :: { pm_lit_id :: Id
 , pm_lit_not :: [PmLit] } > PmPat 'VA
 PmGrd :: { pm_grd_pv :: PatVec
 , pm_grd_expr :: PmExpr } > PmPat 'PAT
  A fake guard pattern (True < _) used to represent cases we cannot handle.
 PmFake :: PmPat 'PAT
+ PmFake :: PmPat
instance Outputable (PmPat a) where
 ppr = pprPmPatDebug
+  Should not be userfacing.
+instance Outputable PmPat where
+ ppr (PmCon alt _arg_tys _con_tvs con_args)
+ = cparen (notNull con_args) (hsep [ppr alt, hsep (map ppr con_args)])
+ ppr (PmVar vid) = ppr vid
+ ppr (PmGrd pv ge) = hsep (map ppr pv) <+> text "<" <+> ppr ge
+ ppr PmFake = text ""
 data T a where
 MkT :: forall p q. (Eq p, Ord q) => p > q > T [p]
 or MkT :: forall p q r. (Eq p, Ord q, [p] ~ r) => p > q > T r
type Pattern = PmPat 'PAT  ^ Patterns
type ValAbs = PmPat 'VA  ^ Value Abstractions

type PatVec = [Pattern]  ^ Pattern Vectors
data ValVec = ValVec [ValAbs] Delta  ^ Value Vector Abstractions
+  Pattern Vectors. The *arity* of a PatVec [p1,..,pn] is
+ the number of p1..pn that are not Guards. See 'patternArity'.
+type PatVec = [PmPat]
+type ValVec = [Id]  ^ Value Vector Abstractions
  Term and type constraints to accompany each value vector abstraction.
 For efficiency, we store the term oracle state instead of the term
 constraints. TODO: Do the same for the type constraints?
data Delta = MkDelta { delta_ty_cs :: Bag EvVar
 , delta_tm_cs :: TmState }

type ValSetAbs = [ValVec]  ^ Value Set Abstractions
type Uncovered = ValSetAbs
+  Each 'Delta' is proof (i.e., a model of the fact) that some values are not
+ covered by a pattern match. E.g. @f Nothing = @ might be given an
+ uncovered set @[x :> Just y]@ or @[x /= Nothing]@, where @x@ is the variable
+ matching against @f@'s first argument.
+type Uncovered = [Delta]
 Instead of keeping the whole sets in memory, we keep a boolean for both the
 covered and the divergent set (we store the uncovered set though, since we
@@ 199,8 +147,7 @@ data Covered = Covered  NotCovered
deriving Show
instance Outputable Covered where
 ppr (Covered) = text "Covered"
 ppr (NotCovered) = text "NotCovered"
+ ppr = text . show
 Like the or monoid for booleans
 Covered = True, Uncovered = False
@@ 217,8 +164,7 @@ data Diverged = Diverged  NotDiverged
deriving Show
instance Outputable Diverged where
 ppr Diverged = text "Diverged"
 ppr NotDiverged = text "NotDiverged"
+ ppr = text . show
instance Semi.Semigroup Diverged where
Diverged <> _ = Diverged
@@ 229,55 +175,36 @@ instance Monoid Diverged where
mempty = NotDiverged
mappend = (Semi.<>)
  When we learned that a given match group is complete
data Provenance =
 FromBuiltin  ^ From the original definition of the type
  constructor.
  FromComplete  ^ From a userprovided @COMPLETE@ pragma
 deriving (Show, Eq, Ord)

instance Outputable Provenance where
 ppr = text . show

instance Semi.Semigroup Provenance where
 FromComplete <> _ = FromComplete
 _ <> FromComplete = FromComplete
 _ <> _ = FromBuiltin

instance Monoid Provenance where
 mempty = FromBuiltin
 mappend = (Semi.<>)

+  A triple of covered, uncovered, and divergent sets.
data PartialResult = PartialResult {
 presultProvenance :: Provenance
  keep track of provenance because we don't want
  to warn about redundant matches if the result
  is contaminated with a COMPLETE pragma
 , presultCovered :: Covered
+ presultCovered :: Covered
, presultUncovered :: Uncovered
, presultDivergent :: Diverged }
instance Outputable PartialResult where
 ppr (PartialResult prov c vsa d)
 = text "PartialResult" <+> ppr prov <+> ppr c
 <+> ppr d <+> ppr vsa
+emptyPartialResult :: PartialResult
+emptyPartialResult = PartialResult { presultUncovered = mempty
+ , presultCovered = mempty
+ , presultDivergent = mempty }
+combinePartialResults :: PartialResult > PartialResult > PartialResult
+combinePartialResults (PartialResult cs1 vsa1 ds1) (PartialResult cs2 vsa2 ds2)
+ = PartialResult (cs1 Semi.<> cs2)
+ (vsa1 Semi.<> vsa2)
+ (ds1 Semi.<> ds2)
instance Semi.Semigroup PartialResult where
 (PartialResult prov1 cs1 vsa1 ds1)
 <> (PartialResult prov2 cs2 vsa2 ds2)
 = PartialResult (prov1 Semi.<> prov2)
 (cs1 Semi.<> cs2)
 (vsa1 Semi.<> vsa2)
 (ds1 Semi.<> ds2)
+instance Outputable PartialResult where
+ ppr (PartialResult c vsa d)
+ = hang (text "PartialResult" <+> ppr c <+> ppr d) 2 (ppr_vsa vsa)
+ where
+ ppr_vsa = braces . fsep . punctuate comma . map ppr
+instance Semi.Semigroup PartialResult where
+ (<>) = combinePartialResults
instance Monoid PartialResult where
 mempty = PartialResult mempty mempty [] mempty
+ mempty = emptyPartialResult
mappend = (Semi.<>)
 newtype ChoiceOf a = ChoiceOf [a]

  Pattern check result

 * Redundant clauses
@@ 291,15 +218,13 @@ instance Monoid PartialResult where

data PmResult =
PmResult {
 pmresultProvenance :: Provenance
 , pmresultRedundant :: [Located [LPat GhcTc]]
+ pmresultRedundant :: [Located [LPat GhcTc]]
, pmresultUncovered :: UncoveredCandidates
, pmresultInaccessible :: [Located [LPat GhcTc]] }
instance Outputable PmResult where
ppr pmr = hang (text "PmResult") 2 $ vcat
 [ text "pmresultProvenance" <+> ppr (pmresultProvenance pmr)
 , text "pmresultRedundant" <+> ppr (pmresultRedundant pmr)
+ [ text "pmresultRedundant" <+> ppr (pmresultRedundant pmr)
, text "pmresultUncovered" <+> ppr (pmresultUncovered pmr)
, text "pmresultInaccessible" <+> ppr (pmresultInaccessible pmr)
]
@@ 315,21 +240,13 @@ instance Outputable PmResult where
 but we don't want to issue just a wildcard as missing. Instead, we print a
 type annotated wildcard, so that the user knows what kind of patterns is
 expected (e.g. (_ :: Int), or (_ :: F Int), where F Int does not reduce).
data UncoveredCandidates = UncoveredPatterns Uncovered
+data UncoveredCandidates = UncoveredPatterns [Id] [Delta]
 TypeOfUncovered Type
instance Outputable UncoveredCandidates where
 ppr (UncoveredPatterns uc) = text "UnPat" <+> ppr uc
+ ppr (UncoveredPatterns vva deltas) = text "UnPat" <+> ppr vva $$ ppr deltas
ppr (TypeOfUncovered ty) = text "UnTy" <+> ppr ty
  The empty pattern check result
emptyPmResult :: PmResult
emptyPmResult = PmResult FromBuiltin [] (UncoveredPatterns []) []

  Nonexhaustive empty case with unknown/trivial inhabitants
uncoveredWithTy :: Type > PmResult
uncoveredWithTy ty = PmResult FromBuiltin [] (TypeOfUncovered ty) []

{
%************************************************************************
%* *
@@ 341,27 +258,28 @@ uncoveredWithTy ty = PmResult FromBuiltin [] (TypeOfUncovered ty) []
  Check a single pattern binding (let)
checkSingle :: DynFlags > DsMatchContext > Id > Pat GhcTc > DsM ()
checkSingle dflags ctxt@(DsMatchContext _ locn) var p = do
 tracePmD "checkSingle" (vcat [ppr ctxt, ppr var, ppr p])
 mb_pm_res < tryM (getResult (checkSingle' locn var p))
+ tracePm "checkSingle" (vcat [ppr ctxt, ppr var, ppr p])
+ mb_pm_res < tryM (checkSingle' locn var p)
case mb_pm_res of
Left _ > warnPmIters dflags ctxt
Right res > dsPmWarn dflags ctxt res
  Check a single pattern binding (let)
checkSingle' :: SrcSpan > Id > Pat GhcTc > PmM PmResult
+checkSingle' :: SrcSpan > Id > Pat GhcTc > DsM PmResult
checkSingle' locn var p = do
 liftD resetPmIterDs  set the iterno to zero
 fam_insts < liftD dsGetFamInstEnvs
 clause < liftD $ translatePat fam_insts p
 missing < mkInitialUncovered [var]
 tracePm "checkSingle': missing" (vcat (map pprValVecDebug missing))
  no guards
 PartialResult prov cs us ds < runMany (pmcheckI clause []) missing
 let us' = UncoveredPatterns us
+ resetPmIterDs  set the iterno to zero
+ fam_insts < dsGetFamInstEnvs
+ clause < translatePat fam_insts p
+ missing < getPmDelta
+ tracePm "checkSingle': missing" (ppr missing)
+ PartialResult cs us ds < pmcheckI clause [] [var] 1 missing
+ dflags < getDynFlags
+ us' < getNFirstUncovered [var] (maxUncoveredPatterns dflags + 1) us
+ let uc = UncoveredPatterns [var] us'
return $ case (cs,ds) of
 (Covered, _ ) > PmResult prov [] us' []  useful
 (NotCovered, NotDiverged) > PmResult prov m us' []  redundant
 (NotCovered, Diverged ) > PmResult prov [] us' m  inaccessible rhs
+ (Covered, _ ) > PmResult [] uc []  useful
+ (NotCovered, NotDiverged) > PmResult m uc []  redundant
+ (NotCovered, Diverged ) > PmResult [] uc m  inaccessible rhs
where m = [cL locn [cL locn p]]
  Exhaustive for guard matches, is used for guards in pattern bindings and
@@ 385,14 +303,14 @@ checkGuardMatches _ (XGRHSs nec) = noExtCon nec
checkMatches :: DynFlags > DsMatchContext
> [Id] > [LMatch GhcTc (LHsExpr GhcTc)] > DsM ()
checkMatches dflags ctxt vars matches = do
 tracePmD "checkMatches" (hang (vcat [ppr ctxt
+ tracePm "checkMatches" (hang (vcat [ppr ctxt
, ppr vars
, text "Matches:"])
2
(vcat (map ppr matches)))
 mb_pm_res < tryM $ getResult $ case matches of
+ mb_pm_res < tryM $ case matches of
 Check EmptyCase separately
  See Note [Checking EmptyCase Expressions]
+  See Note [Checking EmptyCase Expressions] in PmOracle
[]  [var] < vars > checkEmptyCase' var
_normal_match > checkMatches' vars matches
case mb_pm_res of
@@ 401,42 +319,42 @@ checkMatches dflags ctxt vars matches = do
  Check a matchgroup (case, functions, etc.). To be called on a nonempty
 list of matches. For empty case expressions, use checkEmptyCase' instead.
checkMatches' :: [Id] > [LMatch GhcTc (LHsExpr GhcTc)] > PmM PmResult
+checkMatches' :: [Id] > [LMatch GhcTc (LHsExpr GhcTc)] > DsM PmResult
checkMatches' vars matches
 null matches = panic "checkMatches': EmptyCase"
 otherwise = do
 liftD resetPmIterDs  set the iterno to zero
 missing < mkInitialUncovered vars
 tracePm "checkMatches': missing" (vcat (map pprValVecDebug missing))
 (prov, rs,us,ds) < go matches missing
+ resetPmIterDs  set the iterno to zero
+ missing < getPmDelta
+ tracePm "checkMatches': missing" (ppr missing)
+ (rs,us,ds) < go matches [missing]
+ dflags < getDynFlags
+ us' < getNFirstUncovered vars (maxUncoveredPatterns dflags + 1) us
+ let up = UncoveredPatterns vars us'
return $ PmResult {
 pmresultProvenance = prov
 , pmresultRedundant = map hsLMatchToLPats rs
 , pmresultUncovered = UncoveredPatterns us
+ pmresultRedundant = map hsLMatchToLPats rs
+ , pmresultUncovered = up
, pmresultInaccessible = map hsLMatchToLPats ds }
where
go :: [LMatch GhcTc (LHsExpr GhcTc)] > Uncovered
 > PmM (Provenance
 , [LMatch GhcTc (LHsExpr GhcTc)]
+ > DsM ( [LMatch GhcTc (LHsExpr GhcTc)]
, Uncovered
, [LMatch GhcTc (LHsExpr GhcTc)])
 go [] missing = return (mempty, [], missing, [])
+ go [] missing = return ([], missing, [])
go (m:ms) missing = do
 tracePm "checkMatches': go" (ppr m $$ ppr missing)
 fam_insts < liftD dsGetFamInstEnvs
 (clause, guards) < liftD $ translateMatch fam_insts m
 r@(PartialResult prov cs missing' ds)
 < runMany (pmcheckI clause guards) missing
+ tracePm "checkMatches': go" (ppr m)
+ fam_insts < dsGetFamInstEnvs
+ (clause, guards) < translateMatch fam_insts m
+ r@(PartialResult cs missing' ds)
+ < runMany (pmcheckI clause guards vars (length missing)) missing
tracePm "checkMatches': go: res" (ppr r)
 (ms_prov, rs, final_u, is) < go ms missing'
 let final_prov = prov `mappend` ms_prov
+ (rs, final_u, is) < go ms missing'
return $ case (cs, ds) of
 useful
 (Covered, _ ) > (final_prov, rs, final_u, is)
+ (Covered, _ ) > (rs, final_u, is)
 redundant
 (NotCovered, NotDiverged) > (final_prov, m:rs, final_u,is)
+ (NotCovered, NotDiverged) > (m:rs, final_u,is)
 inaccessible
 (NotCovered, Diverged ) > (final_prov, rs, final_u, m:is)
+ (NotCovered, Diverged ) > (rs, final_u, m:is)
hsLMatchToLPats :: LMatch id body > Located [LPat id]
hsLMatchToLPats (dL>L l (Match { m_pats = pats })) = cL l pats
@@ 444,472 +362,59 @@ checkMatches' vars matches
  Check an empty case expression. Since there are no clauses to process, we
 only compute the uncovered set. See Note [Checking EmptyCase Expressions]
 for details.
checkEmptyCase' :: Id > PmM PmResult
checkEmptyCase' var = do
 tm_ty_css < pmInitialTmTyCs
 mb_candidates < inhabitationCandidates (delta_ty_cs tm_ty_css) (idType var)
 case mb_candidates of
+ in "PmOracle" for details.
+checkEmptyCase' :: Id > DsM PmResult
+checkEmptyCase' x = do
+ delta < getPmDelta
+ us < inhabitants delta (idType x) >>= \case
 Inhabitation checking failed / the type is trivially inhabited
 Left ty > return (uncoveredWithTy ty)

  A list of inhabitant candidates is available: Check for each
  one for the satisfiability of the constraints it gives rise to.
 Right (_, candidates) > do
 missing_m < flip mapMaybeM candidates $
 \InhabitationCandidate{ ic_val_abs = va, ic_tm_ct = tm_ct
 , ic_ty_cs = ty_cs
 , ic_strict_arg_tys = strict_arg_tys } > do
 mb_sat < pmIsSatisfiable tm_ty_css tm_ct ty_cs strict_arg_tys
 pure $ fmap (ValVec [va]) mb_sat
 return $ if null missing_m
 then emptyPmResult
 else PmResult FromBuiltin [] (UncoveredPatterns missing_m) []

  Returns 'True' if the argument 'Type' is a fully saturated application of
 a closed type constructor.

 Closed type constructors are those with a fixed right hand side, as
 opposed to e.g. associated types. These are of particular interest for
 patternmatch coverage checking, because GHC can exhaustively consider all
 possible forms that values of a closed type can take on.

 Note that this function is intended to be used to check types of valuelevel
 patterns, so as a consequence, the 'Type' supplied as an argument to this
 function should be of kind @Type@.
pmIsClosedType :: Type > Bool
pmIsClosedType ty
 = case splitTyConApp_maybe ty of
 Just (tc, ty_args)
  is_algebraic_like tc && not (isFamilyTyCon tc)
 > ASSERT2( ty_args `lengthIs` tyConArity tc, ppr ty ) True
 _other > False
 where
  This returns True for TyCons which /act like/ algebraic types.
  (See "Type#type_classification" for what an algebraic type is.)
 
  This is qualified with \"like\" because of a particular special
  case: TYPE (the underlyind kind behind Type, among others). TYPE
  is conceptually a datatype (and thus algebraic), but in practice it is
  a primitive builtin type, so we must check for it specially.
 
  NB: it makes sense to think of TYPE as a closed type in a valuelevel,
  patternmatching context. However, at the kind level, TYPE is certainly
  not closed! Since this function is specifically tailored towards pattern
  matching, however, it's OK to label TYPE as closed.
 is_algebraic_like :: TyCon > Bool
 is_algebraic_like tc = isAlgTyCon tc  tc == tYPETyCon

pmTopNormaliseType_maybe :: FamInstEnvs > Bag EvVar > Type
 > PmM (Maybe (Type, [DataCon], Type))
 ^ Get rid of *outermost* (or toplevel)
 * type function redex
 * data family redex
 * newtypes

 Behaves exactly like `topNormaliseType_maybe`, but instead of returning a
 coercion, it returns useful information for issuing pattern matching
 warnings. See Note [Type normalisation for EmptyCase] for details.

 NB: Normalisation can potentially change kinds, if the head of the type
 is a type family with a variable result kind. I (Richard E) can't think
 of a way to cause trouble here, though.
pmTopNormaliseType_maybe env ty_cs typ
 = do (_, mb_typ') < liftD $ initTcDsForSolver $ tcNormalise ty_cs typ
  Before proceeding, we chuck typ into the constraint solver, in case
  solving for given equalities may reduce typ some. See
  "Wrinkle: local equalities" in
  Note [Type normalisation for EmptyCase].
 pure $ do typ' < mb_typ'
 ((ty_f,tm_f), ty) < topNormaliseTypeX stepper comb typ'
  We need to do topNormaliseTypeX in addition to tcNormalise,
  since topNormaliseX looks through newtypes, which
  tcNormalise does not do.
 Just (eq_src_ty ty (typ' : ty_f [ty]), tm_f [], ty)
 where
  Find the first type in the sequence of rewrites that is a data type,
  newtype, or a data family application (not the representation tycon!).
  This is the one that is equal (in source Haskell) to the initial type.
  If none is found in the list, then all of them are type family
  applications, so we simply return the last one, which is the *simplest*.
 eq_src_ty :: Type > [Type] > Type
 eq_src_ty ty tys = maybe ty id (find is_closed_or_data_family tys)

 is_closed_or_data_family :: Type > Bool
 is_closed_or_data_family ty = pmIsClosedType ty  isDataFamilyAppType ty

  For efficiency, represent both lists as difference lists.
  comb performs the concatenation, for both lists.
 comb (tyf1, tmf1) (tyf2, tmf2) = (tyf1 . tyf2, tmf1 . tmf2)

 stepper = newTypeStepper `composeSteppers` tyFamStepper

  A 'NormaliseStepper' that unwraps newtypes, careful not to fall into
  a loop. If it would fall into a loop, it produces 'NS_Abort'.
 newTypeStepper :: NormaliseStepper ([Type] > [Type],[DataCon] > [DataCon])
 newTypeStepper rec_nts tc tys
  Just (ty', _co) < instNewTyCon_maybe tc tys
 = case checkRecTc rec_nts tc of
 Just rec_nts' > let tyf = ((TyConApp tc tys):)
 tmf = ((tyConSingleDataCon tc):)
 in NS_Step rec_nts' ty' (tyf, tmf)
 Nothing > NS_Abort
  otherwise
 = NS_Done

 tyFamStepper :: NormaliseStepper ([Type] > [Type], [DataCon] > [DataCon])
 tyFamStepper rec_nts tc tys  Try to step a type/data family
 = let (_args_co, ntys, _res_co) = normaliseTcArgs env Representational tc tys in
  NB: It's OK to use normaliseTcArgs here instead of
  normalise_tc_args (which takes the LiftingContext described
  in Note [Normalising types]) because the reduceTyFamApp below
  works only at top level. We'll never recur in this function
  after reducing the kind of a bound tyvar.

 case reduceTyFamApp_maybe env Representational tc ntys of
 Just (_co, rhs) > NS_Step rec_nts rhs ((rhs:), id)
 _ > NS_Done

  Determine suitable constraints to use at the beginning of patternmatch
 coverage checking by consulting the sets of term and type constraints
 currently in scope. If one of these sets of constraints is unsatisfiable,
 use an empty set in its place. (See
 @Note [Recovering from unsatisfiable patternmatching constraints]@
 for why this is done.)
pmInitialTmTyCs :: PmM Delta
pmInitialTmTyCs = do
 ty_cs < liftD getDictsDs
 tm_cs < bagToList <$> liftD getTmCsDs
 sat_ty < tyOracle ty_cs
 let initTyCs = if sat_ty then ty_cs else emptyBag
 initTmState = fromMaybe initialTmState (tmOracle initialTmState tm_cs)
 pure $ MkDelta{ delta_tm_cs = initTmState, delta_ty_cs = initTyCs }
+ Left ty > pure (TypeOfUncovered ty)
+  A list of oracle states for the different satisfiable constructors is
+  available. Turn this into a value set abstraction.
+ Right (va, deltas) > pure (UncoveredPatterns [va] deltas)
+ pure (PmResult [] us [])
+
+getNFirstUncovered :: [Id] > Int > [Delta] > DsM [Delta]
+getNFirstUncovered _ 0 _ = pure []
+getNFirstUncovered _ _ [] = pure []
+getNFirstUncovered vars n (delta:deltas) = do
+ front < provideEvidenceForEquation vars n delta
+ back < getNFirstUncovered vars (n  length front) deltas
+ pure (front ++ back)
+
+  The maximum successive number of refinements ('refineToAltCon') we allow
+ per representative. See Note [Limit the number of refinements].
+mAX_REFINEMENTS :: Int
+ The current number is chosen so that PrelRules is still checked with
+ reasonable performance. If this is set to below 2, ds022 will start to fail.
+ Although that is probably due to the fact that we always increase the
+ refinement counter instead of just increasing it when the contraposition
+ is satisfiable (when the not taken case 'addRefutableAltCon' is
+ satisfiable, that is). That would be the first thing I'd try if we have
+ performance problems on one test while decreasing the threshold leads to
+ other tests being broken like ds022 above.
+mAX_REFINEMENTS = 3
+
+  The threshold for detecting exponential blowup in the number of 'Delta's
+ to check introduced by guards.
+tHRESHOLD_GUARD_DELTAS :: Int
+tHRESHOLD_GUARD_DELTAS = 100
+
+  Multiply the estimated number of 'Delta's to process by a constant
+ branching factor induced by a guard and return the new estimate if it
+ doesn't exceed a constant threshold.
+ See 6. in Note [Guards and Approximation].
+tryMultiplyDeltas :: Int > Int > Maybe Int
+tryMultiplyDeltas multiplier n_delta
+  The ^2 below is intentional: We want to give up on guards with a large
+  branching factor rather quickly, still leaving room for small informative
+  ones later on.
+  n_delta*multiplier^(2::Int) < tHRESHOLD_GUARD_DELTAS
+ = Just (n_delta*multiplier)
+  otherwise
+ = Nothing
{
Note [Recovering from unsatisfiable patternmatching constraints]
~~~~~~~~~~~~~~~~
Consider the following code (see #12957 and #15450):

 f :: Int ~ Bool => ()
 f = case True of { False > () }

We want to warn that the patternmatching in `f` is nonexhaustive. But GHC
used not to do this; in fact, it would warn that the match was /redundant/!
This is because the constraint (Int ~ Bool) in `f` is unsatisfiable, and the
coverage checker deems any matches with unsatifiable constraint sets to be
unreachable.

We decide to better than this. When beginning coverage checking, we first
check if the constraints in scope are unsatisfiable, and if so, we start
afresh with an empty set of constraints. This way, we'll get the warnings
that we expect.
}

  Given a conlike's term constraints, type constraints, and strict argument
 types, check if they are satisfiable.
 (In other words, this is the ⊢_Sat oracle judgment from the GADTs Meet
 Their Match paper.)

 For the purposes of efficiency, this takes as separate arguments the
 ambient term and type constraints (which are known beforehand to be
 satisfiable), as well as the new term and type constraints (which may not
 be satisfiable). This lets us implement two minioptimizations:

 * If there are no new type constraints, then don't bother initializing
 the type oracle, since it's redundant to do so.
 * Since the new term constraint is a separate argument, we only need to
 execute one iteration of the term oracle (instead of traversing the
 entire set of term constraints).

 Taking strict argument types into account is something which was not
 discussed in GADTs Meet Their Match. For an explanation of what role they
 serve, see @Note [Extensions to GADTs Meet Their Match]@.
pmIsSatisfiable
 :: Delta  ^ The ambient term and type constraints
  (known to be satisfiable).
 > TmVarCt  ^ The new term constraint.
 > Bag EvVar  ^ The new type constraints.
 > [Type]  ^ The strict argument types.
 > PmM (Maybe Delta)
  ^ @'Just' delta@ if the constraints (@delta@) are
  satisfiable, and each strict argument type is inhabitable.
  'Nothing' otherwise.
pmIsSatisfiable amb_cs new_tm_c new_ty_cs strict_arg_tys = do
 mb_sat < tmTyCsAreSatisfiable amb_cs new_tm_c new_ty_cs
 case mb_sat of
 Nothing > pure Nothing
 Just delta > do
  We know that the term and type constraints are inhabitable, so now
  check if each strict argument type is inhabitable.
 all_non_void < checkAllNonVoid initRecTc delta strict_arg_tys
 pure $ if all_non_void  Check if each strict argument type
  is inhabitable
 then Just delta
 else Nothing

  Like 'pmIsSatisfiable', but only checks if term and type constraints are
 satisfiable, and doesn't bother checking anything related to strict argument
 types.
tmTyCsAreSatisfiable
 :: Delta  ^ The ambient term and type constraints
  (known to be satisfiable).
 > TmVarCt  ^ The new term constraint.
 > Bag EvVar  ^ The new type constraints.
 > PmM (Maybe Delta)
  ^ @'Just' delta@ if the constraints (@delta@) are
  satisfiable. 'Nothing' otherwise.
tmTyCsAreSatisfiable
 (MkDelta{ delta_tm_cs = amb_tm_cs, delta_ty_cs = amb_ty_cs })
 new_tm_c new_ty_cs = do
 let ty_cs = new_ty_cs `unionBags` amb_ty_cs
 sat_ty < if isEmptyBag new_ty_cs
 then pure True
 else tyOracle ty_cs
 pure $ case (sat_ty, solveOneEq amb_tm_cs new_tm_c) of
 (True, Just term_cs) > Just $ MkDelta{ delta_ty_cs = ty_cs
 , delta_tm_cs = term_cs }
 _unsat > Nothing

  Implements two performance optimizations, as described in the
 \"Strict argument type constraints\" section of
 @Note [Extensions to GADTs Meet Their Match]@.
checkAllNonVoid :: RecTcChecker > Delta > [Type] > PmM Bool
checkAllNonVoid rec_ts amb_cs strict_arg_tys = do
 fam_insts < liftD dsGetFamInstEnvs
 let definitely_inhabited =
 definitelyInhabitedType fam_insts (delta_ty_cs amb_cs)
 tys_to_check < filterOutM definitely_inhabited strict_arg_tys
 let rec_max_bound  tys_to_check `lengthExceeds` 1
 = 1
  otherwise
 = defaultRecTcMaxBound
 rec_ts' = setRecTcMaxBound rec_max_bound rec_ts
 allM (nonVoid rec_ts' amb_cs) tys_to_check

  Checks if a strict argument type of a conlike is inhabitable by a
 terminating value (i.e, an 'InhabitationCandidate').
 See @Note [Extensions to GADTs Meet Their Match]@.
nonVoid
 :: RecTcChecker  ^ The per'TyCon' recursion depth limit.
 > Delta  ^ The ambient term/type constraints (known to be
  satisfiable).
 > Type  ^ The strict argument type.
 > PmM Bool  ^ 'True' if the strict argument type might be inhabited by
  a terminating value (i.e., an 'InhabitationCandidate').
  'False' if it is definitely uninhabitable by anything
  (except bottom).
nonVoid rec_ts amb_cs strict_arg_ty = do
 mb_cands < inhabitationCandidates (delta_ty_cs amb_cs) strict_arg_ty
 case mb_cands of
 Right (tc, cands)
  Just rec_ts' < checkRecTc rec_ts tc
 > anyM (cand_is_inhabitable rec_ts' amb_cs) cands
  A strict argument type is inhabitable by a terminating value if
  at least one InhabitationCandidate is inhabitable.
 _ > pure True
  Either the type is trivially inhabited or we have exceeded the
  recursion depth for some TyCon (so bail out and conservatively
  claim the type is inhabited).
 where
  Checks if an InhabitationCandidate for a strict argument type:
 
  (1) Has satisfiable term and type constraints.
  (2) Has 'nonVoid' strict argument types (we bail out of this
  check if recursion is detected).
 
  See Note [Extensions to GADTs Meet Their Match]
 cand_is_inhabitable :: RecTcChecker > Delta
 > InhabitationCandidate > PmM Bool
 cand_is_inhabitable rec_ts amb_cs
 (InhabitationCandidate{ ic_tm_ct = new_term_c
 , ic_ty_cs = new_ty_cs
 , ic_strict_arg_tys = new_strict_arg_tys }) = do
 mb_sat < tmTyCsAreSatisfiable amb_cs new_term_c new_ty_cs
 case mb_sat of
 Nothing > pure False
 Just new_delta > do
 checkAllNonVoid rec_ts new_delta new_strict_arg_tys

  @'definitelyInhabitedType' ty@ returns 'True' if @ty@ has at least one
 constructor @C@ such that:

 1. @C@ has no equality constraints.
 2. @C@ has no strict argument types.

 See the \"Strict argument type constraints\" section of
 @Note [Extensions to GADTs Meet Their Match]@.
definitelyInhabitedType :: FamInstEnvs > Bag EvVar > Type > PmM Bool
definitelyInhabitedType env ty_cs ty = do
 mb_res < pmTopNormaliseType_maybe env ty_cs ty
 pure $ case mb_res of
 Just (_, cons, _) > any meets_criteria cons
 Nothing > False
 where
 meets_criteria :: DataCon > Bool
 meets_criteria con =
 null (dataConEqSpec con) &&  (1)
 null (dataConImplBangs con)  (2)

{ Note [Type normalisation for EmptyCase]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
EmptyCase is an exception for pattern matching, since it is strict. This means
that it boils down to checking whether the type of the scrutinee is inhabited.
Function pmTopNormaliseType_maybe gets rid of the outermost type function/data
family redex and newtypes, in search of an algebraic type constructor, which is
easier to check for inhabitation.

It returns 3 results instead of one, because there are 2 subtle points:
1. Newtypes are isomorphic to the underlying type in core but not in the source
 language,
2. The representational data family tycon is used internally but should not be
 shown to the user

Hence, if pmTopNormaliseType_maybe env ty_cs ty = Just (src_ty, dcs, core_ty),
then
 (a) src_ty is the rewritten type which we can show to the user. That is, the
 type we get if we rewrite type families but not data families or
 newtypes.
 (b) dcs is the list of data constructors "skipped", every time we normalise a
 newtype to its core representation, we keep track of the source data
 constructor.
 (c) core_ty is the rewritten type. That is,
 pmTopNormaliseType_maybe env ty_cs ty = Just (src_ty, dcs, core_ty)
 implies
 topNormaliseType_maybe env ty = Just (co, core_ty)
 for some coercion co.

To see how all cases come into play, consider the following example:

 data family T a :: *
 data instance T Int = T1  T2 Bool
  Which gives rise to FC:
  data T a
  data R:TInt = T1  T2 Bool
  axiom ax_ti : T Int ~R R:TInt

 newtype G1 = MkG1 (T Int)
 newtype G2 = MkG2 G1

 type instance F Int = F Char
 type instance F Char = G2

In this case pmTopNormaliseType_maybe env ty_cs (F Int) results in

 Just (G2, [MkG2,MkG1], R:TInt)

Which means that in source Haskell:
  G2 is equivalent to F Int (in contrast, G1 isn't).
  if (x : R:TInt) then (MkG2 (MkG1 x) : F Int).


 Wrinkle: Local equalities


Given the following type family:

 type family F a
 type instance F Int = Void

Should the following program (from #14813) be considered exhaustive?

 f :: (i ~ Int) => F i > a
 f x = case x of {}

You might think "of course, since `x` is obviously of type Void". But the
idType of `x` is technically F i, not Void, so if we pass F i to
inhabitationCandidates, we'll mistakenly conclude that `f` is nonexhaustive.
In order to avoid this pitfall, we need to normalise the type passed to
pmTopNormaliseType_maybe, using the constraint solver to solve for any local
equalities (such as i ~ Int) that may be in scope.
}

  Generate all 'InhabitationCandidate's for a given type. The result is
 either @'Left' ty@, if the type cannot be reduced to a closed algebraic type
 (or if it's one trivially inhabited, like 'Int'), or @'Right' candidates@,
 if it can. In this case, the candidates are the signature of the tycon, each
 one accompanied by the term and type constraints it gives rise to.
 See also Note [Checking EmptyCase Expressions]
inhabitationCandidates :: Bag EvVar > Type
 > PmM (Either Type (TyCon, [InhabitationCandidate]))
inhabitationCandidates ty_cs ty = do
 fam_insts < liftD dsGetFamInstEnvs
 mb_norm_res < pmTopNormaliseType_maybe fam_insts ty_cs ty
 case mb_norm_res of
 Just (src_ty, dcs, core_ty) > alts_to_check src_ty core_ty dcs
 Nothing > alts_to_check ty ty []
 where
  All these types are trivially inhabited
 trivially_inhabited = [ charTyCon, doubleTyCon, floatTyCon
 , intTyCon, wordTyCon, word8TyCon ]

  Note: At the moment we leave all the typing and constraint fields of
  PmCon empty, since we know that they are not gonna be used. Is the
  rightthingtodo to actually create them, even if they are never used?
 build_tm :: ValAbs > [DataCon] > ValAbs
 build_tm = foldr (\dc e > PmCon (RealDataCon dc) [] [] [] [e])

  Inhabitation candidates, using the result of pmTopNormaliseType_maybe
 alts_to_check :: Type > Type > [DataCon]
 > PmM (Either Type (TyCon, [InhabitationCandidate]))
 alts_to_check src_ty core_ty dcs = case splitTyConApp_maybe core_ty of
 Just (tc, tc_args)
  tc `elem` trivially_inhabited
 > case dcs of
 [] > return (Left src_ty)
 (_:_) > do var < liftD $ mkPmId core_ty
 let va = build_tm (PmVar var) dcs
 return $ Right (tc, [InhabitationCandidate
 { ic_val_abs = va, ic_tm_ct = mkIdEq var
 , ic_ty_cs = emptyBag, ic_strict_arg_tys = [] }])

  pmIsClosedType core_ty && not (isAbstractTyCon tc)
  Don't consider abstract tycons since we don't know what their
  constructors are, which makes the results of coverage checking
  them extremely misleading.
 > liftD $ do
 var < mkPmId core_ty  it would be wrong to unify x
 alts < mapM (mkOneConFull var tc_args . RealDataCon) (tyConDataCons tc)
 return $ Right
 (tc, [ alt{ic_val_abs = build_tm (ic_val_abs alt) dcs}
  alt < alts ])
  For other types conservatively assume that they are inhabited.
 _other > return (Left src_ty)

{ Note [Checking EmptyCase Expressions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Empty case expressions are strict on the scrutinee. That is, `case x of {}`
will force argument `x`. Hence, `checkMatches` is not sufficient for checking
empty cases, because it assumes that the match is not strict (which is true
for all other cases, apart from EmptyCase). This gave rise to #10746. Instead,
we do the following:

1. We normalise the outermost type family redex, data family redex or newtype,
 using pmTopNormaliseType_maybe (in types/FamInstEnv.hs). This computes 3
 things:
 (a) A normalised type src_ty, which is equal to the type of the scrutinee in
 source Haskell (does not normalise newtypes or data families)
 (b) The actual normalised type core_ty, which coincides with the result
 topNormaliseType_maybe. This type is not necessarily equal to the input
 type in source Haskell. And this is precicely the reason we compute (a)
 and (c): the reasoning happens with the underlying types, but both the
 patterns and types we print should respect newtypes and also show the
 family type constructors and not the representation constructors.

 (c) A list of all newtype data constructors dcs, each one corresponding to a
 newtype rewrite performed in (b).

 For an example see also Note [Type normalisation for EmptyCase]
 in types/FamInstEnv.hs.

2. Function checkEmptyCase' performs the check:
  If core_ty is not an algebraic type, then we cannot check for
 inhabitation, so we emit (_ :: src_ty) as missing, conservatively assuming
 that the type is inhabited.
  If core_ty is an algebraic type, then we unfold the scrutinee to all
 possible constructor patterns, using inhabitationCandidates, and then
 check each one for constraint satisfiability, same as we for normal
 pattern match checking.

%************************************************************************
%* *
Transform source syntax to *our* syntax
@@ 920,14 +425,14 @@ we do the following:
 
 * Utilities
nullaryConPattern :: ConLike > Pattern
+nullaryConPattern :: ConLike > PmPat
 Nullary data constructor and nullary type constructor
nullaryConPattern con =
 PmCon { pm_con_con = con, pm_con_arg_tys = []
 , pm_con_tvs = [], pm_con_dicts = [], pm_con_args = [] }
+ PmCon { pm_con_con = (PmAltConLike con), pm_con_arg_tys = []
+ , pm_con_tvs = [], pm_con_args = [] }
{# INLINE nullaryConPattern #}
truePattern :: Pattern
+truePattern :: PmPat
truePattern = nullaryConPattern (RealDataCon trueDataCon)
{# INLINE truePattern #}
@@ 937,35 +442,54 @@ mkCanFailPmPat ty = do
var < mkPmVar ty
return [var, PmFake]
vanillaConPattern :: ConLike > [Type] > PatVec > Pattern
+vanillaConPattern :: ConLike > [Type] > PatVec > PmPat
 ADT constructor pattern => no existentials, no local constraints
vanillaConPattern con arg_tys args =
 PmCon { pm_con_con = con, pm_con_arg_tys = arg_tys
 , pm_con_tvs = [], pm_con_dicts = [], pm_con_args = args }
+ PmCon { pm_con_con = PmAltConLike con, pm_con_arg_tys = arg_tys
+ , pm_con_tvs = [], pm_con_args = args }
{# INLINE vanillaConPattern #}
  Create an empty list pattern of a given type
nilPattern :: Type > Pattern
+nilPattern :: Type > PmPat
nilPattern ty =
 PmCon { pm_con_con = RealDataCon nilDataCon, pm_con_arg_tys = [ty]
 , pm_con_tvs = [], pm_con_dicts = []
 , pm_con_args = [] }
+ PmCon { pm_con_con = PmAltConLike (RealDataCon nilDataCon)
+ , pm_con_arg_tys = [ty], pm_con_tvs = [], pm_con_args = [] }
{# INLINE nilPattern #}
mkListPatVec :: Type > PatVec > PatVec > PatVec
mkListPatVec ty xs ys = [PmCon { pm_con_con = RealDataCon consDataCon
+mkListPatVec ty xs ys = [PmCon { pm_con_con = PmAltConLike (RealDataCon consDataCon)
, pm_con_arg_tys = [ty]
 , pm_con_tvs = [], pm_con_dicts = []
+ , pm_con_tvs = []
, pm_con_args = xs++ys }]
{# INLINE mkListPatVec #}
  Create a (nonoverloaded) literal pattern
mkLitPattern :: HsLit GhcTc > Pattern
mkLitPattern lit = PmLit { pm_lit_lit = PmSLit lit }
{# INLINE mkLitPattern #}
+  Create a literal pattern
+mkPmLitPattern :: PmLit > PatVec
+mkPmLitPattern lit@(PmLit _ val)
+  We translate String literals to list literals for better overlap reasoning.
+  It's a little unfortunate we do this here rather than in
+  'PmOracle.trySolve' and 'PmOracle.addRefutableAltCon', but it's so much
+  simpler here.
+  See Note [Representation of Strings in TmState] in PmOracle
+  PmLitString s < val
+ , let mk_char_lit c = mkPmLitPattern (PmLit charTy (PmLitChar c))
+ = foldr (\c p > mkListPatVec charTy (mk_char_lit c) p)
+ [nilPattern charTy]
+ (unpackFS s)
+  otherwise
+ = [PmCon { pm_con_con = PmAltLit lit
+ , pm_con_arg_tys = []
+ , pm_con_tvs = []
+ , pm_con_args = [] }]
+{# INLINE mkPmLitPattern #}
 
 * Transform (Pat Id) into of (PmPat Id)
+ * Transform (Pat Id) into [PmPat]
+ The arity of the [PmPat] is always 1, but it may be a combination
+ of a vanilla pattern and a guard pattern.
+ Example: view pattern (f y > Just x)
+ becomes [PmVar z, PmGrd [PmPat (Just x), f y]]
+ where z is fresh
translatePat :: FamInstEnvs > Pat GhcTc > DsM PatVec
translatePat fam_insts pat = case pat of
@@ 977,12 +501,10 @@ translatePat fam_insts pat = case pat of
 ignore strictness annotations for now
BangPat _ p > translatePat fam_insts (unLoc p)
 AsPat _ lid p > do
  Note [Translating As Patterns]
 ps < translatePat fam_insts (unLoc p)
 let [e] = map vaToPmExpr (coercePatVec ps)
 g = PmGrd [PmVar (unLoc lid)] e
 return (ps ++ [g])
+  (x@pat) ===> x (pat < x)
+ AsPat _ (dL>L _ x) p > do
+ pat < translatePat fam_insts (unLoc p)
+ pure [PmVar x, PmGrd pat (Var x)]
SigPat _ p _ty > translatePat fam_insts (unLoc p)
@@ 991,10 +513,10 @@ translatePat fam_insts pat = case pat of
 isIdHsWrapper wrapper > translatePat fam_insts p
 WpCast co < wrapper, isReflexiveCo co > translatePat fam_insts p
 otherwise > do
 ps < translatePat fam_insts p
+ ps < translatePat fam_insts p
(xp,xe) < mkPmId2Forms ty
g < mkGuard ps (mkHsWrap wrapper (unLoc xe))
 return [xp,g]
+ pure [xp,g]
 (n + k) ===> x (True < x >= k) (n < xk)
NPlusKPat ty (dL>L _ _n) _k1 _k2 _ge _minus > mkCanFailPmPat ty
@@ 1008,20 +530,20 @@ translatePat fam_insts pat = case pat of
True > do
(xp,xe) < mkPmId2Forms arg_ty
g < mkGuard ps (HsApp noExtField lexpr xe)
 return [xp,g]
+ return [xp, g]
False > mkCanFailPmPat arg_ty
 list
ListPat (ListPatTc ty Nothing) ps > do
 foldr (mkListPatVec ty) [nilPattern ty]
 <$> translatePatVec fam_insts (map unLoc ps)
+ pv < translatePatVec fam_insts (map unLoc ps)
+ return (foldr (mkListPatVec ty) [nilPattern ty] pv)
 overloaded list
ListPat (ListPatTc _elem_ty (Just (pat_ty, _to_list))) lpats > do
dflags < getDynFlags
if xopt LangExt.RebindableSyntax dflags
 then mkCanFailPmPat pat_ty
 else case splitListTyConApp_maybe pat_ty of
+ then mkCanFailPmPat pat_ty
+ else case splitListTyConApp_maybe pat_ty of
Just e_ty > translatePat fam_insts
(ListPat (ListPatTc e_ty Nothing) lpats)
Nothing > mkCanFailPmPat pat_ty
@@ 1046,43 +568,50 @@ translatePat fam_insts pat = case pat of
ConPatOut { pat_con = (dL>L _ con)
, pat_arg_tys = arg_tys
, pat_tvs = ex_tvs
 , pat_dicts = dicts
, pat_args = ps } > do
 groups < allCompleteMatches con arg_tys
+ let ty = conLikeResTy con arg_tys
+ groups < allCompleteMatches ty
case groups of
 [] > mkCanFailPmPat (conLikeResTy con arg_tys)
+ [] > mkCanFailPmPat ty
_ > do
args < translateConPatVec fam_insts arg_tys ex_tvs con ps
 return [PmCon { pm_con_con = con
+ return [PmCon { pm_con_con = PmAltConLike con
, pm_con_arg_tys = arg_tys
, pm_con_tvs = ex_tvs
 , pm_con_dicts = dicts
, pm_con_args = args }]
  See Note [Translate Overloaded Literal for Exhaustiveness Checking]
 NPat _ (dL>L _ olit) mb_neg _
  OverLit (OverLitTc False ty) (HsIsString src s) _ < olit
 , isStringTy ty >
 foldr (mkListPatVec charTy) [nilPattern charTy] <$>
 translatePatVec fam_insts
 (map (LitPat noExtField . HsChar src) (unpackFS s))
  otherwise > return [PmLit { pm_lit_lit = PmOLit (isJust mb_neg) olit }]

  See Note [Translate Overloaded Literal for Exhaustiveness Checking]
 LitPat _ lit
  HsString src s < lit >
 foldr (mkListPatVec charTy) [nilPattern charTy] <$>
 translatePatVec fam_insts
 (map (LitPat noExtField . HsChar src) (unpackFS s))
  otherwise > return [mkLitPattern lit]
+ NPat ty (dL>L _ olit) mb_neg _ > do
+  See Note [Literal short cut] in MatchLit.hs
+  We inline the Literal short cut for @ty@ here, because @ty@ is more
+  precise than the field of OverLitTc, which is all that dsOverLit (which
+  normally does the literal short cut) can look at. Also @ty@ matches the
+  type of the scrutinee, so info on both pattern and scrutinee (for which
+  short cutting in dsOverLit works properly) is overloaded iff either is.
+ dflags < getDynFlags
+ core_expr < case olit of
+ OverLit{ ol_val = val, ol_ext = OverLitTc rebindable _ }
+  not rebindable
+ , Just expr < shortCutLit dflags val ty
+ > dsExpr expr
+ _ > dsOverLit olit
+ let lit = expectJust "failed to detect OverLit" (coreExprAsPmLit core_expr)
+ let lit' = case mb_neg of
+ Just _ > expectJust "failed to negate lit" (negatePmLit lit)
+ Nothing > lit
+ return (mkPmLitPattern lit')
+
+ LitPat _ lit > do
+ core_expr < dsLit (convertLit lit)
+ let lit = expectJust "failed to detect Lit" (coreExprAsPmLit core_expr)
+ return (mkPmLitPattern lit)
TuplePat tys ps boxity > do
tidy_ps < translatePatVec fam_insts (map unLoc ps)
let tuple_con = RealDataCon (tupleDataCon boxity (length ps))
tys' = case boxity of
 Boxed > tys
  See Note [Unboxed tuple RuntimeRep vars] in TyCon
 Unboxed > map getRuntimeRep tys ++ tys
+ Boxed > tys
+  See Note [Unboxed tuple RuntimeRep vars] in TyCon
+ Unboxed > map getRuntimeRep tys ++ tys
return [vanillaConPattern tuple_con tys' (concat tidy_ps)]
SumPat ty p alt arity > do
@@ 1097,91 +626,6 @@ translatePat fam_insts pat = case pat of
SplicePat {} > panic "Check.translatePat: SplicePat"
XPat {} > panic "Check.translatePat: XPat"
{ Note [Translate Overloaded Literal for Exhaustiveness Checking]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The translation of @NPat@ in exhaustiveness checker is a bit different
from translation in pattern matcher.

 * In pattern matcher (see `tidyNPat' in deSugar/MatchLit.hs), we
 translate integral literals to HsIntPrim or HsWordPrim and translate
 overloaded strings to HsString.

 * In exhaustiveness checker, in `genCaseTmCs1/genCaseTmCs2`, we use
 `lhsExprToPmExpr` to generate uncovered set. In `hsExprToPmExpr`,
 however we generate `PmOLit` for HsOverLit, rather than refine
 `HsOverLit` inside `NPat` to HsIntPrim/HsWordPrim. If we do
 the same thing in `translatePat` as in `tidyNPat`, the exhaustiveness
 checker will fail to match the literals patterns correctly. See
 #14546.

 In Note [Undecidable Equality for Overloaded Literals], we say: "treat
 overloaded literals that look different as different", but previously we
 didn't do such things.

 Now, we translate the literal value to match and the literal patterns
 consistently:

 * For integral literals, we parse both the integral literal value and
 the patterns as OverLit HsIntegral. For example:

 case 0::Int of
 0 > putStrLn "A"
 1 > putStrLn "B"
 _ > putStrLn "C"

 When checking the exhaustiveness of pattern matching, we translate the 0
 in value position as PmOLit, but translate the 0 and 1 in pattern position
 as PmSLit. The inconsistency leads to the failure of eqPmLit to detect the
 equality and report warning of "Pattern match is redundant" on pattern 0,
 as reported in #14546. In this patch we remove the specialization of
 OverLit patterns, and keep the overloaded number literal in pattern as it
 is to maintain the consistency. We know nothing about the `fromInteger`
 method (see Note [Undecidable Equality for Overloaded Literals]). Now we
 can capture the exhaustiveness of pattern 0 and the redundancy of pattern
 1 and _.

 * For string literals, we parse the string literals as HsString. When
 OverloadedStrings is enabled, it further be turned as HsOverLit HsIsString.
 For example:

 case "foo" of
 "foo" > putStrLn "A"
 "bar" > putStrLn "B"
 "baz" > putStrLn "C"

 Previously, the overloaded string values are translated to PmOLit and the
 nonoverloaded string values are translated to PmSLit. However the string
 patterns, both overloaded and nonoverloaded, are translated to list of
 characters. The inconsistency leads to wrong warnings about redundant and
 nonexhaustive pattern matching warnings, as reported in #14546.

 In order to catch the redundant pattern in following case:

 case "foo" of
 ('f':_) > putStrLn "A"
 "bar" > putStrLn "B"

 in this patch, we translate nonoverloaded string literals, both in value
 position and pattern position, as list of characters. For overloaded string
 literals, we only translate it to list of characters only when it's type
 is stringTy, since we know nothing about the toString methods. But we know
 that if two overloaded strings are syntax equal, then they are equal. Then
 if it's type is not stringTy, we just translate it to PmOLit. We can still
 capture the exhaustiveness of pattern "foo" and the redundancy of pattern
 "bar" and "baz" in the following code:

 {# LANGUAGE OverloadedStrings #}
 main = do
 case "foo" of
 "foo" > putStrLn "A"
 "bar" > putStrLn "B"
 "baz" > putStrLn "C"

 We must ensure that doing the same translation to literal values and patterns
 in `translatePat` and `hsExprToPmExpr`. The previous inconsistent work led to
 #14546.
}

  Translate a list of patterns (Note: each pattern is translated
 to a pattern vector but we do not concatenate the results).
translatePatVec :: FamInstEnvs > [Pat GhcTc] > DsM [PatVec]
@@ 1189,7 +633,8 @@ translatePatVec fam_insts pats = mapM (translatePat fam_insts) pats
  Translate a constructor pattern
translateConPatVec :: FamInstEnvs > [Type] > [TyVar]
 > ConLike > HsConPatDetails GhcTc > DsM PatVec
+ > ConLike > HsConPatDetails GhcTc
+ > DsM PatVec
translateConPatVec fam_insts _univ_tys _ex_tvs _ (PrefixCon ps)
= concat <$> translatePatVec fam_insts (map unLoc ps)
translateConPatVec fam_insts _univ_tys _ex_tvs _ (InfixCon p1 p2)
@@ 1221,7 +666,7 @@ translateConPatVec fam_insts univ_tys ex_tvs c (RecCon (HsRecFields fs _))
let zipped = zip orig_lbls [ x  PmVar x < arg_var_pats ]
guards = map (\(name,pvec) > case lookup name zipped of
 Just x > PmGrd pvec (PmExprVar (idName x))
+ Just x > PmGrd pvec (Var x)
Nothing > panic "translateConPatVec: lookup")
translated_pats
@@ 1245,19 +690,20 @@ translateConPatVec fam_insts univ_tys ex_tvs c (RecCon (HsRecFields fs _))
 Translate a single match
translateMatch :: FamInstEnvs > LMatch GhcTc (LHsExpr GhcTc)
 > DsM (PatVec,[PatVec])
translateMatch fam_insts (dL>L _ (Match { m_pats = lpats, m_grhss = grhss })) =
 do
 pats' < concat <$> translatePatVec fam_insts pats
 guards' < mapM (translateGuards fam_insts) guards
 return (pats', guards')
 where
 extractGuards :: LGRHS GhcTc (LHsExpr GhcTc) > [GuardStmt GhcTc]
 extractGuards (dL>L _ (GRHS _ gs _)) = map unLoc gs
 extractGuards _ = panic "translateMatch"

 pats = map unLoc lpats
 guards = map extractGuards (grhssGRHSs grhss)
+ > DsM (PatVec, [PatVec])
+translateMatch fam_insts (dL>L _ (Match { m_pats = lpats, m_grhss = grhss }))
+ = do
+ pats' < concat <$> translatePatVec fam_insts pats
+ guards' < mapM (translateGuards fam_insts) guards
+  tracePm "translateMatch" (vcat [ppr pats, ppr pats', ppr guards, ppr guards'])
+ return (pats', guards')
+ where
+ extractGuards :: LGRHS GhcTc (LHsExpr GhcTc) > [GuardStmt GhcTc]
+ extractGuards (dL>L _ (GRHS _ gs _)) = map unLoc gs
+ extractGuards _ = panic "translateMatch"
+
+ pats = map unLoc lpats
+ guards = map extractGuards (grhssGRHSs grhss)
translateMatch _ _ = panic "translateMatch"
 
@@ 1265,35 +711,11 @@ translateMatch _ _ = panic "translateMatch"
  Translate a list of guard statements to a pattern vector
translateGuards :: FamInstEnvs > [GuardStmt GhcTc] > DsM PatVec
translateGuards fam_insts guards = do
 all_guards < concat <$> mapM (translateGuard fam_insts) guards
 let
 shouldKeep :: Pattern > DsM Bool
 shouldKeep p
  PmVar {} < p = pure True
  PmCon {} < p = (&&)
 <$> singleMatchConstructor (pm_con_con p) (pm_con_arg_tys p)
 <*> allM shouldKeep (pm_con_args p)
 shouldKeep (PmGrd pv e)
  isNotPmExprOther e = pure True  expensive but we want it
  otherwise = allM shouldKeep pv
 shouldKeep _other_pat = pure False  let the rest..

 all_handled < allM shouldKeep all_guards
  It should have been @pure all_guards@ but it is too expressive.
  Since the term oracle does not handle all constraints we generate,
  we (hackily) replace all constraints the oracle cannot handle with a
  single one (we need to know if there is a possibility of failure).
  See Note [Guards and Approximation] for all guardrelated approximations
  we implement.
 if all_handled
 then pure all_guards
 else do
 kept < filterM shouldKeep all_guards
 pure (PmFake : kept)
+translateGuards fam_insts guards =
+ concat <$> mapM (translateGuard fam_insts) guards
  Check whether a pattern can fail to match
cantFailPattern :: Pattern > DsM Bool
+cantFailPattern :: PmPat > DsM Bool
cantFailPattern PmVar {} = pure True
cantFailPattern PmCon { pm_con_con = c, pm_con_arg_tys = tys, pm_con_args = ps}
= (&&) <$> singleMatchConstructor c tys <*> allM cantFailPattern ps
@@ 1395,11 +817,10 @@ below is the *right thing to do*:
The case with literals is a bit different. a literal @l@ should be translated
to @x (True < x == from l)@. Since we want to have better warnings for
overloaded literals as it is a very common feature, we treat them differently.
They are mainly covered in Note [Undecidable Equality for Overloaded Literals]
in PmExpr.
+They are mainly covered in Note [Undecidable Equality for PmAltCons] in PmExpr.
4. N+K Patterns & Pattern Synonyms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
An n+k pattern (n+k) should be translated to @x (True < x >= k) (n < xk)@.
Since the only pattern of the three that causes failure is guard @(n < xk)@,
and has two possible outcomes. Hence, there is no benefit in using a dummy and
@@ 1418,18 +839,85 @@ Additionally, toplevel guard translation (performed by @translateGuards@)
replaces guards that cannot be reasoned about (like the ones we described in
14) with a single @PmFake@ to record the possibility of failure to match.
+6. Combinatorial explosion
+
+Function with many clauses and deeply nested guards like in #11195 tend to
+overwhelm the checker because they lead to exponential splitting behavior.
+See the comments on #11195 on refinement trees. Every guard refines the
+disjunction of Deltas by another split. This no different than the ConVar case,
+but in stark contrast we mostly don't get any useful information out of that
+split! Hence splitting kfold just means having kfold more work. The problem
+exacerbates for larger k, because it gets even more unlikely that we can handle
+all of the arising Deltas better than just continue working on the original
+Delta.
+Long story short: At each point we estimate the number of Deltas we possibly
+have to check in the same manner as the current Delta. If we hit a guard that
+splits the current Delta kfold, we check whether this split would get us beyond
+a certain threshold ('tryMultiplyDeltas') and continue to check the other guards
+with the original Delta.
+
+Note [Limit the number of refinements]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In PrelRules, we have a huge case with relatively deep matches on pattern
+synonyms. Since we allow multiple compatible solutions in the oracle
+(i.e. @x ~ [I# y, 42]@), and every pattern synonym is compatible according to
+'eqPmAltCon' with every other (no generativity as with DataCons), what would
+usually result in a ConVar split where only one branch is satisfiable results
+in a blowup of Deltas. Here's an example:
+ case x of
+ A (A _) > ()
+ B (B _) > ()
+ ...
+By the time we hit the first clause's RHS, we have split the initial Delta twice
+and handled the {x~A y, y ~ A z} case, leaving {x/~A} and {x~A y, y/~A} models
+for the second clause to check.
+
+Now consider what happens if A=Left, B=Right. We get x~B y' from the match,
+which contradicts with {x~A y, y/~A}, because A and B are incompatible due to
+the generative nature of DataCons. This leaves only {x/~A} for checking B's
+clause, which ultimately leaves {x/~[A,B]} and {x~B y', y'/~B} uncovered.
+Resulting in three models to check for the next clause. That's only linear
+growth in the number of models for each clause.
+
+Consider A and B were arbitrary pattern synonyms instead. We still get x~B y'
+from the match, but this no longer refutes {x~A y, y/~A}, because we don't
+assume generativity for pattern synonyms. Ergo, @eqPmAltCon A B == Nothing@
+and we get to check the second clause's inner match with {x~B y', x/~A} and
+{x~[A y,B y'], y/~A}, splitting both in turn. That makes 4 instead of 3 deltas.
+If we keep on doing this, we see that in the nth clause we'd have O(2^n) models
+to check instead of just O(n) as above!
+
+Clearly we have to put a stop to this. So we count in the oracle the number of
+times we refined x to some constructor. If the number of splits exceeds the
+'mAX_REFINEMENTS', we check the next clause using the original Delta rather
+than the union of Deltas arising from the ConVar split.
+
+If for the above example we had mAX_REFINEMENTS=1, then in the second clause
+we would still check the inner match with {x~B y', x/~A} and {x~[A y,B y'], y/~A}
+but *discard* the two Deltas arising from splitting {x~[A y,B y'], y/~A},
+checking the next clause with {x~A y, y/~A} instead of its two refinements.
+In addition to {x~B y', y'~B z', x/~A} (which arose from the other split) and
+{x/~[A,B]} that makes 3 models for the third equation, so linear :).
+
Note [Translate CoPats]
~~~~~~~~~~~~~~~~~~~~~~~
The pattern match checker did not know how to handle coerced patterns `CoPat`
efficiently, which gave rise to #11276. The original approach translated
`CoPat`s:
 pat > co ===> x (pat < (e > co))
+ pat > co ===> x (pat < (x > co))
Instead, we now check whether the coercion is a hole or if it is just refl, in
which case we can drop it. Unfortunately, data families generate useful
coercions so guards are still generated in these cases and checking data
families is not really efficient.
+Why did we do this seemingly unnecessary expansion in the first place?
+The reason is that the type of @pat > co@ (which is the type of the value
+abstraction we match against) might be different than that of @pat@. Data
+instances such as @Sing (a :: Bool)@ are a good example of this: If we would
+just drop the coercion, we'd get a type error when matching @pat@ against its
+value abstraction, with the result being that pmIsSatisfiable decides that every
+possible data constructor fitting @pat@ is rejected as uninhabitated, leading to
+a lot of false warnings.
+
+But we can check whether the coercion is a hole or if it is just refl, in
+which case we can drop it.
%************************************************************************
%* *
@@ 1443,31 +931,15 @@ families is not really efficient.
  Get the type out of a PmPat. For guard patterns (ps < e) we use the type
 of the first (or the single WHEREVER IT IS valid to use?) pattern
pmPatType :: PmPat p > Type
+pmPatType :: PmPat > Type
pmPatType (PmCon { pm_con_con = con, pm_con_arg_tys = tys })
 = conLikeResTy con tys
+ = pmAltConType con tys
pmPatType (PmVar { pm_var_id = x }) = idType x
pmPatType (PmLit { pm_lit_lit = l }) = pmLitType l
pmPatType (PmNLit { pm_lit_id = x }) = idType x
pmPatType (PmGrd { pm_grd_pv = pv })
= ASSERT(patVecArity pv == 1) (pmPatType p)
where Just p = find ((==1) . patternArity) pv
pmPatType PmFake = pmPatType truePattern
  Information about a conlike that is relevant to coverage checking.
 It is called an \"inhabitation candidate\" since it is a value which may
 possibly inhabit some type, but only if its term constraint ('ic_tm_ct')
 and type constraints ('ic_ty_cs') are permitting, and if all of its strict
 argument types ('ic_strict_arg_tys') are inhabitable.
 See @Note [Extensions to GADTs Meet Their Match]@.
data InhabitationCandidate =
 InhabitationCandidate
 { ic_val_abs :: ValAbs
 , ic_tm_ct :: TmVarCt
 , ic_ty_cs :: Bag EvVar
 , ic_strict_arg_tys :: [Type]
 }

{
Note [Extensions to GADTs Meet Their Match]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ 1476,275 +948,45 @@ checker adheres to. Since the paper's publication, there have been some
additional features added to the coverage checker which are not described in
the paper. This Note serves as a reference for these new features.

 Strict argument type constraints


In the ConVar case of clause processing, each conlike K traditionally
generates two different forms of constraints:

* A term constraint (e.g., x ~ K y1 ... yn)
* Type constraints from the conlike's context (e.g., if K has type
 forall bs. Q => s1 .. sn > T tys, then Q would be its type constraints)

As it turns out, these alone are not enough to detect a certain class of
unreachable code. Consider the following example (adapted from #15305):

 data K = K1  K2 !Void

 f :: K > ()
 f K1 = ()

Even though `f` doesn't match on `K2`, `f` is exhaustive in its patterns. Why?
Because it's impossible to construct a terminating value of type `K` using the
`K2` constructor, and thus it's impossible for `f` to ever successfully match
on `K2`.

The reason is because `K2`'s field of type `Void` is //strict//. Because there
are no terminating values of type `Void`, any attempt to construct something
using `K2` will immediately loop infinitely or throw an exception due to the
strictness annotation. (If the field were not strict, then `f` could match on,
say, `K2 undefined` or `K2 (let x = x in x)`.)

Since neither the term nor type constraints mentioned above take strict
argument types into account, we make use of the `nonVoid` function to
determine whether a strict type is inhabitable by a terminating value or not.

`nonVoid ty` returns True when either:
1. `ty` has at least one InhabitationCandidate for which both its term and type
 constraints are satifiable, and `nonVoid` returns `True` for all of the
 strict argument types in that InhabitationCandidate.
2. We're unsure if it's inhabited by a terminating value.

`nonVoid ty` returns False when `ty` is definitely uninhabited by anything
(except bottom). Some examples:

* `nonVoid Void` returns False, since Void has no InhabitationCandidates.
 (This is what lets us discard the `K2` constructor in the earlier example.)
* `nonVoid (Int :~: Int)` returns True, since it has an InhabitationCandidate
 (through the Refl constructor), and its term constraint (x ~ Refl) and
 type constraint (Int ~ Int) are satisfiable.
* `nonVoid (Int :~: Bool)` returns False. Although it has an
 InhabitationCandidate (by way of Refl), its type constraint (Int ~ Bool) is
 not satisfiable.
* Given the following definition of `MyVoid`:

 data MyVoid = MkMyVoid !Void

 `nonVoid MyVoid` returns False. The InhabitationCandidate for the MkMyVoid
 constructor contains Void as a strict argument type, and since `nonVoid Void`
 returns False, that InhabitationCandidate is discarded, leaving no others.

* Performance considerations

We must be careful when recursively calling `nonVoid` on the strict argument
types of an InhabitationCandidate, because doing so naïvely can cause GHC to
fall into an infinite loop. Consider the following example:

 data Abyss = MkAbyss !Abyss

 stareIntoTheAbyss :: Abyss > a
 stareIntoTheAbyss x = case x of {}

In principle, stareIntoTheAbyss is exhaustive, since there is no way to
construct a terminating value using MkAbyss. However, both the term and type
constraints for MkAbyss are satisfiable, so the only way one could determine
that MkAbyss is unreachable is to check if `nonVoid Abyss` returns False.
There is only one InhabitationCandidate for Abyss—MkAbyss—and both its term
and type constraints are satisfiable, so we'd need to check if `nonVoid Abyss`
returns False... and now we've entered an infinite loop!

To avoid this sort of conundrum, `nonVoid` uses a simple test to detect the
presence of recursive types (through `checkRecTc`), and if recursion is
detected, we bail out and conservatively assume that the type is inhabited by
some terminating value. This avoids infinite loops at the expense of making
the coverage checker incomplete with respect to functions like
stareIntoTheAbyss above. Then again, the same problem occurs with recursive
newtypes, like in the following code:

 newtype Chasm = MkChasm Chasm

 gazeIntoTheChasm :: Chasm > a
 gazeIntoTheChasm x = case x of {}  Erroneously warned as nonexhaustive

So this limitation is somewhat understandable.

Note that even with this recursion detection, there is still a possibility that
`nonVoid` can run in exponential time. Consider the following data type:

 data T = MkT !T !T !T

If we call `nonVoid` on each of its fields, that will require us to once again
check if `MkT` is inhabitable in each of those three fields, which in turn will
require us to check if `MkT` is inhabitable again... As you can see, the
branching factor adds up quickly, and if the recursion depth limit is, say,
100, then `nonVoid T` will effectively take forever.

To mitigate this, we check the branching factor every time we are about to call
`nonVoid` on a list of strict argument types. If the branching factor exceeds 1
(i.e., if there is potential for exponential runtime), then we limit the
maximum recursion depth to 1 to mitigate the problem. If the branching factor
is exactly 1 (i.e., we have a linear chain instead of a tree), then it's okay
to stick with a larger maximum recursion depth.

Another microoptimization applies to data types like this one:

 data S a = ![a] !T

Even though there is a strict field of type [a], it's quite silly to call
nonVoid on it, since it's "obvious" that it is inhabitable. To make this
intuition formal, we say that a type is definitely inhabitable (DI) if:

 * It has at least one constructor C such that:
 1. C has no equality constraints (since they might be unsatisfiable)
 2. C has no strict argument types (since they might be uninhabitable)

It's relatively cheap to cheap if a type is DI, so before we call `nonVoid`
on a list of strict argument types, we filter out all of the DI ones.
+* Value abstractions are severely simplified to the point where they are just
+ variables. The information about the PmExpr shape of a variable is encoded in
+ the oracle state 'Delta' instead.
+* Handling of uninhabited fields like `!Void`.
+ See Note [Strict argument type constraints] in PmOracle.
+* Efficient handling of literal splitting, large enumerations and accurate
+ redundancy warnings for `COMPLETE` groups through the oracle.
}
instance Outputable InhabitationCandidate where
 ppr (InhabitationCandidate { ic_val_abs = va, ic_tm_ct = tm_ct
 , ic_ty_cs = ty_cs
 , ic_strict_arg_tys = strict_arg_tys }) =
 text "InhabitationCandidate" <+>
 vcat [ text "ic_val_abs =" <+> ppr va
 , text "ic_tm_ct =" <+> ppr tm_ct
 , text "ic_ty_cs =" <+> ppr ty_cs
 , text "ic_strict_arg_tys =" <+> ppr strict_arg_tys ]

  Generate an 'InhabitationCandidate' for a given conlike (generate
 fresh variables of the appropriate type for arguments)
mkOneConFull :: Id > [Type] > ConLike > DsM InhabitationCandidate
 * 'con' K is a conlike of algebraic data type 'T tys'

 * 'tc_args' are the type arguments of the 'con's TyCon T

 * 'x' is the variable for which we encode an equality constraint
 in the term oracle

 After instantiating the universal tyvars of K to tc_args we get
 K @tys :: forall bs. Q => s1 .. sn > T tys

 Suppose y1 is a strict field. Then we get
 Results: ic_val_abs: K (y1::s1) .. (yn::sn)
 ic_tm_ct: x ~ K y1..yn
 ic_ty_cs: Q
 ic_strict_arg_tys: [s1]
mkOneConFull x tc_args con = do
 let (univ_tvs, ex_tvs, eq_spec, thetas, _req_theta , arg_tys, _con_res_ty)
 = conLikeFullSig con
 arg_is_banged = map isBanged $ conLikeImplBangs con
 subst1 = zipTvSubst univ_tvs tc_args

 (subst, ex_tvs') < cloneTyVarBndrs subst1 ex_tvs <$> getUniqueSupplyM

  Field types
 let arg_tys' = substTys subst arg_tys
  Fresh term variables (VAs) as arguments to the constructor
 arguments < mapM mkPmVar arg_tys'
  All constraints bound by the constructor (alpharenamed)
 let theta_cs = substTheta subst (eqSpecPreds eq_spec ++ thetas)
 evvars < mapM (nameType "pm") theta_cs
 let con_abs = PmCon { pm_con_con = con
 , pm_con_arg_tys = tc_args
 , pm_con_tvs = ex_tvs'
 , pm_con_dicts = evvars
 , pm_con_args = arguments }
 strict_arg_tys = filterByList arg_is_banged arg_tys'
 return $ InhabitationCandidate
 { ic_val_abs = con_abs
 , ic_tm_ct = TVC x (vaToPmExpr con_abs)
 , ic_ty_cs = listToBag evvars
 , ic_strict_arg_tys = strict_arg_tys
 }

 
 * More smart constructors and fresh variable generation
  Create a guard pattern
mkGuard :: PatVec > HsExpr GhcTc > DsM Pattern
mkGuard pv e = do
 res < allM cantFailPattern pv
 let expr = hsExprToPmExpr e
 tracePmD "mkGuard" (vcat [ppr pv, ppr e, ppr res, ppr expr])
 if  res > pure (PmGrd pv expr)
  PmExprOther {} < expr > pure PmFake
  otherwise > pure (PmGrd pv expr)

  Create a term equality of the form: `(x ~ lit)`
mkPosEq :: Id > PmLit > TmVarCt
mkPosEq x l = TVC x (PmExprLit l)
{# INLINE mkPosEq #}

  Create a term equality of the form: `(x ~ x)`
 (always discharged by the term oracle)
mkIdEq :: Id > TmVarCt
mkIdEq x = TVC x (PmExprVar (idName x))
{# INLINE mkIdEq #}
+mkGuard :: PatVec > HsExpr GhcTc > DsM PmPat
+mkGuard pv e = PmGrd pv <$> dsExpr e
  Generate a variable pattern of a given type
mkPmVar :: Type > DsM (PmPat p)
+mkPmVar :: Type > DsM PmPat
mkPmVar ty = PmVar <$> mkPmId ty
{# INLINE mkPmVar #}
  Generate many variable patterns, given a list of types
mkPmVars :: [Type] > DsM PatVec
mkPmVars tys = mapM mkPmVar tys
{# INLINE mkPmVars #}

  Generate a fresh `Id` of a given type
mkPmId :: Type > DsM Id
mkPmId ty = getUniqueM >>= \unique >
 let occname = mkVarOccFS $ fsLit "$pm"
 name = mkInternalName unique occname noSrcSpan
 in return (mkLocalId name ty)
  Generate a fresh term variable of a given and return it in two forms:
 * A variable pattern
 * A variable expression
mkPmId2Forms :: Type > DsM (Pattern, LHsExpr GhcTc)
+mkPmId2Forms :: Type > DsM (PmPat, LHsExpr GhcTc)
mkPmId2Forms ty = do
x < mkPmId ty
return (PmVar x, noLoc (HsVar noExtField (noLoc x)))
 
 * Converting between Value Abstractions, Patterns and PmExpr

  Convert a value abstraction an expression
vaToPmExpr :: ValAbs > PmExpr
vaToPmExpr (PmCon { pm_con_con = c, pm_con_args = ps })
 = PmExprCon c (map vaToPmExpr ps)
vaToPmExpr (PmVar { pm_var_id = x }) = PmExprVar (idName x)
vaToPmExpr (PmLit { pm_lit_lit = l }) = PmExprLit l
vaToPmExpr (PmNLit { pm_lit_id = x }) = PmExprVar (idName x)

  Convert a pattern vector to a list of value abstractions by dropping the
 guards (See Note [Translating As Patterns])
coercePatVec :: PatVec > [ValAbs]
coercePatVec pv = concatMap coercePmPat pv

  Convert a pattern to a list of value abstractions (will be either an empty
 list if the pattern is a guard pattern, or a singleton list in all other
 cases) by dropping the guards (See Note [Translating As Patterns])
coercePmPat :: Pattern > [ValAbs]
coercePmPat (PmVar { pm_var_id = x }) = [PmVar { pm_var_id = x }]
coercePmPat (PmLit { pm_lit_lit = l }) = [PmLit { pm_lit_lit = l }]
coercePmPat (PmCon { pm_con_con = con, pm_con_arg_tys = arg_tys
 , pm_con_tvs = tvs, pm_con_dicts = dicts
 , pm_con_args = args })
 = [PmCon { pm_con_con = con, pm_con_arg_tys = arg_tys
 , pm_con_tvs = tvs, pm_con_dicts = dicts
 , pm_con_args = coercePatVec args }]
coercePmPat (PmGrd {}) = []  drop the guards
coercePmPat PmFake = []  drop the guards

  Check whether a 'ConLike' has the /single match/ property, i.e. whether
+  Check whether a 'PmAltCon' has the /single match/ property, i.e. whether
 it is the only possible match in the given context. See also
 'allCompleteMatches' and Note [Single match constructors].
singleMatchConstructor :: ConLike > [Type] > DsM Bool
singleMatchConstructor cl tys =
 any (isSingleton . snd) <$> allCompleteMatches cl tys
+singleMatchConstructor :: PmAltCon > [Type] > DsM Bool
+singleMatchConstructor PmAltLit{} _ = pure False
+singleMatchConstructor (PmAltConLike cl) tys =
+ any isSingleton <$> allCompleteMatches (conLikeResTy cl tys)
{
Note [Single match constructors]
@@ 1771,42 +1013,46 @@ the singlematch property. We currently don't (can't) check this in the
translation step. See #15753 for why this yields surprising results.
}
  For a given conlike, finds all the sets of patterns which could
 be relevant to that conlike by consulting the result type.
+  For a given type, finds all the COMPLETE sets of conlikes that inhabit it.
+
+ Note that for a data family instance, this must be the *representation* type.
+ e.g. data instance T (a,b) = T1 a b
+ leads to
+ data TPair a b = T1 a b  The "representation" type
+ It is TPair a b, not T (a, b), that is given to allCompleteMatches

 These come from two places.
 1. From data constructors defined with the result type constructor.
 2. From `COMPLETE` pragmas which have the same type as the result
 type constructor. Note that we only use `COMPLETE` pragmas
 *all* of whose pattern types match. See #14135
allCompleteMatches :: ConLike > [Type] > DsM [(Provenance, [ConLike])]
allCompleteMatches cl tys = do
 let fam = case cl of
 RealDataCon dc >
 [(FromBuiltin, map RealDataCon (tyConDataCons (dataConTyCon dc)))]
 PatSynCon _ > []
 ty = conLikeResTy cl tys
 pragmas < case splitTyConApp_maybe ty of
 Just (tc, _) > dsGetCompleteMatches tc
 Nothing > return []
 let fams cm = (FromComplete,) <$>
 mapM dsLookupConLike (completeMatchConLikes cm)
 from_pragma < filter (\(_,m) > isValidCompleteMatch ty m) <$>
 mapM fams pragmas
 let final_groups = fam ++ from_pragma
 return final_groups
 where
  Check that all the pattern synonym return types in a `COMPLETE`
  pragma subsume the type we're matching.
  See Note [Filtering out nonmatching COMPLETE sets]
 isValidCompleteMatch :: Type > [ConLike] > Bool
 isValidCompleteMatch ty = all go
 where
 go (RealDataCon {}) = True
 go (PatSynCon psc) = isJust $ flip tcMatchTy ty $ patSynResTy
 $ patSynSig psc

 patSynResTy (_, _, _, _, _, res_ty) = res_ty
+allCompleteMatches :: Type > DsM [[ConLike]]
+allCompleteMatches ty = case splitTyConApp_maybe ty of
+ Nothing > pure []  NB: We don't know any COMPLETE set, as opposed to [[]]
+ Just (tc, tc_args) > do
+  Look into the representation type of a data family instance, too.
+ env < dsGetFamInstEnvs
+ let (tc', _tc_args', _co) = tcLookupDataFamInst env tc tc_args
+ let mb_rdcs = map RealDataCon <$> tyConDataCons_maybe tc'
+ let maybe_to_list = maybe [] (:[])
+ let rdcs = maybe_to_list mb_rdcs
+  NB: tc, because COMPLETE sets are associated with the parent data family
+  TyCon
+ pragmas < dsGetCompleteMatches tc
+ let fams = mapM dsLookupConLike . completeMatchConLikes
+ pscs < mapM fams pragmas
+ let candidates = rdcs ++ pscs
+  Check that all the pattern synonym return types in a `COMPLETE`
+  pragma subsume the type we're matching.
+  See Note [Filtering out nonmatching COMPLETE sets]
+ pure (filter (isValidCompleteMatch ty) candidates)
+ where
+ isValidCompleteMatch :: Type > [ConLike] > Bool
+ isValidCompleteMatch ty = all p
+ where
+ p (RealDataCon _) = True
+ p (PatSynCon ps) = isJust (tcMatchTy (projResTy (patSynSig ps)) ty)
+ projResTy (_, _, _, _, _, res_ty) = res_ty
{
Note [Filtering out nonmatching COMPLETE sets]
@@ 1858,37 +1104,24 @@ OK. Consider this example (from #14059):
In the incomplete pattern match for `wobble`, we /do/ want to warn that SFalse
should be matched against, even though its type, SBool False, does not match
the scrutinee type, SBool z.
}
 
 * Types and constraints
+SG: Another angle at this is that the implied constraints when we instantiate
+universal type variables in the return type of a GADT will lead to *provided*
+thetas, whereas when we instantiate the return type of a pattern synonym that
+corresponds to a *required* theta. See Note [Pattern synonym result type] in
+PatSyn. Note how isValidCompleteMatches will successfully filter out
newEvVar :: Name > Type > EvVar
newEvVar name ty = mkLocalId name ty
+ pattern Just42 :: Maybe Int
+ pattern Just42 = Just 42
nameType :: String > Type > DsM EvVar
nameType name ty = do
 unique < getUniqueM
 let occname = mkVarOccFS (fsLit (name++"_"++show unique))
 idname = mkInternalName unique occname noSrcSpan
 return (newEvVar idname ty)
+But fail to filter out the equivalent
{
%************************************************************************
%* *
 The type oracle
%* *
%************************************************************************
}
+ pattern Just'42 :: (a ~ Int) => Maybe a
+ pattern Just'42 = Just 42
  Check whether a set of type constraints is satisfiable.
tyOracle :: Bag EvVar > PmM Bool
tyOracle evs
 = liftD $
 do { ((_warns, errs), res) < initTcDsForSolver $ tcCheckSatisfiability evs
 ; case res of
 Just sat > return sat
 Nothing > pprPanic "tyOracle" (vcat $ pprErrMsgBagWithLoc errs) }
+Which seems fine as far as tcMatchTy is concerned, but it raises a few eye
+brows.
+}
{
%************************************************************************
@@ 1907,8 +1140,9 @@ patVecArity :: PatVec > PmArity
patVecArity = sum . map patternArity
  Compute the arity of a pattern
patternArity :: Pattern > PmArity
+patternArity :: PmPat > PmArity
patternArity (PmGrd {}) = 0
+patternArity PmFake = 0
patternArity _other_pat = 1
{
@@ 1920,416 +1154,152 @@ patternArity _other_pat = 1
Main functions are:
* mkInitialUncovered :: [Id] > PmM Uncovered

 Generates the initial uncovered set. Term and type constraints in scope
 are checked, if they are inconsistent, the set is empty, otherwise, the
 set contains only a vector of variables with the constraints in scope.

* pmcheck :: PatVec > [PatVec] > ValVec > PmM PartialResult
+* pmcheck :: PatVec > [PatVec] > ValVec > Delta > DsM PartialResult
 Checks redundancy, coverage and inaccessibility, using auxilary functions
 `pmcheckGuards` and `pmcheckHd`. Mainly handles the guard case which is
 common in all three checks (see paper) and calls `pmcheckGuards` when the
 whole clause is checked, or `pmcheckHd` when the pattern vector does not
 start with a guard.
+ This function implements functions `covered`, `uncovered` and
+ `divergent` from the paper at once. Calls out to the auxilary function
+ `pmcheckGuards` for handling (possibly multiple) guarded RHSs when the whole
+ clause is checked. Slightly different from the paper because it does not even
+ produce the covered and uncovered sets. Since we only care about whether a
+ clause covers SOMETHING or if it may forces ANY argument, we only store a
+ boolean in both cases, for efficiency.
* pmcheckGuards :: [PatVec] > ValVec > PmM PartialResult
+* pmcheckGuards :: [PatVec] > ValVec > Delta > DsM PartialResult
Processes the guards.

* pmcheckHd :: Pattern > PatVec > [PatVec]
 > ValAbs > ValVec > PmM PartialResult

 Worker: This function implements functions `covered`, `uncovered` and
 `divergent` from the paper at once. Slightly different from the paper because
 it does not even produce the covered and uncovered sets. Since we only care
 about whether a clause covers SOMETHING or if it may forces ANY argument, we
 only store a boolean in both cases, for efficiency.
}
  Lift a pattern matching action from a single value vector abstration to a
 value set abstraction, but calling it on every vector and the combining the
+ value set abstraction, but calling it on every vector and combining the
 results.
runMany :: (ValVec > PmM PartialResult) > (Uncovered > PmM PartialResult)
runMany _ [] = return mempty
runMany pm (m:ms) = mappend <$> pm m <*> runMany pm ms

  Generate the initial uncovered set. It initializes the
 delta with all term and type constraints in scope.
mkInitialUncovered :: [Id] > PmM Uncovered
mkInitialUncovered vars = do
 delta < pmInitialTmTyCs
 let patterns = map PmVar vars
 return [ValVec patterns delta]
+runMany :: (Delta > DsM PartialResult) > Uncovered > DsM PartialResult
+runMany _ [] = return emptyPartialResult
+runMany pm (m:ms) = do
+ res < pm m
+ combinePartialResults res <$> runMany pm ms
  Increase the counter for elapsed algorithm iterations, check that the
 limit is not exceeded and call `pmcheck`
pmcheckI :: PatVec > [PatVec] > ValVec > PmM PartialResult
pmcheckI ps guards vva = do
 n < liftD incrCheckPmIterDs
 tracePm "pmCheck" (ppr n <> colon <+> pprPatVec ps
 $$ hang (text "guards:") 2 (vcat (map pprPatVec guards))
 $$ pprValVecDebug vva)
 res < pmcheck ps guards vva
+pmcheckI :: PatVec > [PatVec] > ValVec > Int > Delta > DsM PartialResult
+pmcheckI ps guards vva n delta = do
+ m < incrCheckPmIterDs
+ tracePm "pmCheck" (ppr m <> colon
+ $$ hang (text "patterns:") 2 (ppr ps)
+ $$ hang (text "guards:") 2 (ppr guards)
+ $$ ppr vva
+ $$ ppr delta)
+ res < pmcheck ps guards vva n delta
tracePm "pmCheckResult:" (ppr res)
return res
{# INLINE pmcheckI #}
  Increase the counter for elapsed algorithm iterations, check that the
 limit is not exceeded and call `pmcheckGuards`
pmcheckGuardsI :: [PatVec] > ValVec > PmM PartialResult
pmcheckGuardsI gvs vva = liftD incrCheckPmIterDs >> pmcheckGuards gvs vva
+pmcheckGuardsI :: [PatVec] > Int > Delta > DsM PartialResult
+pmcheckGuardsI gvs n delta = incrCheckPmIterDs >> pmcheckGuards gvs n delta
{# INLINE pmcheckGuardsI #}
  Increase the counter for elapsed algorithm iterations, check that the
 limit is not exceeded and call `pmcheckHd`
pmcheckHdI :: Pattern > PatVec > [PatVec] > ValAbs > ValVec
 > PmM PartialResult
pmcheckHdI p ps guards va vva = do
 n < liftD incrCheckPmIterDs
 tracePm "pmCheckHdI" (ppr n <> colon <+> pprPmPatDebug p
 $$ pprPatVec ps
 $$ hang (text "guards:") 2 (vcat (map pprPatVec guards))
 $$ pprPmPatDebug va
 $$ pprValVecDebug vva)

 res < pmcheckHd p ps guards va vva
 tracePm "pmCheckHdI: res" (ppr res)
 return res
{# INLINE pmcheckHdI #}
+  Check the list of mutually exclusive guards
+pmcheckGuards :: [PatVec] > Int > Delta > DsM PartialResult
+pmcheckGuards [] _ delta = return (usimple delta)
+pmcheckGuards (gv:gvs) n delta = do
+ (PartialResult cs unc ds) < pmcheckI gv [] [] n delta
+ let (n', unc')
+  See 6. in Note [Guards and Approximation]
+  Just n' < tryMultiplyDeltas (length unc) n = (n', unc)
+  otherwise = (n, [delta])
+ (PartialResult css uncs dss) < runMany (pmcheckGuardsI gvs n') unc'
+ return $ PartialResult (cs `mappend` css)
+ uncs
+ (ds `mappend` dss)
  Matching function: Check simultaneously a clause (takes separately the
 patterns and the list of guards) for exhaustiveness, redundancy and
 inaccessibility.
pmcheck :: PatVec > [PatVec] > ValVec > PmM PartialResult
pmcheck [] guards vva@(ValVec [] _)
+pmcheck
+ :: PatVec  ^ Patterns of the clause
+ > [PatVec]  ^ (Possibly multiple) guards of the clause
+ > ValVec  ^ The value vector abstraction to match against
+ > Int  ^ Estimate on the number of similar 'Delta's to handle.
+  See 6. in Note [Guards and Approximation]
+ > Delta  ^ Oracle state giving meaning to the identifiers in the ValVec
+ > DsM PartialResult
+pmcheck [] guards [] n delta
 null guards = return $ mempty { presultCovered = Covered }
  otherwise = pmcheckGuardsI guards vva
+  otherwise = pmcheckGuardsI guards n delta
 Guard
pmcheck (PmFake : ps) guards vva =
+pmcheck (PmFake : ps) guards vva n delta =
 shortcircuit if the guard pattern is useless.
 we just have two possible outcomes: fail here or match and recurse
 none of the two contains any useful information about the failure
 though. So just have these two cases but do not do all the boilerplate
 forces . mkCons vva <$> pmcheckI ps guards vva
pmcheck (p : ps) guards (ValVec vas delta)
  PmGrd { pm_grd_pv = pv, pm_grd_expr = e } < p
 = do
 y < liftD $ mkPmId (pmPatType p)
 let tm_state = extendSubst y e (delta_tm_cs delta)
 delta' = delta { delta_tm_cs = tm_state }
 utail <$> pmcheckI (pv ++ ps) guards (ValVec (PmVar y : vas) delta')

pmcheck [] _ (ValVec (_:_) _) = panic "pmcheck: nilcons"
pmcheck (_:_) _ (ValVec [] _) = panic "pmcheck: consnil"

pmcheck (p:ps) guards (ValVec (va:vva) delta)
 = pmcheckHdI p ps guards va (ValVec vva delta)

  Check the list of guards
pmcheckGuards :: [PatVec] > ValVec > PmM PartialResult
pmcheckGuards [] vva = return (usimple [vva])
pmcheckGuards (gv:gvs) vva = do
 (PartialResult prov1 cs vsa ds) < pmcheckI gv [] vva
 (PartialResult prov2 css vsas dss) < runMany (pmcheckGuardsI gvs) vsa
 return $ PartialResult (prov1 `mappend` prov2)
 (cs `mappend` css)
 vsas
 (ds `mappend` dss)

  Worker function: Implements all cases described in the paper for all three
 functions (`covered`, `uncovered` and `divergent`) apart from the `Guard`
 cases which are handled by `pmcheck`
pmcheckHd :: Pattern > PatVec > [PatVec] > ValAbs > ValVec
 > PmM PartialResult

 Var
pmcheckHd (PmVar x) ps guards va (ValVec vva delta)
  Just tm_state < solveOneEq (delta_tm_cs delta) (TVC x (vaToPmExpr va))
 = ucon va <$> pmcheckI ps guards (ValVec vva (delta {delta_tm_cs = tm_state}))
  otherwise = return mempty

 ConCon
pmcheckHd ( p@(PmCon { pm_con_con = c1, pm_con_tvs = ex_tvs1
 , pm_con_args = args1})) ps guards
 (va@(PmCon { pm_con_con = c2, pm_con_tvs = ex_tvs2
 , pm_con_args = args2})) (ValVec vva delta)
  c1 /= c2 =
 return (usimple [ValVec (va:vva) delta])
  otherwise = do
 let to_evvar tv1 tv2 = nameType "pmConCon" $
 mkPrimEqPred (mkTyVarTy tv1) (mkTyVarTy tv2)
 mb_to_evvar tv1 tv2
  If we have identical constructors but different existential
  tyvars, then generate extra equality constraints to ensure the
  existential tyvars.
  See Note [Coverage checking and existential tyvars].
  tv1 == tv2 = pure Nothing
  otherwise = Just <$> to_evvar tv1 tv2
 evvars < (listToBag . catMaybes) <$>
 ASSERT(ex_tvs1 `equalLength` ex_tvs2)
 liftD (zipWithM mb_to_evvar ex_tvs1 ex_tvs2)
 let delta' = delta { delta_ty_cs = evvars `unionBags` delta_ty_cs delta }
 kcon c1 (pm_con_arg_tys p) (pm_con_tvs p) (pm_con_dicts p)
 <$> pmcheckI (args1 ++ ps) guards (ValVec (args2 ++ vva) delta')

 LitLit
pmcheckHd (PmLit l1) ps guards (va@(PmLit l2)) vva =
 case eqPmLit l1 l2 of
 True > ucon va <$> pmcheckI ps guards vva
 False > return $ ucon va (usimple [vva])
+  TODO: I don't think this should mkCons delta, rather than just replace the
+  presultUncovered by [delta] completely. Note that the uncovered set
+  returned from the recursive call can only be a refinement of the
+  original delta.
+ forces . mkCons delta <$> pmcheckI ps guards vva n delta
+pmcheck (p@PmGrd { pm_grd_pv = pv, pm_grd_expr = e } : ps) guards vva n delta = do
+ tracePm "PmGrd: pmPatType" (vcat [ppr p, ppr (pmPatType p)])
+ x < mkPmId (exprType e)
+ delta' < expectJust "x is fresh" <$> addVarCoreCt delta x e
+ pmcheckI (pv ++ ps) guards (x : vva) n delta'
+
+ Var: Add x :> y to the oracle and recurse
+pmcheck (PmVar x : ps) guards (y : vva) n delta = do
+ delta' < expectJust "x is fresh" <$> addTmCt delta (TmVarVar x y)
+ pmcheckI ps guards vva n delta'
 ConVar
pmcheckHd (p@(PmCon { pm_con_con = con, pm_con_arg_tys = tys }))
 ps guards
 (PmVar x) (ValVec vva delta) = do
 (prov, complete_match) < select =<< liftD (allCompleteMatches con tys)

 cons_cs < mapM (liftD . mkOneConFull x tys) complete_match

 inst_vsa < flip mapMaybeM cons_cs $
 \InhabitationCandidate{ ic_val_abs = va, ic_tm_ct = tm_ct
 , ic_ty_cs = ty_cs
 , ic_strict_arg_tys = strict_arg_tys } > do
 mb_sat < pmIsSatisfiable delta tm_ct ty_cs strict_arg_tys
 pure $ fmap (ValVec (va:vva)) mb_sat

 set_provenance prov .
 force_if (canDiverge (idName x) (delta_tm_cs delta)) <$>
 runMany (pmcheckI (p:ps) guards) inst_vsa

 LitVar
pmcheckHd (p@(PmLit l)) ps guards (PmVar x) (ValVec vva delta)
 = force_if (canDiverge (idName x) (delta_tm_cs delta)) <$>
 mkUnion non_matched <$>
 case solveOneEq (delta_tm_cs delta) (mkPosEq x l) of
 Just tm_state > pmcheckHdI p ps guards (PmLit l) $
 ValVec vva (delta {delta_tm_cs = tm_state})
 Nothing > return mempty
 where
  See Note [Refutable shapes] in TmOracle
 us  Just tm_state < addSolveRefutableAltCon (delta_tm_cs delta) x (PmAltLit l)
 = [ValVec (PmNLit x [l] : vva) (delta { delta_tm_cs = tm_state })]
  otherwise = []

 non_matched = usimple us

 LitNLit
pmcheckHd (p@(PmLit l)) ps guards
 (PmNLit { pm_lit_id = x, pm_lit_not = lits }) (ValVec vva delta)
  all (not . eqPmLit l) lits
 , Just tm_state < solveOneEq (delta_tm_cs delta) (mkPosEq x l)
  Both guards check the same so it would be sufficient to have only
  the second one. Nevertheless, it is much cheaper to check whether
  the literal is in the list so we check it first, to avoid calling
  the term oracle (`solveOneEq`) if possible
 = mkUnion non_matched <$>
 pmcheckHdI p ps guards (PmLit l)
 (ValVec vva (delta { delta_tm_cs = tm_state }))
  otherwise = return non_matched
 where
  See Note [Refutable shapes] in TmOracle
 us  Just tm_state < addSolveRefutableAltCon (delta_tm_cs delta) x (PmAltLit l)
 = [ValVec (PmNLit x (l:lits) : vva) (delta { delta_tm_cs = tm_state })]
  otherwise = []

 non_matched = usimple us

 
 The following three can happen only in cases like #322 where constructors
 and overloaded literals appear in the same match. The general strategy is
 to replace the literal (positive/negative) by a variable and recurse. The
 fact that the variable is equal to the literal is recorded in `delta` so
 no information is lost

 LitCon
pmcheckHd p@PmLit{} ps guards va@PmCon{} (ValVec vva delta)
 = do y < liftD $ mkPmId (pmPatType va)
  Analogous to the ConVar case, we have to case split the value
  abstraction on possible literals. We do so by introducing a fresh
  variable that is equated to the constructor. LitVar will then take
  care of the case split by resorting to NLit.
 let tm_state = extendSubst y (vaToPmExpr va) (delta_tm_cs delta)
 delta' = delta { delta_tm_cs = tm_state }
 pmcheckHdI p ps guards (PmVar y) (ValVec vva delta')

 ConLit
pmcheckHd p@PmCon{} ps guards (PmLit l) (ValVec vva delta)
 = do y < liftD $ mkPmId (pmPatType p)
  This desugars to the ConVar case by introducing a fresh variable that
  is equated to the literal via a constraint. ConVar will then properly
  case split on all possible constructors.
 let tm_state = extendSubst y (PmExprLit l) (delta_tm_cs delta)
 delta' = delta { delta_tm_cs = tm_state }
 pmcheckHdI p ps guards (PmVar y) (ValVec vva delta')

 ConNLit
pmcheckHd (p@(PmCon {})) ps guards (PmNLit { pm_lit_id = x }) vva
 = pmcheckHdI p ps guards (PmVar x) vva

 Impossible: handled by pmcheck
pmcheckHd PmFake _ _ _ _ = panic "pmcheckHd: Fake"
pmcheckHd (PmGrd {}) _ _ _ _ = panic "pmcheckHd: Guard"

{
Note [Coverage checking and existential tyvars]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GHC's implementation of the patternmatch coverage algorithm (as described in
the GADTs Meet Their Match paper) must take some care to emit enough type
constraints when handling data constructors with exisentially quantified type
variables. To better explain what the challenge is, consider a constructor K
of the form:

 K @e_1 ... @e_m ev_1 ... ev_v ty_1 ... ty_n :: T u_1 ... u_p

Where:

* e_1, ..., e_m are the existentially bound type variables.
* ev_1, ..., ev_v are evidence variables, which may inhabit a dictionary type
 (e.g., Eq) or an equality constraint (e.g., e_1 ~ Int).
* ty_1, ..., ty_n are the types of K's fields.
* T u_1 ... u_p is the return type, where T is the data type constructor, and
 u_1, ..., u_p are the universally quantified type variables.

In the ConVar case, the coverage algorithm will have in hand the constructor
K as well as a list of type arguments [t_1, ..., t_n] to substitute T's
universally quantified type variables u_1, ..., u_n for. It's crucial to take
these in as arguments, as it is nontrivial to derive them just from the result
type of a pattern synonym and the ambient type of the match (#11336, #17112).
The type checker already did the hard work, so we should just make use of it.

The presence of existentially quantified type variables adds a significant
wrinkle. We always grab e_1, ..., e_m from the definition of K to begin with,
but we don't want them to appear in the final PmCon, because then
calling (mkOneConFull K) for other pattern variables might reuse the same
existential tyvars, which is certainly wrong.

Previously, GHC's solution to this wrinkle was to always create fresh names
for the existential tyvars and put them into the PmCon. This works well for
many cases, but it can break down if you nest GADT pattern matches in just
the right way. For instance, consider the following program:

 data App f a where
 App :: f a > App f (Maybe a)

 data Ty a where
 TBool :: Ty Bool
 TInt :: Ty Int

 data T f a where
 C :: T Ty (Maybe Bool)

 foo :: T f a > App f a > ()
 foo C (App TBool) = ()

foo is a total program, but with the previous approach to handling existential
tyvars, GHC would mark foo's patterns as nonexhaustive.

When foo is desugared to Core, it looks roughly like so:

 foo @f @a (C co1 _co2) (App @a1 _co3 (TBool > co1)) = ()

(Where `a1` is an existential tyvar.)

That, in turn, is processed by the coverage checker to become:

 foo @f @a (C co1 _co2) (App @a1 _co3 (pmvar123 :: f a1))
  TBool < pmvar123 > co1
 = ()

Note that the type of pmvar123 is `f a1`—this will be important later.

Now, we proceed with coveragechecking as usual. When we come to the
ConVar case for App, we create a fresh variable `a2` to represent its
existential tyvar. At this point, we have the equality constraints
`(a ~ Maybe a2, a ~ Maybe Bool, f ~ Ty)` in scope.

However, when we check the guard, it will use the type of pmvar123, which is
`f a1`. Thus, when considering if pmvar123 can match the constructor TInt,
it will generate the constraint `a1 ~ Int`. This means our final set of
equality constraints would be:

 f ~ Ty
 a ~ Maybe Bool
 a ~ Maybe a2
 a1 ~ Int

Which is satisfiable! Freshening the existential tyvar `a` to `a2` doomed us,
because GHC is unable to relate `a2` to `a1`, which really should be the same
tyvar.

Luckily, we can avoid this pitfall. Recall that the ConVar case was where we
generated a PmCon with toofresh existentials. But after ConVar, we have the
ConCon case, which considers whether each constructor of a particular data type
can be matched on in a particular spot.

In the case of App, when we get to the ConCon case, we will compare our
original App PmCon (from the source program) to the App PmCon created from the
ConVar case. In the former PmCon, we have `a1` in hand, which is exactly the
existential tyvar we want! Thus, we can force `a1` to be the same as `a2` here
by emitting an additional `a1 ~ a2` constraint. Now our final set of equality
constraints will be:

 f ~ Ty
 a ~ Maybe Bool
 a ~ Maybe a2
 a1 ~ Int
 a1 ~ a2

Which is unsatisfiable, as we desired, since we now have that
Int ~ a1 ~ a2 ~ Bool.

In general, App might have more than one constructor, in which case we
couldn't reuse the existential tyvar for App for a different constructor. This
means that we can only use this trick in ConCon when the constructors are the
same. But this is fine, since this is the only scenario where this situation
arises in the first place!
}
+pmcheck (p@PmCon{ pm_con_con = con, pm_con_args = args
+ , pm_con_arg_tys = arg_tys, pm_con_tvs = ex_tvs } : ps)
+ guards (x : vva) n delta = do
+  E.g f (K p q) =
+ 
+  Split the value vector into two value vectors:
+  * one for , binding x to (K p q)
+  * one for , recording that x is /not/ (K _ _)
+
+  Stuff for
+ pr_pos < refineToAltCon delta x con arg_tys ex_tvs >>= \case
+ Nothing > pure mempty
+ Just (delta', arg_vas) >
+ pmcheckI (args ++ ps) guards (arg_vas ++ vva) n delta'
+
+  Stuff for
+  The var is forced regardless of whether @con@ was satisfiable
+ let pr_pos' = forceIfCanDiverge delta x pr_pos
+ pr_neg < addRefutableAltCon delta x con >>= \case
+ Nothing > pure mempty
+ Just delta' > pure (usimple delta')
+
+ tracePm "ConVar" (vcat [ppr p, ppr x, ppr pr_pos', ppr pr_neg])
+
+  Combine both into a single PartialResult
+ let pr = mkUnion pr_pos' pr_neg
+ case (presultUncovered pr_pos', presultUncovered pr_neg) of
+ ([], _) > pure pr
+ (_, []) > pure pr
+  See Note [Limit the number of refinements]
+ _  lookupNumberOfRefinements delta x < mAX_REFINEMENTS
+ > pure pr
+  otherwise > pure pr{ presultUncovered = [delta] }
+
+pmcheck [] _ (_:_) _ _ = panic "pmcheck: nilcons"
+pmcheck (_:_) _ [] _ _ = panic "pmcheck: consnil"
 
 * Utilities for main checking
updateVsa :: (ValSetAbs > ValSetAbs) > (PartialResult > PartialResult)
updateVsa f p@(PartialResult { presultUncovered = old })
+updateUncovered :: (Uncovered > Uncovered) > (PartialResult > PartialResult)
+updateUncovered f p@(PartialResult { presultUncovered = old })
= p { presultUncovered = f old }
  Initialise with default values for covering and divergent information.
usimple :: ValSetAbs > PartialResult
usimple vsa = mempty { presultUncovered = vsa }

  Take the tail of all value vector abstractions in the uncovered set
utail :: PartialResult > PartialResult
utail = updateVsa upd
 where upd vsa = [ ValVec vva delta  ValVec (_:vva) delta < vsa ]

  Prepend a value abstraction to all value vector abstractions in the
 uncovered set
ucon :: ValAbs > PartialResult > PartialResult
ucon va = updateVsa upd
 where
 upd vsa = [ ValVec (va:vva) delta  ValVec vva delta < vsa ]

  Given a data constructor of arity `a` and an uncovered set containing
 value vector abstractions of length `(a+n)`, pass the first `n` value
 abstractions to the constructor (Hence, the resulting value vector
 abstractions will have length `n+1`)
kcon :: ConLike > [Type] > [TyVar] > [EvVar]
 > PartialResult > PartialResult
kcon con arg_tys ex_tvs dicts
 = let n = conLikeArity con
 upd vsa =
 [ ValVec (va:vva) delta
  ValVec vva' delta < vsa
 , let (args, vva) = splitAt n vva'
 , let va = PmCon { pm_con_con = con
 , pm_con_arg_tys = arg_tys
 , pm_con_tvs = ex_tvs
 , pm_con_dicts = dicts
 , pm_con_args = args } ]
 in updateVsa upd
+  Initialise with default values for covering and divergent information and
+ a singleton uncovered set.
+usimple :: Delta > PartialResult
+usimple delta = mempty { presultUncovered = [delta] }
  Get the union of two covered, uncovered and divergent value set
 abstractions. Since the covered and divergent sets are represented by a
@@ 2339,21 +1309,19 @@ kcon con arg_tys ex_tvs dicts
mkUnion :: PartialResult > PartialResult > PartialResult
mkUnion = mappend
  Add a value vector abstraction to a value set abstraction (uncovered).
mkCons :: ValVec > PartialResult > PartialResult
mkCons vva = updateVsa (vva:)
+  Add a model to the uncovered set.
+mkCons :: Delta > PartialResult > PartialResult
+mkCons model = updateUncovered (model:)
  Set the divergent set to not empty
forces :: PartialResult > PartialResult
forces pres = pres { presultDivergent = Diverged }
  Set the divergent set to nonempty if the flag is `True`
force_if :: Bool > PartialResult > PartialResult
force_if True pres = forces pres
force_if False pres = pres

set_provenance :: Provenance > PartialResult > PartialResult
set_provenance prov pr = pr { presultProvenance = prov }
+  Set the divergent set to nonempty if the variable has not been forced yet
+forceIfCanDiverge :: Delta > Id > PartialResult > PartialResult
+forceIfCanDiverge delta x
+  canDiverge delta x = forces
+  otherwise = id
 
 * Propagation of term constraints inwards when checking nested matches
@@ 2362,7 +1330,7 @@ set_provenance prov pr = pr { presultProvenance = prov }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When checking a match it would be great to have all type and term information
available so we can get more precise results. For this reason we have functions
`addDictsDs' and `addTmCsDs' in PmMonad that store in the environment type and
+`addDictsDs' and `addTmVarCsDs' in DsMonad that store in the environment type and
term constraints (respectively) as we go deeper.
The type constraints we propagate inwards are collected by `collectEvVarsPats'
@@ 2384,80 +1352,88 @@ f x = case x of
(_:_) > True
[] > False  can't happen
Functions `genCaseTmCs1' and `genCaseTmCs2' are responsible for generating
+Functions `addScrutTmCs' and `addPatTmCs' are responsible for generating
these constraints.
}
  Generate equalities when checking a case expression:
 case x of { p1 > e1; ... pn > en }
 When we go deeper to check e.g. e1 we record two equalities:
 (x ~ y), where y is the initial uncovered when checking (p1; .. ; pn)
 and (x ~ p1).
genCaseTmCs2 :: Maybe (LHsExpr GhcTc)  Scrutinee
 > [Pat GhcTc]  LHS (should have length 1)
 > [Id]  MatchVars (should have length 1)
 > DsM (Bag TmVarCt)
genCaseTmCs2 Nothing _ _ = return emptyBag
genCaseTmCs2 (Just scr) [p] [var] = do
 fam_insts < dsGetFamInstEnvs
 [e] < map vaToPmExpr . coercePatVec <$> translatePat fam_insts p
 let scr_e = lhsExprToPmExpr scr
 return $ listToBag [(TVC var e), (TVC var scr_e)]
genCaseTmCs2 _ _ _ = panic "genCaseTmCs2: HsCase"

  Generate a simple equality when checking a case expression:
 case x of { matches }
 When checking matches we record that (x ~ y) where y is the initial
+locallyExtendPmDelta :: (Delta > DsM (Maybe Delta)) > DsM a > DsM a
+locallyExtendPmDelta ext k = getPmDelta >>= ext >>= \case
+  If adding a constraint would lead to a contradiction, don't add it.
+  See @Note [Recovering from unsatisfiable patternmatching constraints]@
+  for why this is done.
+ Nothing > k
+ Just delta' > updPmDelta delta' k
+
+  Add inscope type constraints
+addTyCsDs :: Bag EvVar > DsM a > DsM a
+addTyCsDs ev_vars =
+ locallyExtendPmDelta (\delta > addTypeEvidence delta ev_vars)
+
+  Add equalities for the scrutinee to the local 'DsM' environment when
+ checking a case expression:
+ case e of x { matches }
+ When checking matches we record that (x ~ e) where x is the initial
 uncovered. All matches will have to satisfy this equality.
genCaseTmCs1 :: Maybe (LHsExpr GhcTc) > [Id] > Bag TmVarCt
genCaseTmCs1 Nothing _ = emptyBag
genCaseTmCs1 (Just scr) [var] = unitBag (TVC var (lhsExprToPmExpr scr))
genCaseTmCs1 _ _ = panic "genCaseTmCs1: HsCase"

{ Note [Literals in PmPat]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Instead of translating a literal to a variable accompanied with a guard, we
treat them like constructor patterns. The following example from
"./libraries/base/GHC/IO/Encoding.hs" shows why:

mkTextEncoding' :: CodingFailureMode > String > IO TextEncoding
mkTextEncoding' cfm enc = case [toUpper c  c < enc, c /= ''] of
 "UTF8" > return $ UTF8.mkUTF8 cfm
 "UTF16" > return $ UTF16.mkUTF16 cfm
 "UTF16LE" > return $ UTF16.mkUTF16le cfm
 ...

Each clause gets translated to a list of variables with an equal number of
guards. For every guard we generate two cases (equals True/equals False) which
means that we generate 2^n cases to feed the oracle with, where n is the sum of
the length of all strings that appear in the patterns. For this particular
example this means over 2^40 cases. Instead, by representing them like with
constructor we get the following:
 1. We exploit the common prefix with our representation of VSAs
 2. We prune immediately nonreachable cases
 (e.g. False == (x == "U"), True == (x == "U"))

Note [Translating As Patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Instead of translating x@p as: x (p < x)
we instead translate it as: p (x < coercePattern p)
for performance reasons. For example:

 f x@True = 1
 f y@False = 2

Gives the following with the first translation:

 x > {x == False, x == y, y == True}

If we use the second translation we get an empty set, independently of the
oracle. Since the pattern `p' may contain guard patterns though, it cannot be
used as an expression. That's why we call `coercePatVec' to drop the guard and
`vaToPmExpr' to transform the value abstraction to an expression in the
guard pattern (value abstractions are a subset of expressions). We keep the
guards in the first pattern `p' though.
+addScrutTmCs :: Maybe (LHsExpr GhcTc) > [Id] > DsM a > DsM a
+addScrutTmCs Nothing _ k = k
+addScrutTmCs (Just scr) [x] k = do
+ scr_e < dsLExpr scr
+ locallyExtendPmDelta (\delta > addVarCoreCt delta x scr_e) k
+addScrutTmCs _ _ _ = panic "addScrutTmCs: HsCase with more than one case binder"
+
+  Add equalities to the local 'DsM' environment when checking the RHS of a
+ case expression:
+ case e of x { p1 > e1; ... pn > en }
+ When we go deeper to check e.g. e1 we record (x ~ p1).
+addPatTmCs :: [Pat GhcTc]  LHS (should have length 1)
+ > [Id]  MatchVars (should have length 1)
+ > DsM a
+ > DsM a
+ Morally, this computes an approximation of the Covered set for p1
+ (which pmcheck currently discards). TODO: Reuse pmcheck instead of calling
+ out to awkard addVarPatVecCt.
+addPatTmCs ps xs k = do
+ fam_insts < dsGetFamInstEnvs
+ pv < concat <$> translatePatVec fam_insts ps
+ locallyExtendPmDelta (\delta > addVarPatVecCt delta xs pv) k
+
+ 
+ * Converting between Value Abstractions, Patterns and PmExpr
+  Add a constraint equating a variable to a 'PatVec'. Picks out the single
+ 'PmPat' of arity 1 and equates x to it. Returns the original Delta if that
+ fails. Otherwise it returns Nothing when the resulting Delta would be
+ unsatisfiable, or @Just delta'@ when the extended @delta'@ is still possibly
+ satisfiable.
+addVarPatVecCt :: Delta > [Id] > PatVec > DsM (Maybe Delta)
+ This is just a simple version of pmcheck to compute the Covered Delta
+ (which pmcheck doesn't even attempt to keep).
+ Also PmGrd, although having pattern arity 0, really stores important info.
+ For example, aspatterns desugar to a plain variable match and an associated
+ PmGrd for the RHS of the @. We don't currently look into that PmGrd and I'm
+ not willing to duplicate any more of pmcheck.
+addVarPatVecCt delta (x:xs) (pat:pv)
+  patternArity pat == 1  PmVar or PmCon
+ = runMaybeT $ do
+ delta' < MaybeT (addVarPatCt delta x pat)
+ MaybeT (addVarPatVecCt delta' xs pv)
+  otherwise  PmGrd or PmFake
+ = addVarPatVecCt delta (x:xs) pv
+addVarPatVecCt delta [] pv = ASSERT( patVecArity pv == 0 ) pure (Just delta)
+addVarPatVecCt _ (_:_) [] = panic "More match vars than patterns"
+
+  Convert a pattern to a 'PmExpr' (will be either 'Nothing' if the pattern is
+ a guard pattern, or 'Just' an expression in all other cases) by dropping the
+ guards
+addVarPatCt :: Delta > Id > PmPat > DsM (Maybe Delta)
+addVarPatCt delta x (PmVar { pm_var_id = y }) = addTmCt delta (TmVarVar x y)
+addVarPatCt delta x (PmCon { pm_con_con = con, pm_con_args = args }) = runMaybeT $ do
+ arg_ids < traverse (lift . mkPmId . pmPatType) args
+ delta' < foldlM (\delta (y, arg) > MaybeT (addVarPatCt delta y arg)) delta (zip arg_ids args)
+ MaybeT (addTmCt delta' (TmVarCon x con arg_ids))
+addVarPatCt delta _ _pat = ASSERT( patternArity _pat == 0 ) pure (Just delta)
+{
%************************************************************************
%* *
Pretty printing of exhaustiveness/redundancy check warnings
@@ 2465,32 +1441,34 @@ guards in the first pattern `p' though.
%************************************************************************
}
  Check whether any part of pattern match checking is enabled (does not
 matter whether it is the redundancy check or the exhaustiveness check).
isAnyPmCheckEnabled :: DynFlags > DsMatchContext > Bool
isAnyPmCheckEnabled dflags (DsMatchContext kind _loc)
+  Check whether any part of pattern match checking is enabled for this
+ 'HsMatchContext' (does not matter whether it is the redundancy check or the
+ exhaustiveness check).
+isMatchContextPmChecked :: DynFlags > Origin > HsMatchContext id > Bool
+isMatchContextPmChecked dflags origin kind
+  isGenerated origin
+ = False
+  otherwise
= wopt Opt_WarnOverlappingPatterns dflags  exhaustive dflags kind
instance Outputable ValVec where
 ppr (ValVec vva delta)
 = let (subst, refuts) = wrapUpTmState (delta_tm_cs delta)
 vector = substInValAbs subst vva
 in pprUncovered (vector, refuts)

  Apply a term substitution to a value vector abstraction. All VAs are
 transformed to PmExpr (used only before pretty printing).
substInValAbs :: TmVarCtEnv > [ValAbs] > [PmExpr]
substInValAbs subst = map (exprDeepLookup subst . vaToPmExpr)
+  Return True when any of the pattern match warnings ('allPmCheckWarnings')
+ are enabled, in which case we need to run the pattern match checker.
+needToRunPmCheck :: DynFlags > Origin > Bool
+needToRunPmCheck dflags origin
+  isGenerated origin
+ = False
+  otherwise
+ = notNull (filter (`wopt` dflags) allPmCheckWarnings)
  Issue all the warnings (coverage, exhaustiveness, inaccessibility)
dsPmWarn :: DynFlags > DsMatchContext > PmResult > DsM ()
dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result
= when (flag_i  flag_u) $ do
 let exists_r = flag_i && notNull redundant && onlyBuiltin
 exists_i = flag_i && notNull inaccessible && onlyBuiltin && not is_rec_upd
+ let exists_r = flag_i && notNull redundant
+ exists_i = flag_i && notNull inaccessible && not is_rec_upd
exists_u = flag_u && (case uncovered of
 TypeOfUncovered _ > True
 UncoveredPatterns u > notNull u)
+ TypeOfUncovered _ > True
+ UncoveredPatterns _ unc > notNull unc)
when exists_r $ forM_ redundant $ \(dL>L l q) > do
putSrcSpanDs l (warnDs (Reason Opt_WarnOverlappingPatterns)
@@ 2500,12 +1478,11 @@ dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result
(pprEqn q "has inaccessible right hand side"))
when exists_u $ putSrcSpanDs loc $ warnDs flag_u_reason $
case uncovered of
 TypeOfUncovered ty > warnEmptyCase ty
 UncoveredPatterns candidates > pprEqns candidates
+ TypeOfUncovered ty > warnEmptyCase ty
+ UncoveredPatterns vars unc > pprEqns vars unc
where
PmResult
 { pmresultProvenance = prov
 , pmresultRedundant = redundant
+ { pmresultRedundant = redundant
, pmresultUncovered = uncovered
, pmresultInaccessible = inaccessible } = pm_result
@@ 2516,8 +1493,6 @@ dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result
is_rec_upd = case kind of { RecUpd > True; _ > False }
 See Note [Inaccessible warnings for record updates]
 onlyBuiltin = prov == FromBuiltin

maxPatterns = maxUncoveredPatterns dflags
 Print a single clause (for redundant/withinaccessiblerhs)
@@ 2525,14 +1500,12 @@ dsPmWarn dflags ctx@(DsMatchContext kind loc) pm_result
f (pprPats kind (map unLoc q))
 Print several clauses (for uncovered clauses)
 pprEqns qs = pprContext False ctx (text "are nonexhaustive") $ \_ >
 case qs of  See #11245
 [ValVec [] _]
 > text "Guards do not cover entire pattern space"
 _missing > let us = map ppr qs
 in hang (text "Patterns not matched:") 4
 (vcat (take maxPatterns us)
 $$ dots maxPatterns us)
+ pprEqns vars deltas = pprContext False ctx (text "are nonexhaustive") $ \_ >
+ case vars of  See #11245
+ [] > text "Guards do not cover entire pattern space"
+ _ > let us = map (\delta > pprUncovered delta vars) deltas
+ in hang (text "Patterns not matched:") 4
+ (vcat (take maxPatterns us) $$ dots maxPatterns us)
 Print a typeannotated wildcard (for nonexhaustive `EmptyCase`s for
 which we only know the type and have no inhabitants at hand)
@@ 2583,6 +1556,15 @@ dots maxPatterns qs
 qs `lengthExceeds` maxPatterns = text "..."
 otherwise = empty
+  All warning flags that need to run the pattern match checker.
+allPmCheckWarnings :: [WarningFlag]
+allPmCheckWarnings =
+ [ Opt_WarnIncompletePatterns
+ , Opt_WarnIncompleteUniPatterns
+ , Opt_WarnIncompletePatternsRecUpd
+ , Opt_WarnOverlappingPatterns
+ ]
+
  Check whether the exhaustiveness checker should run (exhaustiveness only)
exhaustive :: DynFlags > HsMatchContext id > Bool
exhaustive dflags = maybe False (`wopt` dflags) . exhaustiveWarningFlag
@@ 2626,43 +1608,3 @@ pprContext singular (DsMatchContext kind _loc) msg rest_of_msg_fun
pprPats :: HsMatchContext Name > [Pat GhcTc] > SDoc
pprPats kind pats
= sep [sep (map ppr pats), matchSeparator kind, text "..."]

 Debugging Infrastructre

tracePm :: String > SDoc > PmM ()
tracePm herald doc = liftD $ tracePmD herald doc


tracePmD :: String > SDoc > DsM ()
tracePmD herald doc = do
 dflags < getDynFlags
 printer < mkPrintUnqualifiedDs
 liftIO $ dumpIfSet_dyn_printer printer dflags
 Opt_D_dump_ec_trace (text herald $$ (nest 2 doc))


pprPmPatDebug :: PmPat a > SDoc
pprPmPatDebug (PmCon cc _arg_tys _con_tvs _con_dicts con_args)
 = hsep [text "PmCon", ppr cc, hsep (map pprPmPatDebug con_args)]
pprPmPatDebug (PmVar vid) = text "PmVar" <+> ppr vid
pprPmPatDebug (PmLit li) = text "PmLit" <+> ppr li
pprPmPatDebug (PmNLit i nl) = text "PmNLit" <+> ppr i <+> ppr nl
pprPmPatDebug (PmGrd pv ge) = text "PmGrd" <+> hsep (map pprPmPatDebug pv)
 <+> ppr ge
pprPmPatDebug PmFake = text "PmFake"

pprPatVec :: PatVec > SDoc
pprPatVec ps = hang (text "Pattern:") 2
 (brackets $ sep
 $ punctuate (comma <> char '\n') (map pprPmPatDebug ps))

pprValAbs :: [ValAbs] > SDoc
pprValAbs ps = hang (text "ValAbs:") 2
 (brackets $ sep
 $ punctuate (comma) (map pprPmPatDebug ps))

pprValVecDebug :: ValVec > SDoc
pprValVecDebug (ValVec vas _d) = text "ValVec" <+>
 parens (pprValAbs vas)
  $$ ppr (delta_tm_cs _d)
  $$ ppr (delta_ty_cs _d)
diff git a/compiler/deSugar/DsBinds.hs b/compiler/deSugar/DsBinds.hs
index 4a6a463b2dfc8719bfe22adfca53d2df52339987..cea7f3215bc28d3a1afe1647e8ab0a58a4d79136 100644
 a/compiler/deSugar/DsBinds.hs
+++ b/compiler/deSugar/DsBinds.hs
@@ 29,7 +29,7 @@ import {# SOURCE #} Match( matchWrapper )
import DsMonad
import DsGRHSs
import DsUtils
import Check ( checkGuardMatches )
+import Check ( needToRunPmCheck, addTyCsDs, checkGuardMatches )
import HsSyn  lots of things
import CoreSyn  lots of things
@@ 186,11 +186,15 @@ dsHsBind dflags (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
, abs_exports = exports
, abs_ev_binds = ev_binds
, abs_binds = binds, abs_sig = has_sig })
 = do { ds_binds < addDictsDs (listToBag dicts) $
 dsLHsBinds binds
  addDictsDs: push type constraints deeper
  for inner pattern match check
  See Check, Note [Type and Term Equality Propagation]
+ = do { ds_binds < applyWhen (needToRunPmCheck dflags FromSource)
+  FromSource might not be accurate, but at worst
+  we do superfluous calls to the pattern match
+  oracle.
+  addTyCsDs: push type constraints deeper
+  for inner pattern match check
+  See Check, Note [Type and Term Equality Propagation]
+ (addTyCsDs (listToBag dicts))
+ (dsLHsBinds binds)
; ds_ev_binds < dsTcEvBinds_s ev_binds
diff git a/compiler/deSugar/DsGRHSs.hs b/compiler/deSugar/DsGRHSs.hs
index 5adc661388f39c673c48a0099a7a303c438b7758..b0d35d0b2a0c572c50ff458a5c996eb391e1180e 100644
 a/compiler/deSugar/DsGRHSs.hs
+++ b/compiler/deSugar/DsGRHSs.hs
@@ 23,7 +23,9 @@ import MkCore
import CoreSyn
import CoreUtils (bindNonRec)
import Check (genCaseTmCs2)
+import BasicTypes (Origin(FromSource))
+import DynFlags
+import Check (needToRunPmCheck, addTyCsDs, addPatTmCs, addScrutTmCs)
import DsMonad
import DsUtils
import Type ( Type )
@@ 122,11 +124,16 @@ matchGuards (BindStmt _ pat bind_rhs _ _ : stmts) ctx rhs rhs_ty = do
let upat = unLoc pat
dicts = collectEvVarsPat upat
match_var < selectMatchVar upat
 tm_cs < genCaseTmCs2 (Just bind_rhs) [upat] [match_var]
 match_result < addDictsDs dicts $
 addTmCsDs tm_cs $
  See Note [Type and Term Equality Propagation] in Check
 matchGuards stmts ctx rhs rhs_ty
+
+ dflags < getDynFlags
+ match_result <
+  See Note [Type and Term Equality Propagation] in Check
+ applyWhen (needToRunPmCheck dflags FromSource)
+  FromSource might not be accurate, but at worst
+  we do superfluous calls to the pattern match
+  oracle.
+ (addTyCsDs dicts . addScrutTmCs (Just bind_rhs) [match_var] . addPatTmCs [upat] [match_var])
+ (matchGuards stmts ctx rhs rhs_ty)
core_rhs < dsLExpr bind_rhs
match_result' < matchSinglePatVar match_var (StmtCtxt ctx) pat rhs_ty
match_result
diff git a/compiler/deSugar/DsMonad.hs b/compiler/deSugar/DsMonad.hs
index 9534b4e20b7efee218baf946f86f839553e24e06..d937b3b134c576aa7f499d184627e568679d5946 100644
 a/compiler/deSugar/DsMonad.hs
+++ b/compiler/deSugar/DsMonad.hs
@@ 29,8 +29,8 @@ module DsMonad (
DsMetaEnv, DsMetaVal(..), dsGetMetaEnv, dsLookupMetaEnv, dsExtendMetaEnv,
  Getting and setting EvVars and term constraints in local environment
 getDictsDs, addDictsDs, getTmCsDs, addTmCsDs,
+  Getting and setting pattern match oracle states
+ getPmDelta, updPmDelta,
 Iterations for pm checking
incrCheckPmIterDs, resetPmIterDs, dsGetCompleteMatches,
@@ 70,7 +70,7 @@ import BasicTypes ( Origin )
import DataCon
import ConLike
import TyCon
import PmExpr
+import {# SOURCE #} PmOracle
import Id
import Module
import Outputable
@@ 82,7 +82,6 @@ import NameEnv
import DynFlags
import ErrUtils
import FastString
import Var (EvVar)
import UniqFM ( lookupWithDefaultUFM )
import Literal ( mkLitString )
import CostCentreState
@@ 285,8 +284,7 @@ mkDsEnvs dflags mod rdr_env type_env fam_inst_env msg_var pmvar cc_st_var
}
lcl_env = DsLclEnv { dsl_meta = emptyNameEnv
, dsl_loc = real_span
 , dsl_dicts = emptyBag
 , dsl_tm_cs = emptyBag
+ , dsl_delta = initDelta
, dsl_pm_iter = pmvar
}
in (gbl_env, lcl_env)
@@ 386,23 +384,14 @@ the @SrcSpan@ being carried around.
getGhcModeDs :: DsM GhcMode
getGhcModeDs = getDynFlags >>= return . ghcMode
  Get inscope type constraints (pm check)
getDictsDs :: DsM (Bag EvVar)
getDictsDs = do { env < getLclEnv; return (dsl_dicts env) }
+  Get the current pattern match oracle state. See 'dsl_delta'.
+getPmDelta :: DsM Delta
+getPmDelta = do { env < getLclEnv; return (dsl_delta env) }
  Add inscope type constraints (pm check)
addDictsDs :: Bag EvVar > DsM a > DsM a
addDictsDs ev_vars
 = updLclEnv (\env > env { dsl_dicts = unionBags ev_vars (dsl_dicts env) })

  Get inscope term constraints (pm check)
getTmCsDs :: DsM (Bag TmVarCt)
getTmCsDs = do { env < getLclEnv; return (dsl_tm_cs env) }

  Add inscope term constraints (pm check)
addTmCsDs :: Bag TmVarCt > DsM a > DsM a
addTmCsDs tm_cs
 = updLclEnv (\env > env { dsl_tm_cs = unionBags tm_cs (dsl_tm_cs env) })
+  Set the pattern match oracle state within the scope of the given action.
+ See 'dsl_delta'.
+updPmDelta :: Delta > DsM a > DsM a
+updPmDelta delta = updLclEnv (\env > env { dsl_delta = delta })
  Increase the counter for elapsed pattern match check iterations.
 If the current counter is already over the limit, fail
diff git a/compiler/deSugar/ExtractDocs.hs b/compiler/deSugar/ExtractDocs.hs
index ce5299443bb68ff5f7bf6977aa72377d890f2b54..4d7f11507427e9a5140e77200bdae083543dda62 100644
 a/compiler/deSugar/ExtractDocs.hs
+++ b/compiler/deSugar/ExtractDocs.hs
@@ 289,7 +289,7 @@ ungroup group_ =
mkDecls (typesigs . hs_valds) (SigD noExtField) group_ ++
mkDecls (valbinds . hs_valds) (ValD noExtField) group_
where
 typesigs (XValBindsLR (NValBinds _ sigs)) = filter (isUserSig . unLoc) sigs
+ typesigs (XValBindsLR (NValBinds _ sig)) = filter (isUserSig . unLoc) sig
typesigs ValBinds{} = error "expected XValBindsLR"
valbinds (XValBindsLR (NValBinds binds _)) =
diff git a/compiler/deSugar/Match.hs b/compiler/deSugar/Match.hs
index 921b829fb9c8ef0cb26c937a89d8eed2e361309b..a0576494a0e1c29f1312dab07892b43c23846f5c 100644
 a/compiler/deSugar/Match.hs
+++ b/compiler/deSugar/Match.hs
@@ 690,7 +690,13 @@ Call @match@ with all of this information!
matchWrapper
:: HsMatchContext Name  ^ For shadowing warning messages
 > Maybe (LHsExpr GhcTc)  ^ Scrutinee, if we check a case expr
+ > Maybe (LHsExpr GhcTc)  ^ Scrutinee. (Just scrut) for a case expr
+  case scrut of { p1 > e1 ... }
+  (and in this case the MatchGroup will
+  have all singleton patterns)
+  Nothing for a function definition
+  f p1 q1 = ...  No "scrutinee"
+  f p2 q2 = ...  in this case
> MatchGroup GhcTc (LHsExpr GhcTc)  ^ Matches being desugared
> DsM ([Id], CoreExpr)  ^ Results (usually passed to 'match')
@@ 730,25 +736,30 @@ matchWrapper ctxt mb_scr (MG { mg_alts = (dL>L _ matches)
; eqns_info < mapM (mk_eqn_info new_vars) matches
  pattern match check warnings
 ; unless (isGenerated origin) $
 when (isAnyPmCheckEnabled dflags (DsMatchContext ctxt locn)) $
 addTmCsDs (genCaseTmCs1 mb_scr new_vars) $
  See Note [Type and Term Equality Propagation]
 checkMatches dflags (DsMatchContext ctxt locn) new_vars matches
+  Pattern match check warnings for /this matchgroup/
+ ; when (isMatchContextPmChecked dflags origin ctxt) $
+ addScrutTmCs mb_scr new_vars $
+  See Note [Type and Term Equality Propagation]
+ checkMatches dflags (DsMatchContext ctxt locn) new_vars matches
; result_expr < handleWarnings $
matchEquations ctxt new_vars eqns_info rhs_ty
; return (new_vars, result_expr) }
where
+  Called once per equation in the match, or alternative in the case
mk_eqn_info vars (dL>L _ (Match { m_pats = pats, m_grhss = grhss }))
= do { dflags < getDynFlags
; let upats = map (unLoc . decideBangHood dflags) pats
dicts = collectEvVarsPats upats
 ; tm_cs < genCaseTmCs2 mb_scr upats vars
 ; match_result < addDictsDs dicts $  See Note [Type and Term Equality Propagation]
 addTmCsDs tm_cs $  See Note [Type and Term Equality Propagation]
 dsGRHSs ctxt grhss rhs_ty
+
+ ; match_result <
+  Extend the environment with knowledge about
+  the matches before desguaring the RHS
+  See Note [Type and Term Equality Propagation]
+ applyWhen (needToRunPmCheck dflags origin)
+ (addTyCsDs dicts . addScrutTmCs mb_scr vars . addPatTmCs upats vars)
+ (dsGRHSs ctxt grhss rhs_ty)
+
; return (EqnInfo { eqn_pats = upats
, eqn_orig = FromSource
, eqn_rhs = match_result }) }
diff git a/compiler/deSugar/PmExpr.hs b/compiler/deSugar/PmExpr.hs
index 3697dd8cc16e12e49f4421a88049206d22a21016..f83c63e4fd61315ff2db4c8135c6226fe77f482f 100644
 a/compiler/deSugar/PmExpr.hs
+++ b/compiler/deSugar/PmExpr.hs
@@ 6,29 +6,38 @@ Haskell expressions (as used by the pattern matching checker) and utilities.
{# LANGUAGE CPP #}
{# LANGUAGE ViewPatterns #}
{# LANGUAGE GeneralizedNewtypeDeriving #}
module PmExpr (
 PmExpr(..), PmLit(..), PmAltCon(..), TmVarCt(..),
 eqPmLit, isNotPmExprOther, lhsExprToPmExpr, hsExprToPmExpr
+ PmLit(..), PmLitValue(..), PmAltCon(..),
+ pmAltConType, PmEquality(..), eqPmAltCon,
+ pmLitType, literalToPmLit, negatePmLit, overloadPmLit,
+ pmLitAsStringLit, coreExprAsPmLit
) where
#include "HsVersions.h"
import GhcPrelude
import BasicTypes (SourceText)
import FastString (FastString, unpackFS)
import HsSyn
+import Util
+import FastString
import Id
import Name
import DataCon
import ConLike
import TcEvidence (isErasableHsWrapper)
import TcType (isStringTy)
import TysWiredIn
import Outputable
import SrcLoc
+import Maybes
+import Type
+import TyCon
+import Literal
+import CoreSyn
+import CoreUtils (exprType)
+import PrelNames
+import TysWiredIn
+import TysPrim
+
+import Numeric (fromRat)
+import Data.Foldable (find)
+import Data.Ratio
{
%************************************************************************
@@ 38,52 +47,135 @@ import SrcLoc
%************************************************************************
}
{ Note [PmExprOther in PmExpr]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since there is no plan to extend the (currently pretty naive) term oracle in
the near future, instead of playing with the verbose (HsExpr Id), we lift it to
PmExpr. All expressions the term oracle does not handle are wrapped by the
constructor PmExprOther. Note that we do not perform substitution in
PmExprOther. Because of this, we do not even print PmExprOther, since they may
refer to variables that are otherwise substituted away.
}

 
 ** Types
  Lifted expressions for pattern match checking.
data PmExpr = PmExprVar Name
  PmExprCon ConLike [PmExpr]
  PmExprLit PmLit
  PmExprOther (HsExpr GhcTc)  Note [PmExprOther in PmExpr]


mkPmExprData :: DataCon > [PmExpr] > PmExpr
mkPmExprData dc args = PmExprCon (RealDataCon dc) args

  Literals (simple and overloaded ones) for pattern match checking.
data PmLit = PmSLit (HsLit GhcTc)  simple
  PmOLit Bool { is it negated? } (HsOverLit GhcTc)  overloaded

  Equality between literals for pattern match checking.
eqPmLit :: PmLit > PmLit > Bool
eqPmLit (PmSLit l1) (PmSLit l2) = l1 == l2
eqPmLit (PmOLit b1 l1) (PmOLit b2 l2) = b1 == b2 && l1 == l2
  See Note [Undecidable Equality for Overloaded Literals]
eqPmLit _ _ = False

  Represents a match against a literal. We mostly use it to to encode shapes
 for a variable that immediately lead to a refutation.

 See Note [Refutable shapes] in TmOracle. Really similar to 'CoreSyn.AltCon'.
newtype PmAltCon = PmAltLit PmLit
 deriving Outputable

+ See Note [Undecidable Equality for PmAltCons]
+data PmLit = PmLit
+ { pm_lit_ty :: Type
+ , pm_lit_val :: PmLitValue }
+
+data PmLitValue
+ = PmLitInt Integer
+  PmLitRat Rational
+  PmLitChar Char
+  We won't actually see PmLitString in the oracle since we desugar strings to
+  lists
+  PmLitString FastString
+  PmLitOverInt Int { How often Negated? } Integer
+  PmLitOverRat Int { How often Negated? } Rational
+  PmLitOverString FastString
+
+  Undecidable semantic equality result.
+ See Note [Undecidable Equality for PmAltCons]
+data PmEquality
+ = Equal
+  Disjoint
+  PossiblyOverlap
+ deriving (Eq, Show)
+
+  When 'PmEquality' can be decided. @True <=> Equal@, @False <=> Disjoint@.
+decEquality :: Bool > PmEquality
+decEquality True = Equal
+decEquality False = Disjoint
+
+  Undecidable equality for values represented by 'PmLit's.
+ See Note [Undecidable Equality for PmAltCons]
+
+ * @Just True@ ==> Surely equal
+ * @Just False@ ==> Surely different (nonoverlapping, even!)
+ * @Nothing@ ==> Equality relation undecidable
+eqPmLit :: PmLit > PmLit > PmEquality
+eqPmLit (PmLit t1 v1) (PmLit t2 v2)
+  no haddock  pprTrace "eqPmLit" (ppr t1 <+> ppr v1 $$ ppr t2 <+> ppr v2) False = undefined
+  not (t1 `eqType` t2) = Disjoint
+  otherwise = go v1 v2
+ where
+ go (PmLitInt i1) (PmLitInt i2) = decEquality (i1 == i2)
+ go (PmLitRat r1) (PmLitRat r2) = decEquality (r1 == r2)
+ go (PmLitChar c1) (PmLitChar c2) = decEquality (c1 == c2)
+ go (PmLitString s1) (PmLitString s2) = decEquality (s1 == s2)
+ go (PmLitOverInt n1 i1) (PmLitOverInt n2 i2)
+  n1 == n2 && i1 == i2 = Equal
+ go (PmLitOverRat n1 r1) (PmLitOverRat n2 r2)
+  n1 == n2 && r1 == r2 = Equal
+ go (PmLitOverString s1) (PmLitOverString s2)
+  s1 == s2 = Equal
+ go _ _ = PossiblyOverlap
+
+  Syntactic equality.
+instance Eq PmLit where
+ a == b = eqPmLit a b == Equal
+
+  Type of a 'PmLit'
+pmLitType :: PmLit > Type
+pmLitType (PmLit ty _) = ty
+
+  Type of a 'PmAltCon'
+pmAltConType :: PmAltCon > [Type] > Type
+pmAltConType (PmAltLit lit) _arg_tys = ASSERT( null _arg_tys ) pmLitType lit
+pmAltConType (PmAltConLike con) arg_tys = conLikeResTy con arg_tys
+
+  Undecidable equality for values represented by 'ConLike's.
+ See Note [Undecidable Equality for PmAltCons].
+ 'PatSynCon's aren't enforced to be generative, so two syntactically different
+ 'PatSynCon's might match the exact same values. Without looking into and
+ reasoning about the pattern synonym's definition, we can't decide if their
+ sets of matched values is different.
+
+ * @Just True@ ==> Surely equal
+ * @Just False@ ==> Surely different (nonoverlapping, even!)
+ * @Nothing@ ==> Equality relation undecidable
+eqConLike :: ConLike > ConLike > PmEquality
+eqConLike (RealDataCon dc1) (RealDataCon dc2) = decEquality (dc1 == dc2)
+eqConLike (PatSynCon psc1) (PatSynCon psc2)
+  psc1 == psc2
+ = Equal
+eqConLike _ _ = PossiblyOverlap
+
+  Represents the head of a match against a 'ConLike' or literal.
+ Really similar to 'CoreSyn.AltCon'.
+data PmAltCon = PmAltConLike ConLike
+  PmAltLit PmLit
+
+  We can't in general decide whether two 'PmAltCon's match the same set of
+ values. In addition to the reasons in 'eqPmLit' and 'eqConLike', a
+ 'PmAltConLike' might or might not represent the same value as a 'PmAltLit'.
+ See Note [Undecidable Equality for PmAltCons].
+
+ * @Just True@ ==> Surely equal
+ * @Just False@ ==> Surely different (nonoverlapping, even!)
+ * @Nothing@ ==> Equality relation undecidable
+
+ Examples (omitting some constructor wrapping):
+
+ * @eqPmAltCon (LitInt 42) (LitInt 1) == Just False@: Lit equality is
+ decidable
+ * @eqPmAltCon (DataCon A) (DataCon B) == Just False@: DataCon equality is
+ decidable
+ * @eqPmAltCon (LitOverInt 42) (LitOverInt 1) == Nothing@: OverLit equality
+ is undecidable
+ * @eqPmAltCon (PatSyn PA) (PatSyn PB) == Nothing@: PatSyn equality is
+ undecidable
+ * @eqPmAltCon (DataCon I#) (LitInt 1) == Nothing@: DataCon to Lit
+ comparisons are undecidable without reasoning about the wrapped @Int#@
+ * @eqPmAltCon (LitOverInt 1) (LitOverInt 1) == Just True@: We assume
+ reflexivity for overloaded literals
+ * @eqPmAltCon (PatSyn PA) (PatSyn PA) == Just True@: We assume reflexivity
+ for Pattern Synonyms
+eqPmAltCon :: PmAltCon > PmAltCon > PmEquality
+eqPmAltCon (PmAltConLike cl1) (PmAltConLike cl2) = eqConLike cl1 cl2
+eqPmAltCon (PmAltLit l1) (PmAltLit l2) = eqPmLit l1 l2
+eqPmAltCon _ _ = PossiblyOverlap
+
+  Syntactic equality.
instance Eq PmAltCon where
 PmAltLit l1 == PmAltLit l2 = eqPmLit l1 l2
+ a == b = eqPmAltCon a b == Equal
{ Note [Undecidable Equality for Overloaded Literals]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{ Note [Undecidable Equality for PmAltCons]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Equality on overloaded literals is undecidable in the general case. Consider
the following example:
@@ 96,25 +188,19 @@ the following example:
f 1 = ()  Clause A
f 2 = ()  Clause B
Clause B is redundant but to detect this, we should be able to solve the
constraint: False ~ (fromInteger 2 ~ fromInteger 1) which means that we
have to look through function `fromInteger`, whose implementation could
+Clause B is redundant but to detect this, we must decide the constraint:
+@fromInteger 2 ~ fromInteger 1@ which means that we
+have to look through function @fromInteger@, whose implementation could
be anything. This poses difficulties for:
1. The expressive power of the check.
We cannot expect a reasonable implementation of pattern matching to detect
 that fromInteger 2 ~ fromInteger 1 is True, unless we unfold function
+ that @fromInteger 2 ~ fromInteger 1@ is True, unless we unfold function
fromInteger. This puts termination at risk and is undecidable in the
general case.
2. Performance.
 Having an unresolved constraint False ~ (fromInteger 2 ~ fromInteger 1)
 lying around could become expensive really fast. Ticket #11161 illustrates
 how heavy use of overloaded literals can generate plenty of those
 constraints, effectively undermining the term oracle's performance.

3. Error nessages/Warnings.
 What should our message for `f` above be? A reasonable approach would be
+2. Error messages/Warnings.
+ What should our message for @f@ above be? A reasonable approach would be
to issue:
Pattern matches are (potentially) redundant:
@@ 122,8 +208,13 @@ be anything. This poses difficulties for:
but seems to complex and confusing for the user.
We choose to treat overloaded literals that look different as different. The
impact of this is the following:
+We choose to equate only obviously equal overloaded literals, in all other cases
+we signal undecidability by returning Nothing from 'eqPmAltCons'. We do
+better for nonoverloaded literals, because we know their fromInteger/fromString
+implementation is actually injective, allowing us to simplify the constraint
+@fromInteger 1 ~ fromInteger 2@ to @1 ~ 2@, which is trivially unsatisfiable.
+
+The impact of this treatment of overloaded literals is the following:
* Redundancy checking is rather conservative, since it cannot see that clause
B above is redundant.
@@ 136,120 +227,100 @@ impact of this is the following:
* The warnings issued are simpler.
 * We do not play on the safe side, strictly speaking. The assumption that
 1 /= 2 makes the redundancy check more conservative but at the same time
 makes its dual (exhaustiveness check) unsafe. This we can live with, mainly
 for two reasons:
 1. At the moment we do not use the results of the check during compilation
 where this would be a disaster (could result in runtime errors even if
 our function was deemed exhaustive).
 2. Pattern matcing on literals can never be considered exhaustive unless we
 have a catchall clause. Hence, this assumption affects mainly the
 appearance of the warnings and is, in practice safe.
+Similar reasoning applies to pattern synonyms: In contrast to data constructors,
+which are generative, constraints like F a ~ G b for two different pattern
+synonyms F and G aren't immediately unsatisfiable. We assume F a ~ F a, though.
}
  A term constraint. @TVC x e@ encodes that @x@ is equal to @e@.
data TmVarCt = TVC !Id !PmExpr
+literalToPmLit :: Type > Literal > Maybe PmLit
+literalToPmLit ty l = PmLit ty <$> go l
+ where
+ go (LitChar c) = Just (PmLitChar c)
+ go (LitFloat r) = Just (PmLitRat r)
+ go (LitDouble r) = Just (PmLitRat r)
+ go (LitString s) = Just (PmLitString (mkFastStringByteString s))
+ go (LitNumber _ i _) = Just (PmLitInt i)
+ go _ = Nothing
+
+negatePmLit :: PmLit > Maybe PmLit
+negatePmLit (PmLit ty v) = PmLit ty <$> go v
+ where
+ go (PmLitInt i) = Just (PmLitInt (i))
+ go (PmLitRat r) = Just (PmLitRat (r))
+ go (PmLitOverInt n i) = Just (PmLitOverInt (n+1) i)
+ go (PmLitOverRat n r) = Just (PmLitOverRat (n+1) r)
+ go _ = Nothing
+
+overloadPmLit :: Type > PmLit > Maybe PmLit
+overloadPmLit ty (PmLit _ v) = PmLit ty <$> go v
+ where
+ go (PmLitInt i) = Just (PmLitOverInt 0 i)
+ go (PmLitRat r) = Just (PmLitOverRat 0 r)
+ go (PmLitString s)
+  ty `eqType` stringTy = Just v
+  otherwise = Just (PmLitOverString s)
+ go _ = Nothing
instance Outputable TmVarCt where
 ppr (TVC x e) = ppr x <+> char '~' <+> ppr e
+pmLitAsStringLit :: PmLit > Maybe FastString
+pmLitAsStringLit (PmLit _ (PmLitString s)) = Just s
+pmLitAsStringLit _ = Nothing
 
 ** Predicates on PmExpr
  Check if an expression is lifted or not
isNotPmExprOther :: PmExpr > Bool
isNotPmExprOther (PmExprOther _) = False
isNotPmExprOther _expr = True

 
 ** Lift source expressions (HsExpr Id) to PmExpr
lhsExprToPmExpr :: LHsExpr GhcTc > PmExpr
lhsExprToPmExpr (dL>L _ e) = hsExprToPmExpr e

hsExprToPmExpr :: HsExpr GhcTc > PmExpr

 Translating HsVar to flexible meta variables in the unification problem is
 morally wrong, but it does the right thing for now.
 In contrast to the situation in pattern matches, HsVars in expression syntax
 are object language variables, most similar to rigid variables with an
 unknown solution. The correct way would be to handle them through PmExprOther
 and identify syntactically equal occurrences by the same rigid meta variable,
 but we can't compare the wrapped HsExpr for equality. Hence we are stuck with
 this hack.
hsExprToPmExpr (HsVar _ x) = PmExprVar (idName (unLoc x))

 Translating HsConLikeOut to a flexible meta variable is misleading.
 For an example why, consider `consAreRigid` in
 `testsuite/tests/pmcheck/should_compile/PmExprVars.hs`.
 hsExprToPmExpr (HsConLikeOut _ c) = PmExprVar (conLikeName c)

 Desugar literal strings as a list of characters. For other literal values,
 keep it as it is.
 See `translatePat` in Check.hs (the `NPat` and `LitPat` case), and
 Note [Translate Overloaded Literal for Exhaustiveness Checking].
hsExprToPmExpr (HsOverLit _ olit)
  OverLit (OverLitTc False ty) (HsIsString src s) _ < olit, isStringTy ty
 = stringExprToList src s
  otherwise = PmExprLit (PmOLit False olit)
hsExprToPmExpr (HsLit _ lit)
  HsString src s < lit
 = stringExprToList src s
  otherwise = PmExprLit (PmSLit lit)

hsExprToPmExpr e@(NegApp _ (dL>L _ neg_expr) _)
  PmExprLit (PmOLit False olit) < hsExprToPmExpr neg_expr
  NB: DON'T simply @(NegApp (NegApp olit))@ as @x@. when extension
  @RebindableSyntax@ enabled, ((x)) may not equals to x.
 = PmExprLit (PmOLit True olit)
  otherwise = PmExprOther e

hsExprToPmExpr (HsPar _ (dL>L _ e)) = hsExprToPmExpr e

hsExprToPmExpr e@(ExplicitTuple _ ps boxity)
  all tupArgPresent ps = mkPmExprData tuple_con tuple_args
  otherwise = PmExprOther e
 where
 tuple_con = tupleDataCon boxity (length ps)
 tuple_args = [ lhsExprToPmExpr e  (dL>L _ (Present _ e)) < ps ]

hsExprToPmExpr e@(ExplicitList _ mb_ol elems)
  Nothing < mb_ol = foldr cons nil (map lhsExprToPmExpr elems)
  otherwise = PmExprOther e { overloaded list: No PmExprApp }
 where
 cons x xs = mkPmExprData consDataCon [x,xs]
 nil = mkPmExprData nilDataCon []

 we want this but we would have to make everything monadic :/
 ./compiler/deSugar/DsMonad.hs:397:dsLookupDataCon :: Name > DsM DataCon

 hsExprToPmExpr (RecordCon c _ binds) = do
 con < dsLookupDataCon (unLoc c)
 args < mapM lhsExprToPmExpr (hsRecFieldsArgs binds)
 return (PmExprCon con args)
hsExprToPmExpr e@(RecordCon {}) = PmExprOther e

hsExprToPmExpr (HsTick _ _ e) = lhsExprToPmExpr e
hsExprToPmExpr (HsBinTick _ _ _ e) = lhsExprToPmExpr e
hsExprToPmExpr (HsTickPragma _ _ _ _ e) = lhsExprToPmExpr e
hsExprToPmExpr (HsSCC _ _ _ e) = lhsExprToPmExpr e
hsExprToPmExpr (HsCoreAnn _ _ _ e) = lhsExprToPmExpr e
hsExprToPmExpr (ExprWithTySig _ e _) = lhsExprToPmExpr e
hsExprToPmExpr (HsWrap _ w e)
  A dictionary application spoils e and we have no choice but to return an
  PmExprOther. Same thing for other stuff that can't erased in the
  compilation process. Otherwise this bites in
  teststuite/tests/pmcheck/should_compile/PmExprVars.hs.
  isErasableHsWrapper w = hsExprToPmExpr e
hsExprToPmExpr e = PmExprOther e  the rest are not handled by the oracle

stringExprToList :: SourceText > FastString > PmExpr
stringExprToList src s = foldr cons nil (map charToPmExpr (unpackFS s))
+coreExprAsPmLit :: CoreExpr > Maybe PmLit
+ coreExprAsPmLit e  pprTrace "coreExprAsPmLit" (ppr e) False = undefined
+coreExprAsPmLit (Tick _t e) = coreExprAsPmLit e
+coreExprAsPmLit (Lit l) = literalToPmLit (literalType l) l
+coreExprAsPmLit e = case collectArgs e of
+ (Var x, [Lit l])
+  Just dc < isDataConWorkId_maybe x
+ , dc `elem` [intDataCon, wordDataCon, charDataCon, floatDataCon, doubleDataCon]
+ > literalToPmLit (exprType e) l
+ (Var x, [_ty, Lit n, Lit d])
+  Just dc < isDataConWorkId_maybe x
+ , dataConName dc == ratioDataConName
+  HACK: just assume we have a literal double. This case only occurs for
+  overloaded lits anyway, so we immediately override type information
+ > literalToPmLit (exprType e) (mkLitDouble (litValue n % litValue d))
+ (Var x, args)
+  Take care of XRebindableSyntax. The last argument should be the (only)
+  integer literal, otherwise we can't really do much about it.
+  [Lit l] < dropWhile (not . is_lit) args
+  getOccFS because of XRebindableSyntax
+ , getOccFS (idName x) == getOccFS fromIntegerName
+ > literalToPmLit (literalType l) l >>= overloadPmLit (exprType e)
+ (Var x, args)
+  Similar to fromInteger case
+  [r] < dropWhile (not . is_ratio) args
+ , getOccFS (idName x) == getOccFS fromRationalName
+ > coreExprAsPmLit r >>= overloadPmLit (exprType e)
+ (Var x, [Type _ty, _dict, s])
+  idName x == fromStringName
+  NB: Calls coreExprAsPmLit and then overloadPmLit, so that we return PmLitOverStrings
+ > coreExprAsPmLit s >>= overloadPmLit (exprType e)
+  These last two cases handle String literals
+ (Var x, [Type ty])
+  Just dc < isDataConWorkId_maybe x
+ , dc == nilDataCon
+ , ty `eqType` charTy
+ > literalToPmLit stringTy (mkLitString "")
+ (Var x, [Lit l])
+  idName x `elem` [unpackCStringName, unpackCStringUtf8Name]
+ > literalToPmLit stringTy l
+ _ > Nothing
where
 cons x xs = mkPmExprData consDataCon [x,xs]
 nil = mkPmExprData nilDataCon []
 charToPmExpr c = PmExprLit (PmSLit (HsChar src c))
+ is_lit Lit{} = True
+ is_lit _ = False
+ is_ratio (Type _) = False
+ is_ratio r
+  Just (tc, _) < splitTyConApp_maybe (exprType r)
+ = tyConName tc == ratioTyConName
+  otherwise
+ = False
{
%************************************************************************
@@ 259,22 +330,35 @@ stringExprToList src s = foldr cons nil (map charToPmExpr (unpackFS s))
%************************************************************************
}
instance Outputable PmLit where
 ppr (PmSLit l) = pmPprHsLit l
 ppr (PmOLit neg l) = (if neg then char '' else empty) <> ppr l
+instance Outputable PmLitValue where
+ ppr (PmLitInt i) = ppr i
+ ppr (PmLitRat r) = ppr (double (fromRat r))  good enough
+ ppr (PmLitChar c) = pprHsChar c
+ ppr (PmLitString s) = pprHsString s
+ ppr (PmLitOverInt n i) = minuses n (ppr i)
+ ppr (PmLitOverRat n r) = minuses n (ppr (double (fromRat r)))
+ ppr (PmLitOverString s) = pprHsString s
+
+ Take care of negated literals
+minuses :: Int > SDoc > SDoc
+minuses n sdoc = iterate (\sdoc > parens (char '' <> sdoc)) sdoc !! n
instance Outputable PmExpr where
 ppr = go (0 :: Int)
+instance Outputable PmLit where
+ ppr (PmLit ty v) = ppr v <> suffix
where
 go _ (PmExprLit l) = ppr l
 go _ (PmExprVar v) = ppr v
 go _ (PmExprOther e) = angleBrackets (ppr e)
 go _ (PmExprCon (RealDataCon dc) args)
  isTupleDataCon dc = parens $ comma_sep $ map ppr args
  dc == consDataCon = brackets $ comma_sep $ map ppr (list_cells args)
 where
 comma_sep = fsep . punctuate comma
 list_cells (hd:tl) = hd : list_cells tl
 list_cells _ = []
 go prec (PmExprCon cl args)
 = cparen (null args  prec > 0) (hcat (ppr cl:map (go 1) args))
+  Some adhoc hackery for displaying proper lit suffixes based on type
+ tbl = [ (intPrimTy, primIntSuffix)
+ , (int64PrimTy, primInt64Suffix)
+ , (wordPrimTy, primWordSuffix)
+ , (word64PrimTy, primWord64Suffix)
+ , (charPrimTy, primCharSuffix)
+ , (floatPrimTy, primFloatSuffix)
+ , (doublePrimTy, primDoubleSuffix) ]
+ suffix = fromMaybe empty (snd <$> find (eqType ty . fst) tbl)
+
+instance Outputable PmAltCon where
+ ppr (PmAltConLike cl) = ppr cl
+ ppr (PmAltLit l) = ppr l
+
+instance Outputable PmEquality where
+ ppr = text . show
diff git a/compiler/deSugar/PmOracle.hs b/compiler/deSugar/PmOracle.hs
new file mode 100644
index 0000000000000000000000000000000000000000..93c4d1d9594523b5a8c02a891b066c3f6a5b0aea
 /dev/null
+++ b/compiler/deSugar/PmOracle.hs
@@ 0,0 +1,1872 @@
+{
+Authors: George Karachalias
+ Sebastian Graf
+ Ryan Scott
+}
+
+{# LANGUAGE CPP, LambdaCase, TupleSections, PatternSynonyms, ViewPatterns, MultiWayIf #}
+
+  The pattern match oracle. The main export of the module are the functions
+ 'addTmCt', 'refineToAltCon' and 'addRefutableAltCon' for adding
+ facts to the oracle, and 'provideEvidenceForEquation' to turn a 'Delta' into
+ a concrete evidence for an equation.
+module PmOracle (
+
+ DsM, tracePm, mkPmId,
+ Delta, initDelta, canDiverge, lookupRefuts, lookupSolution,
+ lookupNumberOfRefinements,
+
+ TmCt(..),
+ inhabitants,
+ addTypeEvidence,  Add type equalities
+ addRefutableAltCon,  Add a negative term equality
+ addTmCt,  Add a positive term equality x ~ e
+ addVarCoreCt,  Add a positive term equality x ~ core_expr
+ refineToAltCon,  Add a positive refinement x ~ K _ _
+ tmOracle,  Add multiple positive term equalities
+ provideEvidenceForEquation,
+ ) where
+
+#include "HsVersions.h"
+
+import GhcPrelude
+
+import PmExpr
+
+import DynFlags
+import Outputable
+import ErrUtils
+import Util
+import Bag
+import UniqDSet
+import Unique
+import Id
+import VarEnv
+import UniqDFM
+import Var (EvVar)
+import Name
+import CoreSyn
+import CoreOpt (exprIsConApp_maybe)
+import CoreUtils (exprType)
+import MkCore (mkListExpr, mkCharExpr)
+import UniqSupply
+import FastString
+import SrcLoc
+import ListSetOps (unionLists)
+import Maybes
+import ConLike
+import DataCon
+import PatSyn
+import TyCon
+import TysWiredIn
+import TysPrim (tYPETyCon)
+import TyCoRep
+import Type
+import TcSimplify (tcNormalise, tcCheckSatisfiability)
+import Unify (tcMatchTy)
+import TcRnTypes (pprEvVarWithType, completeMatchConLikes)
+import Coercion
+import MonadUtils hiding (foldlM)
+import DsMonad hiding (foldlM)
+import FamInst
+import FamInstEnv
+
+import Control.Monad (zipWithM, guard, mzero)
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.State.Strict
+import Data.Bifunctor (second)
+import Data.Foldable (foldlM)
+import Data.List (find)
+import Data.List.NonEmpty (NonEmpty (..))
+import qualified Data.List.NonEmpty as NonEmpty
+import qualified Data.Semigroup as Semigroup
+
+ Debugging Infrastructre
+
+tracePm :: String > SDoc > DsM ()
+tracePm herald doc = do
+ dflags < getDynFlags
+ printer < mkPrintUnqualifiedDs
+ liftIO $ dumpIfSet_dyn_printer printer dflags
+ Opt_D_dump_ec_trace (text herald $$ (nest 2 doc))
+
+  Generate a fresh `Id` of a given type
+mkPmId :: Type > DsM Id
+mkPmId ty = getUniqueM >>= \unique >
+ let occname = mkVarOccFS $ fsLit "$pm"
+ name = mkInternalName unique occname noSrcSpan
+ in return (mkLocalId name ty)
+
+
+ * Caching possible matches of a COMPLETE set
+
+type ConLikeSet = UniqDSet ConLike
+
+  A data type caching the results of 'completeMatchConLikes' with support for
+ deletion of contructors that were already matched on.
+data PossibleMatches
+ = PM TyCon (NonEmpty ConLikeSet)
+  ^ Each ConLikeSet is a (subset of) the constructors in a COMPLETE pragma
+  'NonEmpty' because the empty case would mean that the type has no COMPLETE
+  set at all, for which we have 'NoPM'
+  NoPM
+  ^ No COMPLETE set for this type (yet). Think of overloaded literals.
+
+instance Outputable PossibleMatches where
+ ppr (PM _tc cs) = ppr (NonEmpty.toList cs)
+ ppr NoPM = text ""
+
+initIM :: Type > DsM (Maybe PossibleMatches)
+initIM ty = case splitTyConApp_maybe ty of
+ Nothing > pure Nothing
+ Just (tc, tc_args) > do
+  Look into the representation type of a data family instance, too.
+ env < dsGetFamInstEnvs
+ let (tc', _tc_args', _co) = tcLookupDataFamInst env tc tc_args
+ let mb_rdcs = map RealDataCon <$> tyConDataCons_maybe tc'
+ let rdcs = maybeToList mb_rdcs
+  NB: tc, because COMPLETE sets are associated with the parent data family
+  TyCon
+ pragmas < dsGetCompleteMatches tc
+ let fams = mapM dsLookupConLike . completeMatchConLikes
+ pscs < mapM fams pragmas
+ pure (PM tc . fmap mkUniqDSet <$> NonEmpty.nonEmpty (rdcs ++ pscs))
+
+markMatched :: ConLike > PossibleMatches > PossibleMatches
+markMatched con (PM tc ms) = PM tc (fmap (`delOneFromUniqDSet` con) ms)
+markMatched _ NoPM = NoPM
+
+  Satisfiability decisions as a data type. The @proof@ can carry a witness
+ for satisfiability and might even be instantiated to 'Data.Void.Void' to
+ degenerate into a semidecision predicate.
+data Satisfiability proof
+ = Unsatisfiable
+  PossiblySatisfiable
+  Satisfiable !proof
+
+maybeSatisfiable :: Maybe a > Satisfiability a
+maybeSatisfiable (Just a) = Satisfiable a
+maybeSatisfiable Nothing = Unsatisfiable
+
+  Tries to return one of the possible 'ConLike's from one of the COMPLETE
+ sets. If the 'PossibleMatches' was inhabited before (cf. 'ensureInhabited')
+ this 'ConLike' is evidence for that assurance.
+getUnmatchedConstructor :: PossibleMatches > Satisfiability ConLike
+getUnmatchedConstructor NoPM = PossiblySatisfiable
+getUnmatchedConstructor (PM _tc ms)
+ = maybeSatisfiable $ NonEmpty.head <$> traverse pick_one_conlike ms
+ where
+ pick_one_conlike cs = case uniqDSetToList cs of
+ [] > Nothing
+ (cl:_) > Just cl
+
+
+ * Instantiating constructors, types and evidence
+
+newEvVar :: Name > Type > EvVar
+newEvVar name ty = mkLocalId name ty
+
+nameType :: String > Type > DsM EvVar
+nameType name ty = do
+ unique < getUniqueM
+ let occname = mkVarOccFS (fsLit (name++"_"++show unique))
+ idname = mkInternalName unique occname noSrcSpan
+ return (newEvVar idname ty)
+
+  Instantiate a 'ConLike' given its universal type arguments. Instantiates
+ existential and term binders with fresh variables of appropriate type.
+ Also returns instantiated evidence variables from the match and the types of
+ strict constructor fields.
+mkOneConFull :: [Type] > ConLike > DsM ([Id], [EvVar], [Type], [TyVar])
+ * 'con' K is a ConLike
+  In the case of DataCons and most PatSynCons, these
+ are associated with a particular TyCon T
+  But there are PatSynCons for this is not the case! See #11336, #17112
+
+ * 'arg_tys' tys are the types K's universally quantified type
+ variables should be instantiated to.
+  For DataCons and most PatSyns these are the arguments of their TyCon
+  For cases like in #11336, #17112, the univ_ts include those variables
+ from the view pattern, so tys will have to come from the type checker.
+ They can't easily be recovered from the result type.
+
+ After instantiating the universal tyvars of K to tys we get
+ K @tys :: forall bs. Q => s1 .. sn > T tys
+ Note that if K is a PatSynCon, depending on arg_tys, T might not necessarily
+ be a concrete TyCon.
+
+ Suppose y1 is a strict field. Then we get
+ Results: [y1,..,yn]
+ Q
+ [s1]
+ [e1,..,en]
+mkOneConFull arg_tys con = do
+ let (univ_tvs, ex_tvs, eq_spec, thetas, _req_theta , field_tys, _con_res_ty)
+ = conLikeFullSig con
+  pprTrace "mkOneConFull" (ppr con $$ ppr arg_tys $$ ppr univ_tvs $$ ppr _con_res_ty) (return ())
+  Substitute universals for type arguments
+ let subst_univ = zipTvSubst univ_tvs arg_tys
+  Instantiate fresh existentials as arguments to the contructor
+ (subst, ex_tvs') < cloneTyVarBndrs subst_univ ex_tvs <$> getUniqueSupplyM
+ let field_tys' = substTys subst field_tys
+  Instantiate fresh term variables (VAs) as arguments to the constructor
+ vars < mapM mkPmId field_tys'
+  All constraints bound by the constructor (alpharenamed), these are added
+  to the type oracle
+ let theta_cs = substTheta subst (eqSpecPreds eq_spec ++ thetas)
+ theta_ev_vars < mapM (nameType "pm") theta_cs
+  Figure out the types of strict constructor fields
+ let arg_is_banged = map isBanged $ conLikeImplBangs con
+ strict_arg_tys = filterByList arg_is_banged field_tys'
+ return (vars, theta_ev_vars, strict_arg_tys, ex_tvs')
+
+equateTyVars :: [TyVar] > [TyVar] > DsM [EvVar]
+equateTyVars ex_tvs1 ex_tvs2
+ = ASSERT(ex_tvs1 `equalLength` ex_tvs2)
+ catMaybes <$> zipWithM mb_to_evvar ex_tvs1 ex_tvs2
+ where
+ mb_to_evvar tv1 tv2
+  tv1 == tv2 = pure Nothing
+  otherwise = Just <$> to_evvar tv1 tv2
+ to_evvar tv1 tv2 = nameType "pmConCon" $
+ mkPrimEqPred (mkTyVarTy tv1) (mkTyVarTy tv2)
+
+
+ * Pattern match oracle
+
+
+  Term and type constraints to accompany each value vector abstraction.
+ For efficiency, we store the term oracle state instead of the term
+ constraints.
+data Delta = MkDelta { delta_ty_cs :: Bag EvVar  Type oracle; things like a~Int
+ , delta_tm_cs :: TmState }  Term oracle; things like x~Nothing
+
+  An initial delta that is always satisfiable
+initDelta :: Delta
+initDelta = MkDelta emptyBag initTmState
+
+instance Outputable Delta where
+ ppr delta = vcat [
+  intentionally formatted this way enable the dev to comment in only
+  the info she needs
+ ppr (delta_tm_cs delta),
+ ppr (pprEvVarWithType <$> delta_ty_cs delta)
+ ppr (delta_ty_cs delta)
+ ]
+
+
+{ Note [Recovering from unsatisfiable patternmatching constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider the following code (see #12957 and #15450):
+
+ f :: Int ~ Bool => ()
+ f = case True of { False > () }
+
+We want to warn that the patternmatching in `f` is nonexhaustive. But GHC
+used not to do this; in fact, it would warn that the match was /redundant/!
+This is because the constraint (Int ~ Bool) in `f` is unsatisfiable, and the
+coverage checker deems any matches with unsatifiable constraint sets to be
+unreachable.
+
+We decide to better than this. When beginning coverage checking, we first
+check if the constraints in scope are unsatisfiable, and if so, we start
+afresh with an empty set of constraints. This way, we'll get the warnings
+that we expect.
+}
+
+
+ * Composable satisfiability checks
+
+  Given a 'Delta', check if it is compatible with new facts encoded in this
+ this check. If so, return 'Just' a potentially extended 'Delta'. Return
+ 'Nothing' if unsatisfiable.
+
+ There are three essential SatisfiabilityChecks:
+ 1. 'tmIsSatisfiable', adding term oracle facts
+ 2. 'tyIsSatisfiable', adding type oracle facts
+ 3. 'tysAreNonVoid', checks if the given types have an inhabitant
+ Functions like 'pmIsSatisfiable', 'nonVoid' and 'testInhabited' plug these
+ together as they see fit.
+newtype SatisfiabilityCheck = SC (Delta > DsM (Maybe Delta))
+
+  Check the given 'Delta' for satisfiability by the the given
+ 'SatisfiabilityCheck'. Return 'Just' a new, potentially extended, 'Delta' if
+ successful, and 'Nothing' otherwise.
+runSatisfiabilityCheck :: Delta > SatisfiabilityCheck > DsM (Maybe Delta)
+runSatisfiabilityCheck delta (SC chk) = chk delta
+
+  Allowing easy composition of 'SatisfiabilityCheck's.
+instance Semigroup SatisfiabilityCheck where
+  This is @a >=> b@ from MaybeT DsM
+ SC a <> SC b = SC c
+ where
+ c delta = a delta >>= \case
+ Nothing > pure Nothing
+ Just delta' > b delta'
+
+instance Monoid SatisfiabilityCheck where
+  We only need this because of mconcat (which we use in place of sconcat,
+  which requires NonEmpty lists as argument, making all call sites ugly)
+ mempty = SC (pure . Just)
+
+
+ * Oracle transition function
+
+  Given a conlike's term constraints, type constraints, and strict argument
+ types, check if they are satisfiable.
+ (In other words, this is the ⊢_Sat oracle judgment from the GADTs Meet
+ Their Match paper.)
+
+ Taking strict argument types into account is something which was not
+ discussed in GADTs Meet Their Match. For an explanation of what role they
+ serve, see @Note [Strict argument type constraints]@.
+pmIsSatisfiable
+ :: Delta  ^ The ambient term and type constraints
+  (known to be satisfiable).
+ > Bag TmCt  ^ The new term constraints.
+ > Bag EvVar  ^ The new type constraints.
+ > [Type]  ^ The strict argument types.
+ > DsM (Maybe Delta)
+  ^ @'Just' delta@ if the constraints (@delta@) are
+  satisfiable, and each strict argument type is inhabitable.
+  'Nothing' otherwise.
+pmIsSatisfiable amb_cs new_tm_cs new_ty_cs strict_arg_tys =
+  The order is important here! Check the new type constraints before we check
+  whether strict argument types are inhabited given those constraints.
+ runSatisfiabilityCheck amb_cs $ mconcat
+ [ tyIsSatisfiable True new_ty_cs
+ , tmIsSatisfiable new_tm_cs
+ , tysAreNonVoid initRecTc strict_arg_tys
+ ]
+
+
+ * Type normalisation
+
+  The return value of 'pmTopNormaliseType'
+data TopNormaliseTypeResult
+ = NoChange Type
+  ^ 'tcNormalise' failed to simplify the type and 'topNormaliseTypeX' was
+  unable to reduce the outermost type application, so the type came out
+  unchanged.
+  NormalisedByConstraints Type
+  ^ 'tcNormalise' was able to simplify the type with some local constraint
+  from the type oracle, but 'topNormaliseTypeX' couldn't identify a type
+  redex.
+  HadRedexes Type [(Type, DataCon)] Type
+  ^ 'tcNormalise' may or may not been able to simplify the type, but
+  'topNormaliseTypeX' made progress either way and got rid of at least one
+  outermost type or data family redex or newtype.
+  The first field is the last type that was reduced solely through type
+  family applications (possibly just the 'tcNormalise'd type). This is the
+  one that is equal (in source Haskell) to the initial type.
+  The third field is the type that we get when also looking through data
+  family applications and newtypes. This would be the representation type in
+  Core (modulo casts).
+  The second field is the list of Newtype 'DataCon's that we looked through
+  in the chain of reduction steps between the Source type and the Core type.
+  We also keep the type of the DataCon application, so that we don't have to
+  reconstruct it in inhabitationCandidates.build_newtype.
+
+  Just give me the potentially normalised source type, unchanged or not!
+normalisedSourceType :: TopNormaliseTypeResult > Type
+normalisedSourceType (NoChange ty) = ty
+normalisedSourceType (NormalisedByConstraints ty) = ty
+normalisedSourceType (HadRedexes ty _ _) = ty
+
+instance Outputable TopNormaliseTypeResult where
+ ppr (NoChange ty) = text "NoChange" <+> ppr ty
+ ppr (NormalisedByConstraints ty) = text "NormalisedByConstraints" <+> ppr ty
+ ppr (HadRedexes src_ty ds core_ty) = text "HadRedexes" <+> braces fields
+ where
+ fields = fsep (punctuate comma [ text "src_ty =" <+> ppr src_ty
+ , text "newtype_dcs =" <+> ppr ds
+ , text "core_ty =" <+> ppr core_ty ])
+
+pmTopNormaliseType :: Bag EvVar > Type > DsM TopNormaliseTypeResult
+ ^ Get rid of *outermost* (or toplevel)
+ * type function redex
+ * data family redex
+ * newtypes
+
+ Behaves like `topNormaliseType_maybe`, but instead of returning a
+ coercion, it returns useful information for issuing pattern matching
+ warnings. See Note [Type normalisation for EmptyCase] for details.
+ It also initially 'tcNormalise's the type with the bag of local constraints.
+
+ See 'TopNormaliseTypeResult' for the meaning of the return value.
+
+ NB: Normalisation can potentially change kinds, if the head of the type
+ is a type family with a variable result kind. I (Richard E) can't think
+ of a way to cause trouble here, though.
+pmTopNormaliseType ty_cs typ
+ = do env < dsGetFamInstEnvs
+  Before proceeding, we chuck typ into the constraint solver, in case
+  solving for given equalities may reduce typ some. See
+  "Wrinkle: local equalities" in Note [Type normalisation for EmptyCase].
+ (_, mb_typ') < initTcDsForSolver $ tcNormalise ty_cs typ
+  If tcNormalise didn't manage to simplify the type, continue anyway.
+  We might be able to reduce type applications nonetheless!
+ let typ' = fromMaybe typ mb_typ'
+  Now we look with topNormaliseTypeX through type and data family
+  applications and newtypes, which tcNormalise does not do.
+  See also 'TopNormaliseTypeResult'.
+ pure $ case topNormaliseTypeX (stepper env) comb typ' of
+ Nothing
+  Nothing < mb_typ' > NoChange typ
+  otherwise > NormalisedByConstraints typ'
+ Just ((ty_f,tm_f), ty) > HadRedexes src_ty newtype_dcs core_ty
+ where
+ src_ty = eq_src_ty ty (typ' : ty_f [ty])
+ newtype_dcs = tm_f []
+ core_ty = ty
+ where
+  Find the first type in the sequence of rewrites that is a data type,
+  newtype, or a data family application (not the representation tycon!).
+  This is the one that is equal (in source Haskell) to the initial type.
+  If none is found in the list, then all of them are type family
+  applications, so we simply return the last one, which is the *simplest*.
+ eq_src_ty :: Type > [Type] > Type
+ eq_src_ty ty tys = maybe ty id (find is_closed_or_data_family tys)
+
+ is_closed_or_data_family :: Type > Bool
+ is_closed_or_data_family ty = pmIsClosedType ty  isDataFamilyAppType ty
+
+  For efficiency, represent both lists as difference lists.
+  comb performs the concatenation, for both lists.
+ comb (tyf1, tmf1) (tyf2, tmf2) = (tyf1 . tyf2, tmf1 . tmf2)
+
+ stepper env = newTypeStepper `composeSteppers` tyFamStepper env
+
+  A 'NormaliseStepper' that unwraps newtypes, careful not to fall into
+  a loop. If it would fall into a loop, it produces 'NS_Abort'.
+ newTypeStepper :: NormaliseStepper ([Type] > [Type],[(Type, DataCon)] > [(Type, DataCon)])
+ newTypeStepper rec_nts tc tys
+  Just (ty', _co) < instNewTyCon_maybe tc tys
+ , let orig_ty = TyConApp tc tys
+ = case checkRecTc rec_nts tc of
+ Just rec_nts' > let tyf = (orig_ty:)
+ tmf = ((orig_ty, tyConSingleDataCon tc):)
+ in NS_Step rec_nts' ty' (tyf, tmf)
+ Nothing > NS_Abort
+  otherwise
+ = NS_Done
+
+ tyFamStepper :: FamInstEnvs > NormaliseStepper ([Type] > [Type], a > a)
+ tyFamStepper env rec_nts tc tys  Try to step a type/data family
+ = let (_args_co, ntys, _res_co) = normaliseTcArgs env Representational tc tys in
+  NB: It's OK to use normaliseTcArgs here instead of
+  normalise_tc_args (which takes the LiftingContext described
+  in Note [Normalising types]) because the reduceTyFamApp below
+  works only at top level. We'll never recur in this function
+  after reducing the kind of a bound tyvar.
+
+ case reduceTyFamApp_maybe env Representational tc ntys of
+ Just (_co, rhs) > NS_Step rec_nts rhs ((rhs:), id)
+ _ > NS_Done
+
+  Returns 'True' if the argument 'Type' is a fully saturated application of
+ a closed type constructor.
+
+ Closed type constructors are those with a fixed right hand side, as
+ opposed to e.g. associated types. These are of particular interest for
+ patternmatch coverage checking, because GHC can exhaustively consider all
+ possible forms that values of a closed type can take on.
+
+ Note that this function is intended to be used to check types of valuelevel
+ patterns, so as a consequence, the 'Type' supplied as an argument to this
+ function should be of kind @Type@.
+pmIsClosedType :: Type > Bool
+pmIsClosedType ty
+ = case splitTyConApp_maybe ty of
+ Just (tc, ty_args)
+  is_algebraic_like tc && not (isFamilyTyCon tc)
+ > ASSERT2( ty_args `lengthIs` tyConArity tc, ppr ty ) True
+ _other > False
+ where
+  This returns True for TyCons which /act like/ algebraic types.
+  (See "Type#type_classification" for what an algebraic type is.)
+ 
+  This is qualified with \"like\" because of a particular special
+  case: TYPE (the underlyind kind behind Type, among others). TYPE
+  is conceptually a datatype (and thus algebraic), but in practice it is
+  a primitive builtin type, so we must check for it specially.
+ 
+  NB: it makes sense to think of TYPE as a closed type in a valuelevel,
+  patternmatching context. However, at the kind level, TYPE is certainly
+  not closed! Since this function is specifically tailored towards pattern
+  matching, however, it's OK to label TYPE as closed.
+ is_algebraic_like :: TyCon > Bool
+ is_algebraic_like tc = isAlgTyCon tc  tc == tYPETyCon
+
+{ Note [Type normalisation for EmptyCase]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+EmptyCase is an exception for pattern matching, since it is strict. This means
+that it boils down to checking whether the type of the scrutinee is inhabited.
+Function pmTopNormaliseType gets rid of the outermost type function/data
+family redex and newtypes, in search of an algebraic type constructor, which is
+easier to check for inhabitation.
+
+It returns 3 results instead of one, because there are 2 subtle points:
+1. Newtypes are isomorphic to the underlying type in core but not in the source
+ language,
+2. The representational data family tycon is used internally but should not be
+ shown to the user
+
+Hence, if pmTopNormaliseType env ty_cs ty = Just (src_ty, dcs, core_ty),
+then
+ (a) src_ty is the rewritten type which we can show to the user. That is, the
+ type we get if we rewrite type families but not data families or
+ newtypes.
+ (b) dcs is the list of newtype constructors "skipped", every time we normalise
+ a newtype to its core representation, we keep track of the source data
+ constructor and the type we unwrap.
+ (c) core_ty is the rewritten type. That is,
+ pmTopNormaliseType env ty_cs ty = Just (src_ty, dcs, core_ty)
+ implies
+ topNormaliseType_maybe env ty = Just (co, core_ty)
+ for some coercion co.
+
+To see how all cases come into play, consider the following example:
+
+ data family T a :: *
+ data instance T Int = T1  T2 Bool
+  Which gives rise to FC:
+  data T a
+  data R:TInt = T1  T2 Bool
+  axiom ax_ti : T Int ~R R:TInt
+
+ newtype G1 = MkG1 (T Int)
+ newtype G2 = MkG2 G1
+
+ type instance F Int = F Char
+ type instance F Char = G2
+
+In this case pmTopNormaliseType env ty_cs (F Int) results in
+
+ Just (G2, [(G2,MkG2),(G1,MkG1)], R:TInt)
+
+Which means that in source Haskell:
+  G2 is equivalent to F Int (in contrast, G1 isn't).
+  if (x : R:TInt) then (MkG2 (MkG1 x) : F Int).
+
+
+ Wrinkle: Local equalities
+
+
+Given the following type family:
+
+ type family F a
+ type instance F Int = Void
+
+Should the following program (from #14813) be considered exhaustive?
+
+ f :: (i ~ Int) => F i > a
+ f x = case x of {}
+
+You might think "of course, since `x` is obviously of type Void". But the
+idType of `x` is technically F i, not Void, so if we pass F i to
+inhabitationCandidates, we'll mistakenly conclude that `f` is nonexhaustive.
+In order to avoid this pitfall, we need to normalise the type passed to
+pmTopNormaliseType, using the constraint solver to solve for any local
+equalities (such as i ~ Int) that may be in scope.
+}
+
+
+ * Type oracle
+
+  Check whether a set of type constraints is satisfiable.
+tyOracle :: Bag EvVar > DsM Bool
+tyOracle evs
+ = do { ((_warns, errs), res) < initTcDsForSolver $ tcCheckSatisfiability evs
+ ; case res of
+ Just sat > return sat
+ Nothing > pprPanic "tyOracle" (vcat $ pprErrMsgBagWithLoc errs) }
+
+  A 'SatisfiabilityCheck' based on new typelevel constraints.
+ Returns a new 'Delta' if the new constraints are compatible with existing
+ ones. Doesn't bother calling out to the type oracle if the bag of new type
+ constraints was empty. Will only recheck 'PossibleMatches' in the term oracle
+ for emptiness if the first argument is 'True'.
+tyIsSatisfiable :: Bool > Bag EvVar > SatisfiabilityCheck
+tyIsSatisfiable recheck_complete_sets new_ty_cs = SC $ \delta > do
+ tracePm "tyIsSatisfiable" (ppr (fmap pprEvVarWithType new_ty_cs))
+ let ty_cs = new_ty_cs `unionBags` delta_ty_cs delta
+ let delta' = delta{ delta_ty_cs = ty_cs }
+ if isEmptyBag new_ty_cs
+ then pure (Just delta)
+ else tyOracle ty_cs >>= \case
+ False > pure Nothing
+ True
+  recheck_complete_sets > ensureAllPossibleMatchesInhabited delta'
+  otherwise > pure (Just delta')
+
+
+{ *********************************************************************
+* *
+ SharedIdEnv
+ DIdEnv with sharing
+* *
+********************************************************************* }
+
+  Either @Indirect x@, meaning the value is represented by that of @x@, or
+ an @Entry@ containing containing the actual value it represents.
+data Shared a
+ = Indirect Id
+  Entry a
+
+  A 'DIdEnv' in which entries can be shared by multiple 'Id's.
+ Merge equivalence classes of two Ids by 'setIndirectSDIE' and set the entry
+ of an Id with 'setEntrySDIE'.
+newtype SharedDIdEnv a
+ = SDIE { unSDIE :: DIdEnv (Shared a) }
+
+emptySDIE :: SharedDIdEnv a
+emptySDIE = SDIE emptyDVarEnv
+
+lookupReprAndEntrySDIE :: SharedDIdEnv a > Id > (Id, Maybe a)
+lookupReprAndEntrySDIE sdie@(SDIE env) x = case lookupDVarEnv env x of
+ Nothing > (x, Nothing)
+ Just (Indirect y) > lookupReprAndEntrySDIE sdie y
+ Just (Entry a) > (x, Just a)
+
+  @lookupSDIE env x@ looks up an entry for @x@, looking through all
+ 'Indirect's until it finds a shared 'Entry'.
+lookupSDIE :: SharedDIdEnv a > Id > Maybe a
+lookupSDIE sdie x = snd (lookupReprAndEntrySDIE sdie x)
+
+  Check if two variables are part of the same equivalence class.
+sameRepresentative :: SharedDIdEnv a > Id > Id > Bool
+sameRepresentative sdie x y =
+ fst (lookupReprAndEntrySDIE sdie x) == fst (lookupReprAndEntrySDIE sdie y)
+
+  @setIndirectSDIE env x y@ sets @x@'s 'Entry' to @Indirect y@, thereby
+ merging @x@'s equivalence class into @y@'s. This will discard all info on
+ @x@!
+setIndirectSDIE :: SharedDIdEnv a > Id > Id > SharedDIdEnv a
+setIndirectSDIE sdie@(SDIE env) x y =
+ SDIE $ extendDVarEnv env (fst (lookupReprAndEntrySDIE sdie x)) (Indirect y)
+
+  @setEntrySDIE env x a@ sets the 'Entry' @x@ is associated with to @a@,
+ thereby modifying its whole equivalence class.
+setEntrySDIE :: SharedDIdEnv a > Id > a > SharedDIdEnv a
+setEntrySDIE sdie@(SDIE env) x a =
+ SDIE $ extendDVarEnv env (fst (lookupReprAndEntrySDIE sdie x)) (Entry a)
+
+traverseSharedIdEnv :: Applicative f => (a > f b) > SharedDIdEnv a > f (SharedDIdEnv b)
+traverseSharedIdEnv f = fmap (SDIE . listToUDFM) . traverse g . udfmToList . unSDIE
+ where
+ g (u, Indirect y) = pure (u,Indirect y)
+ g (u, Entry a) = (u,) . Entry <$> f a
+
+instance Outputable a => Outputable (Shared a) where
+ ppr (Indirect x) = ppr x
+ ppr (Entry a) = ppr a
+
+instance Outputable a => Outputable (SharedDIdEnv a) where
+ ppr (SDIE env) = ppr env
+
+
+{ *********************************************************************
+* *
+ TmState
+ What we know about terms
+* *
+********************************************************************* }
+
+  The term oracle state. Stores 'VarInfo' for encountered 'Id's. These
+ entries are possibly shared when we figure out that two variables must be
+ equal, thus represent the same set of values.
+
+ See Note [TmState invariants].
+newtype TmState = TS (SharedDIdEnv VarInfo)
+  Deterministic so that we generate deterministic error messages
+
+  Information about an 'Id'. Stores positive ('vi_pos') facts, like @x ~ Just 42@,
+ and negative ('vi_neg') facts, like "x is not (:)".
+ Also caches the type ('vi_ty'), the 'PossibleMatches' of a COMPLETE set
+ ('vi_cache') and the number of times each variable was refined
+ ('vi_n_refines').
+
+ Subject to Note [The Pos/Neg invariant].
+data VarInfo
+ = VI
+ { vi_ty :: !Type
+  ^ The type of the variable. Important for rejecting possible GADT
+  constructors or incompatible pattern synonyms (@Just42 :: Maybe Int@).
+
+ , vi_pos :: [(PmAltCon, [Id])]
+  ^ Positive info: 'PmExprCon's it is (i.e. @x ~ [Just y, PatSyn z]@), all
+  at the same time (i.e. conjunctive). We need a list because of nested
+  pattern matches involving pattern synonym
+  case x of { Just y > case x of PatSyn z > ... }
+  However, no more than one RealDataCon in the list, otherwise contradiction
+  because of generativity.
+
+ , vi_neg :: ![PmAltCon]
+  ^ Negative info: A list of 'PmAltCon's that it cannot match.
+  Example, assuming
+ 
+  @
+  data T = Leaf Int  Branch T T  Node Int T
+  @
+ 
+  then @x /~ [Leaf, Node]@ means that @x@ cannot match a @Leaf@ or @Node@,
+  and hence can only match @Branch@. Is orthogonal to anything from 'vi_pos',
+  in the sense that 'eqPmAltCon' returns @PossiblyOverlap@ for any pairing
+  between 'vi_pos' and 'vi_neg'.
+
+  See Note [Why record both positive and negative info?]
+
+ , vi_cache :: !PossibleMatches
+  ^ A cache of the associated COMPLETE sets. At any time a superset of
+  possible constructors of each COMPLETE set. So, if it's not in here, we
+  can't possibly match on it. Complementary to 'vi_neg'. We still need it
+  to recognise completion of a COMPLETE set efficiently for large enums.
+
+ , vi_n_refines :: !Int
+  ^ Purely for Check performance reasons. The number of times this
+  representative was refined ('refineToAltCon') in the Check's ConVar split.
+  Sadly, we can't store this info in the Check module, as it's tightly coupled
+  to the particular 'Delta' and also is per *representative*, not per
+  syntactic variable. Note that this number does not always correspond to the
+  length of solutions: 'addVarConCt' might add a solution without
+  incurring the potential exponential blowup by ConVar.
+ }
+
+{ Note [The Pos/Neg invariant]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Invariant applying to each VarInfo: Whenever we have @(C, [y,z])@ in 'vi_pos',
+any entry in 'vi_neg' must be incomparable to C (return Nothing) according to
+'eqPmAltCons'. Those entries that are comparable either lead to a refutation
+or are redudant. Examples:
+* @x ~ Just y@, @x /~ [Just]@. 'eqPmAltCon' returns @Equal@, so refute.
+* @x ~ Nothing@, @x /~ [Just]@. 'eqPmAltCon' returns @Disjoint@, so negative
+ info is redundant and should be discarded.
+* @x ~ I# y@, @x /~ [4,2]@. 'eqPmAltCon' returns @PossiblyOverlap@, so orthogal.
+ We keep this info in order to be able to refute a redundant match on i.e. 4
+ later on.
+
+This carries over to pattern synonyms and overloaded literals. Say, we have
+ pattern Just42 = Just 42
+ case Just42 of x
+ Nothing > ()
+ Just _ > ()
+Even though we had a solution for the value abstraction called x here in form
+of a PatSynCon (Just42,[]), this solution is incomparable to both Nothing and
+Just. Hence we retain the info in vi_neg, which eventually allows us to detect
+the complete pattern match.
+
+The Pos/Neg invariant extends to vi_cache, which stores essentially positive
+information. We make sure that vi_neg and vi_cache never overlap. This isn't
+strictly necessary since vi_cache is just a cache, so doesn't need to be
+accurate: Every suggestion of a possible ConLike from vi_cache might be
+refutable by the type oracle anyway. But it helps to maintain sanity while
+debugging traces.
+
+Note [Why record both positive and negative info?]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You might think that knowing positive info (like x ~ Just y) would render
+negative info irrelevant, but not so because of pattern synonyms. E.g we might
+know that x cannot match (Foo 4), where pattern Foo p = Just p
+
+Also overloaded literals themselves behave like pattern synonyms. E.g if
+postively we know that (x ~ I# y), we might also negatively want to record that
+x does not match 45 f 45 = e2 f (I# 22#) = e3 f 45 = e4 
+Overlapped
+
+Note [TmState invariants]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+The term oracle state is never obviously (i.e., without consulting the type
+oracle) contradictory. This implies a few invariants:
+* Whenever vi_pos overlaps with vi_neg according to 'eqPmAltCon', we refute.
+ This is implied by the Note [Pos/Neg invariant].
+* Whenever vi_neg subsumes a COMPLETE set, we refute. We consult vi_cache to
+ detect this, but we could just compare whole COMPLETE sets to vi_neg every
+ time, if it weren't for performance.
+
+Maintaining these invariants in 'addVarVarCt' (the core of the term oracle) and
+'addRefutableAltCon' is subtle.
+* Merging VarInfos. Example: Add the fact @x ~ y@ (see 'equate').
+  (COMPLETE) If we had @x /~ True@ and @y /~ False@, then we get
+ @x /~ [True,False]@. This is vacuous by matter of comparing to the vanilla
+ COMPLETE set, so should refute.
+  (Pos/Neg) If we had @x /~ True@ and @y ~ True@, we have to refute.
+* Adding positive information. Example: Add the fact @x ~ K ys@ (see 'addVarConCt')
+  (Neg) If we had @x /~ K@, refute.
+  (Pos) If we had @x ~ K2@, and that contradicts the new solution according to
+ 'eqPmAltCon' (ex. K2 is [] and K is (:)), then refute.
+  (Refine) If we had @x /~ K zs@, unify each y with each z in turn.
+* Adding negative information. Example: Add the fact @x /~ Nothing@ (see 'addRefutableAltCon')
+  (Refut) If we have @x ~ K ys@, refute.
+  (Redundant) If we have @x ~ K2@ and @eqPmAltCon K K2 == Disjoint@
+ (ex. Just and Nothing), the info is redundant and can be
+ discarded.
+  (COMPLETE) If K=Nothing and we had @x /~ Just@, then we get
+ @x /~ [Just,Nothing]@. This is vacuous by matter of comparing to the vanilla
+ COMPLETE set, so should refute.
+
+Note that merging VarInfo in equate can be done by calling out to 'addVarConCt' and
+'addRefutableAltCon' for each of the facts individually.
+
+Note [Representation of Strings in TmState]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Instead of treating regular String literals as a PmLits, we treat it as a list
+of characters in the oracle for better overlap reasoning. The following example
+shows why:
+
+ f :: String > ()
+ f ('f':_) = ()
+ f "foo" = ()
+ f _ = ()
+
+The second case is redundant, and we like to warn about it. Therefore either
+the oracle will have to do some smart conversion between the list and literal
+representation or treat is as the list it really is at runtime.
+
+The "smart conversion" has the advantage of leveraging the more compact literal
+representation wherever possible, but is really nasty to get right with negative
+equalities: Just think of how to encode @x /= "foo"@.
+The "list" option is far simpler, but incurs some overhead in representation and
+warning messages (which can be alleviated by someone with enough dedication).
+}
+
+  Not userfacing.
+instance Outputable TmState where
+ ppr (TS state) = ppr state
+
+  Not userfacing.
+instance Outputable VarInfo where
+ ppr (VI ty pos neg cache n)
+ = braces (hcat (punctuate comma [ppr ty, ppr pos, ppr neg, ppr cache, ppr n]))
+
+  Initial state of the oracle.
+initTmState :: TmState
+initTmState = TS emptySDIE
+
+  A 'SatisfiabilityCheck' based on new termlevel constraints.
+ Returns a new 'Delta' if the new constraints are compatible with existing
+ ones.
+tmIsSatisfiable :: Bag TmCt > SatisfiabilityCheck
+tmIsSatisfiable new_tm_cs = SC $ \delta > tmOracle delta new_tm_cs
+
+  External interface to the term oracle.
+tmOracle :: Foldable f => Delta > f TmCt > DsM (Maybe Delta)
+tmOracle delta = runMaybeT . foldlM go delta
+ where
+ go delta ct = MaybeT (addTmCt delta ct)
+
+
+ * Looking up VarInfo
+
+emptyVarInfo :: Id > VarInfo
+emptyVarInfo x = VI (idType x) [] [] NoPM 0
+
+lookupVarInfo :: TmState > Id > VarInfo
+ (lookupVarInfo tms x) tells what we know about 'x'
+lookupVarInfo (TS env) x = fromMaybe (emptyVarInfo x) (lookupSDIE env x)
+
+initPossibleMatches :: Bag EvVar > VarInfo > DsM VarInfo
+initPossibleMatches ty_cs vi@VI{ vi_ty = ty, vi_cache = NoPM } = do
+  New evidence might lead to refined info on ty, in turn leading to discovery
+  of a COMPLETE set.
+ res < pmTopNormaliseType ty_cs ty
+ let ty' = normalisedSourceType res
+ mb_pm < initIM ty'
+  tracePm "initPossibleMatches" (ppr vi $$ ppr ty' $$ ppr res $$ ppr mb_pm)
+ case mb_pm of
+ Nothing > pure vi
+ Just pm > pure vi{ vi_ty = ty', vi_cache = pm }
+initPossibleMatches _ vi = pure vi
+
+  @initLookupVarInfo ts x@ looks up the 'VarInfo' for @x@ in @ts@ and tries
+ to initialise the 'vi_cache' component if it was 'NoPM' through
+ 'initPossibleMatches'.
+initLookupVarInfo :: Delta > Id > DsM VarInfo
+initLookupVarInfo MkDelta{ delta_tm_cs = ts, delta_ty_cs = ty_cs } x
+ = initPossibleMatches ty_cs (lookupVarInfo ts x)
+
+
+ * Exported utility functions querying 'Delta'
+
+  Check whether a constraint (x ~ BOT) can succeed,
+ given the resulting state of the term oracle.
+canDiverge :: Delta > Id > Bool
+canDiverge MkDelta{ delta_tm_cs = ts } x
+  If the variable seems not evaluated, there is a possibility for
+  constraint x ~ BOT to be satisfiable. That's the case when we haven't found
+  a solution (i.e. some equivalent literal or constructor) for it yet.
+  Even if we don't have a solution yet, it might be involved in a negative
+  constraint, in which case we must already have evaluated it earlier.
+  VI _ [] [] _ _ < lookupVarInfo ts x
+ = True
+  Variable x is already in WHNF or we know some refutable shape, so the
+  constraint is nonsatisfiable
+  otherwise = False
+
+lookupRefuts :: Uniquable k => Delta > k > [PmAltCon]
+ Unfortunately we need the extra bit of polymorphism and the unfortunate
+ duplication of lookupVarInfo here.
+lookupRefuts MkDelta{ delta_tm_cs = ts@(TS (SDIE env)) } k =
+ case lookupUDFM env k of
+ Nothing > []
+ Just (Indirect y) > vi_neg (lookupVarInfo ts y)
+ Just (Entry vi) > vi_neg vi
+
+isDataConSolution :: (PmAltCon, [Id]) > Bool
+isDataConSolution (PmAltConLike (RealDataCon _), _) = True
+isDataConSolution _ = False
+
+ @lookupSolution delta x@ picks a single solution ('vi_pos') of @x@ from
+ possibly many, preferring 'RealDataCon' solutions whenever possible.
+lookupSolution :: Delta > Id > Maybe (PmAltCon, [Id])
+lookupSolution delta x = case vi_pos (lookupVarInfo (delta_tm_cs delta) x) of
+ [] > Nothing
+ pos
+  Just sol < find isDataConSolution pos > Just sol
+  otherwise > Just (head pos)
+
+  @lookupNumberOfRefinements delta x@ Looks up how many times we have refined
+ ('refineToAltCon') @x@ for some 'PmAltCon' to arrive at @delta@. This number
+ is always less or equal to @length (lookupSolution delta x)@!
+lookupNumberOfRefinements :: Delta > Id > Int
+lookupNumberOfRefinements delta x
+ = vi_n_refines (lookupVarInfo (delta_tm_cs delta) x)
+
+
+ * Adding facts to the oracle
+
+  A term constraint. Either equates two variables or a variable with a
+ 'PmAltCon' application.
+data TmCt
+ = TmVarVar !Id !Id
+  TmVarCon !Id !PmAltCon ![Id]
+
+instance Outputable TmCt where
+ ppr (TmVarVar x y) = ppr x <+> char '~' <+> ppr y
+ ppr (TmVarCon x con args) = ppr x <+> char '~' <+> hsep (ppr con : map ppr args)
+
+  Add type equalities to 'Delta'.
+addTypeEvidence :: Delta > Bag EvVar > DsM (Maybe Delta)
+addTypeEvidence delta dicts
+ = runSatisfiabilityCheck delta (tyIsSatisfiable True dicts)
+
+  Tries to equate two representatives in 'Delta'.
+ See Note [TmState invariants].
+addTmCt :: Delta > TmCt > DsM (Maybe Delta)
+addTmCt delta ct = runMaybeT $ case ct of
+ TmVarVar x y > addVarVarCt delta (x, y)
+ TmVarCon x con args > addVarConCt delta x con args
+
+  Record that a particular 'Id' can't take the shape of a 'PmAltCon' in the
+ 'Delta' and return @Nothing@ if that leads to a contradiction.
+ See Note [TmState invariants].
+addRefutableAltCon :: Delta > Id > PmAltCon > DsM (Maybe Delta)
+addRefutableAltCon delta@MkDelta{ delta_tm_cs = TS env } x nalt = runMaybeT $ do
+ vi@(VI _ pos neg pm _) < lift (initLookupVarInfo delta x)
+  1. Bail out quickly when nalt contradicts a solution
+ let contradicts nalt (cl, _args) = eqPmAltCon cl nalt == Equal
+ guard (not (any (contradicts nalt) pos))
+  2. Only record the new fact when it's not already implied by one of the
+  solutions
+ let implies nalt (cl, _args) = eqPmAltCon cl nalt == Disjoint
+ let neg'
+  any (implies nalt) pos = neg
+  See Note [Completeness checking with required Thetas]
+  hasRequiredTheta nalt = neg
+  otherwise = unionLists neg [nalt]
+ let vi_ext = vi{ vi_neg = neg' }
+  3. Make sure there's at least one other possible constructor
+ vi' < case nalt of
+ PmAltConLike cl
+ > MaybeT (ensureInhabited delta vi_ext{ vi_cache = markMatched cl pm })
+ _ > pure vi_ext
+ pure delta{ delta_tm_cs = TS (setEntrySDIE env x vi') }
+
+hasRequiredTheta :: PmAltCon > Bool
+hasRequiredTheta (PmAltConLike cl) = notNull req_theta
+ where
+ (_,_,_,_,req_theta,_,_) = conLikeFullSig cl
+hasRequiredTheta _ = False
+
+{ Note [Completeness checking with required Thetas]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider the situation in #11224
+
+ import Text.Read (readMaybe)
+ pattern PRead :: Read a => () => a > String
+ pattern PRead x < (readMaybe > Just x)
+ f :: String > Int
+ f (PRead x) = x
+ f (PRead xs) = length xs
+ f _ = 0
+
+Is the first match exhaustive on the PRead synonym? Should the second line thus
+deemed redundant? The answer is, of course, No! The required theta is like a
+hidden parameter which must be supplied at the pattern match site, so PRead
+is much more like a view pattern (where behavior depends on the particular value
+passed in).
+The simple solution here is to forget in 'addRefutableAltCon' that we matched
+on synonyms with a required Theta like @PRead@, so that subsequent matches on
+the same constructor are never flagged as redundant. The consequence is that
+we no longer detect the actually redundant match in
+
+ g :: String > Int
+ g (PRead x) = x
+ g (PRead y) = y  redundant!
+ g _ = 0
+
+But that's a small price to pay, compared to the proper solution here involving
+storing required arguments along with the PmAltConLike in 'vi_neg'.
+}
+
+  Guess the universal argument types of a ConLike from an instantiation of
+ its result type. Rather easy for DataCons, but not so much for PatSynCons.
+ See Note [Pattern synonym result type] in PatSyn.hs.
+guessConLikeUnivTyArgsFromResTy :: FamInstEnvs > Type > ConLike > Maybe [Type]
+guessConLikeUnivTyArgsFromResTy env res_ty (RealDataCon _) = do
+ (tc, tc_args) < splitTyConApp_maybe res_ty
+  Consider data families: In case of a DataCon, we need to translate to
+  the representation TyCon. For PatSyns, they are relative to the data
+  family TyCon, so we don't need to translate them.
+ let (_, tc_args', _) = tcLookupDataFamInst env tc tc_args
+ Just tc_args'
+guessConLikeUnivTyArgsFromResTy _ res_ty (PatSynCon ps) = do
+  We were successful if we managed to instantiate *every* univ_tv of con.
+  This is difficult and bound to fail in some cases, see
+  Note [Pattern synonym result type] in PatSyn.hs. So we just try our best
+  here and be sure to return an instantiation when we can substitute every
+  universally quantified type variable.
+  We *could* instantiate all the other univ_tvs just to fresh variables, I
+  suppose, but that means we get weird field types for which we don't know
+  anything. So we prefer to keep it simple here.
+ let (univ_tvs,_,_,_,_,con_res_ty) = patSynSig ps
+ subst < tcMatchTy con_res_ty res_ty
+ traverse (lookupTyVar subst) univ_tvs
+
+ensureInhabited :: Delta > VarInfo > DsM (Maybe VarInfo)
+  Returns (Just vi) guarantees that at least one member
+  of each ConLike in the COMPLETE set satisfies the oracle
+ 
+  Internally uses and updates the ConLikeSets in vi_cache.
+ 
+  NB: Does /not/ filter each ConLikeSet with the oracle; members may
+  remain that do not statisfy it. This lazy approach just
+  avoids doing unnecessary work.
+ensureInhabited delta vi = fmap (set_cache vi) <$> test (vi_cache vi)  This would be much less tedious with lenses
+ where
+ set_cache vi cache = vi { vi_cache = cache }
+
+ test NoPM = pure (Just NoPM)
+ test (PM tc ms) = runMaybeT (PM tc <$> traverse one_set ms)
+
+ one_set cs = find_one_inh cs (uniqDSetToList cs)
+
+ find_one_inh :: ConLikeSet > [ConLike] > MaybeT DsM ConLikeSet
+  (find_one_inh cs cls) iterates over cls, deleting from cs
+  any uninhabited elements of cls. Stop (returning Just cs)
+  when you see an inhabited element; return Nothing if all
+  are uninhabited
+ find_one_inh _ [] = mzero
+ find_one_inh cs (con:cons) = lift (inh_test con) >>= \case
+ True > pure cs
+ False > find_one_inh (delOneFromUniqDSet cs con) cons
+
+ inh_test :: ConLike > DsM Bool
+  @inh_test K@ Returns False if a nonbottom value @v::ty@ cannot possibly
+  be of form @K _ _ _@. Returning True is always sound.
+ 
+  It's like 'DataCon.dataConCannotMatch', but more clever because it takes
+  the facts in Delta into account.
+ inh_test con = do
+ env < dsGetFamInstEnvs
+ case guessConLikeUnivTyArgsFromResTy env (vi_ty vi) con of
+ Nothing > pure True  be conservative about this
+ Just arg_tys > do
+ (_vars, ev_vars, strict_arg_tys, _ex_tyvars) < mkOneConFull arg_tys con
+  No need to run the term oracle compared to pmIsSatisfiable
+ fmap isJust <$> runSatisfiabilityCheck delta $ mconcat
+  Important to pass False to tyIsSatisfiable here, so that we won't
+  recursively call ensureAllPossibleMatchesInhabited, leading to an
+  endless recursion.
+ [ tyIsSatisfiable False (listToBag ev_vars)
+ , tysAreNonVoid initRecTc strict_arg_tys
+ ]
+
+  Checks if every 'VarInfo' in the term oracle has still an inhabited
+ 'vi_cache', considering the current type information in 'Delta'.
+ This check is necessary after having matched on a GADT con to weed out
+ impossible matches.
+ensureAllPossibleMatchesInhabited :: Delta > DsM (Maybe Delta)
+ensureAllPossibleMatchesInhabited delta@MkDelta{ delta_tm_cs = TS env }
+ = runMaybeT (set_tm_cs_env delta <$> traverseSharedIdEnv go env)
+ where
+ set_tm_cs_env delta env = delta{ delta_tm_cs = TS env }
+ go vi = MaybeT (ensureInhabited delta vi)
+
+  @refineToAltCon delta x con arg_tys ex_tyvars@ instantiates @con@ at
+ @arg_tys@ with fresh variables (equating existentials to @ex_tyvars@).
+ It adds a new term equality equating @x@ is to the resulting 'PmExprCon' and
+ new type equalities arising from GADT matches.
+ If successful, returns the new @delta@ and the fresh term variables, or
+ @Nothing@ otherwise.
+refineToAltCon :: Delta > Id > PmAltCon > [Type] > [TyVar] > DsM (Maybe (Delta, [Id]))
+refineToAltCon delta x l@PmAltLit{} _arg_tys _ex_tvs1 = runMaybeT $ do
+ delta' < addVarConCt delta x l []
+ pure (markRefined delta' x, [])
+refineToAltCon delta x alt@(PmAltConLike con) arg_tys ex_tvs1 = do
+  The plan for ConLikes:
+  Suppose K :: forall a b y z. (y,b) > z > T a b
+  where the y,z are the existentials
+  @refineToAltCon delta x K [ex1, ex2]@ extends delta with the
+  positive information x :> K y' z' p q, for some fresh y', z', p, q.
+  This is done by mkOneConFull.
+  We return the fresh [p,q] args, and bind the existentials [y',z'] to
+  [ex1, ex2].
+  Return Nothing if such a match is contradictory with delta.
+
+ (arg_vars, theta_ev_vars, strict_arg_tys, ex_tvs2) < mkOneConFull arg_tys con
+
+  If we have identical constructors but different existential
+  tyvars, then generate extra equality constraints to ensure the
+  existential tyvars.
+  See Note [Coverage checking and existential tyvars].
+ ex_ev_vars < equateTyVars ex_tvs1 ex_tvs2
+
+ let new_ty_cs = listToBag theta_ev_vars `unionBags` listToBag ex_ev_vars
+ let new_tm_cs = unitBag (TmVarCon x alt arg_vars)
+
+  Now check satifiability
+ mb_delta < pmIsSatisfiable delta new_tm_cs new_ty_cs strict_arg_tys
+ tracePm "refineToAltCon" (vcat [ ppr x
+ , ppr new_tm_cs
+ , ppr new_ty_cs
+ , ppr strict_arg_tys
+ , ppr delta
+ , ppr mb_delta ])
+ case mb_delta of
+ Nothing > pure Nothing
+ Just delta' > pure (Just (markRefined delta' x, arg_vars))
+
+  This is the only place that actualy increments 'vi_n_refines'.
+markRefined :: Delta > Id > Delta
+markRefined delta@MkDelta{ delta_tm_cs = ts@(TS env) } x
+ = delta{ delta_tm_cs = TS env' }
+ where
+ vi = lookupVarInfo ts x
+ env' = setEntrySDIE env x vi{ vi_n_refines = vi_n_refines vi + 1 }
+
+{
+Note [Coverage checking and existential tyvars]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+GHC's implementation of the patternmatch coverage algorithm (as described in
+the GADTs Meet Their Match paper) must take some care to emit enough type
+constraints when handling data constructors with exisentially quantified type
+variables. To better explain what the challenge is, consider a constructor K
+of the form:
+
+ K @e_1 ... @e_m ev_1 ... ev_v ty_1 ... ty_n :: T u_1 ... u_p
+
+Where:
+
+* e_1, ..., e_m are the existentially bound type variables.
+* ev_1, ..., ev_v are evidence variables, which may inhabit a dictionary type
+ (e.g., Eq) or an equality constraint (e.g., e_1 ~ Int).
+* ty_1, ..., ty_n are the types of K's fields.
+* T u_1 ... u_p is the return type, where T is the data type constructor, and
+ u_1, ..., u_p are the universally quantified type variables.
+
+In the ConVar case, the coverage algorithm will have in hand the constructor
+K as well as a list of type arguments [t_1, ..., t_n] to substitute T's
+universally quantified type variables u_1, ..., u_n for. It's crucial to take
+these in as arguments, as it is nontrivial to derive them just from the result
+type of a pattern synonym and the ambient type of the match (#11336, #17112).
+The type checker already did the hard work, so we should just make use of it.
+
+The presence of existentially quantified type variables adds a significant
+wrinkle. We always grab e_1, ..., e_m from the definition of K to begin with,
+but we don't want them to appear in the final PmCon, because then
+calling (mkOneConFull K) for other pattern variables might reuse the same
+existential tyvars, which is certainly wrong.
+
+Previously, GHC's solution to this wrinkle was to always create fresh names
+for the existential tyvars and put them into the PmCon. This works well for
+many cases, but it can break down if you nest GADT pattern matches in just
+the right way. For instance, consider the following program:
+
+ data App f a where
+ App :: f a > App f (Maybe a)
+
+ data Ty a where
+ TBool :: Ty Bool
+ TInt :: Ty Int
+
+ data T f a where
+ C :: T Ty (Maybe Bool)
+
+ foo :: T f a > App f a > ()
+ foo C (App TBool) = ()
+
+foo is a total program, but with the previous approach to handling existential
+tyvars, GHC would mark foo's patterns as nonexhaustive.
+
+When foo is desugared to Core, it looks roughly like so:
+
+ foo @f @a (C co1 _co2) (App @a1 _co3 (TBool > co1)) = ()
+
+(Where `a1` is an existential tyvar.)
+
+That, in turn, is processed by the coverage checker to become:
+
+ foo @f @a (C co1 _co2) (App @a1 _co3 (pmvar123 :: f a1))
+  TBool < pmvar123 > co1
+ = ()
+
+Note that the type of pmvar123 is `f a1`—this will be important later.
+
+Now, we proceed with coveragechecking as usual. When we come to the
+ConVar case for App, we create a fresh variable `a2` to represent its
+existential tyvar. At this point, we have the equality constraints
+`(a ~ Maybe a2, a ~ Maybe Bool, f ~ Ty)` in scope.
+
+However, when we check the guard, it will use the type of pmvar123, which is
+`f a1`. Thus, when considering if pmvar123 can match the constructor TInt,
+it will generate the constraint `a1 ~ Int`. This means our final set of
+equality constraints would be:
+
+ f ~ Ty
+ a ~ Maybe Bool
+ a ~ Maybe a2
+ a1 ~ Int
+
+Which is satisfiable! Freshening the existential tyvar `a` to `a2` doomed us,
+because GHC is unable to relate `a2` to `a1`, which really should be the same
+tyvar.
+
+Luckily, we can avoid this pitfall. Recall that the ConVar case was where we
+generated a PmCon with toofresh existentials. But after ConVar, we have the
+ConCon case, which considers whether each constructor of a particular data type
+can be matched on in a particular spot.
+
+In the case of App, when we get to the ConCon case, we will compare our
+original App PmCon (from the source program) to the App PmCon created from the
+ConVar case. In the former PmCon, we have `a1` in hand, which is exactly the
+existential tyvar we want! Thus, we can force `a1` to be the same as `a2` here
+by emitting an additional `a1 ~ a2` constraint. Now our final set of equality
+constraints will be:
+
+ f ~ Ty
+ a ~ Maybe Bool
+ a ~ Maybe a2
+ a1 ~ Int
+ a1 ~ a2
+
+Which is unsatisfiable, as we desired, since we now have that
+Int ~ a1 ~ a2 ~ Bool.
+
+In general, App might have more than one constructor, in which case we
+couldn't reuse the existential tyvar for App for a different constructor. This
+means that we can only use this trick in ConCon when the constructors are the
+same. But this is fine, since this is the only scenario where this situation
+arises in the first place!
+}
+
+
+ * Term oracle unification procedure
+
+  Try to unify two 'Id's and record the gained knowledge in 'Delta'.
+
+ Returns @Nothing@ when there's a contradiction. Returns @Just delta@
+ when the constraint was compatible with prior facts, in which case @delta@
+ has integrated the knowledge from the equality constraint.
+
+ See Note [TmState invariants].
+addVarVarCt :: Delta > (Id, Id) > MaybeT DsM Delta
+addVarVarCt delta@MkDelta{ delta_tm_cs = TS env } (x, y)
+  It's important that we never @equate@ two variables of the same equivalence
+  class, otherwise we might get cyclic substitutions.
+  Cf. 'extendSubstAndSolve' and
+  @testsuite/tests/pmcheck/should_compile/CyclicSubst.hs@.
+  sameRepresentative env x y = pure delta
+  otherwise = equate delta x y
+
+  @equate ts@(TS env) x y@ merges the equivalence classes of @x@ and @y@ by
+ adding an indirection to the environment.
+ Makes sure that the positive and negative facts of @x@ and @y@ are
+ compatible.
+ Preconditions: @not (sameRepresentative env x y)@
+
+ See Note [TmState invariants].
+equate :: Delta > Id > Id > MaybeT DsM Delta
+equate delta@MkDelta{ delta_tm_cs = TS env } x y
+ = ASSERT( not (sameRepresentative env x y) )
+ case (lookupSDIE env x, lookupSDIE env y) of
+ (Nothing, _) > pure (delta{ delta_tm_cs = TS (setIndirectSDIE env x y) })
+ (_, Nothing) > pure (delta{ delta_tm_cs = TS (setIndirectSDIE env y x) })
+  Merge the info we have for x into the info for y
+ (Just vi_x, Just vi_y) > do
+  This assert will probably trigger at some point...
+  We should decide how to break the tie
+ MASSERT2( vi_ty vi_x `eqType` vi_ty vi_y, text "Not same type" )
+  First assume that x and y are in the same equivalence class
+ let env_ind = setIndirectSDIE env x y
+  Then sum up the refinement counters
+ let vi_y' = vi_y{ vi_n_refines = vi_n_refines vi_x + vi_n_refines vi_y }
+ let env_refs = setEntrySDIE env_ind y vi_y'
+ let delta_refs = delta{ delta_tm_cs = TS env_refs }
+  and then gradually merge every positive fact we have on x into y
+ let add_fact delta (cl, args) = addVarConCt delta y cl args
+ delta_pos < foldlM add_fact delta_refs (vi_pos vi_x)
+  Do the same for negative info
+ let add_refut delta nalt = MaybeT (addRefutableAltCon delta y nalt)
+ delta_neg < foldlM add_refut delta_pos (vi_neg vi_x)
+  vi_cache will be updated in addRefutableAltCon, so we are good to
+  go!
+ pure delta_neg
+
+  @addVarConCt x alt args ts@ extends the substitution with a mapping @x: >
+ PmExprCon alt args@ if compatible with refutable shapes of @x@ and its
+ solution, reject (@Nothing@) otherwise.
+
+ See Note [TmState invariants].
+addVarConCt :: Delta > Id > PmAltCon > [Id] > MaybeT DsM Delta
+addVarConCt delta@MkDelta{ delta_tm_cs = TS env } x alt args = do
+ VI ty pos neg cache n < lift (initLookupVarInfo delta x)
+  First try to refute with a negative fact
+ guard (all ((/= Equal) . eqPmAltCon alt) neg)
+  Then see if any of the other solutions (remember: each of them is an
+  additional refinement of the possible values x could take) indicate a
+  contradiction
+ guard (all ((/= Disjoint) . eqPmAltCon alt . fst) pos)
+  Now we should be good! Add (alt, args) as a possible solution, or refine an
+  existing one
+ case find ((== Equal) . eqPmAltCon alt . fst) pos of
+ Just (_, other_args) > do
+ foldlM addVarVarCt delta (zip args other_args)
+ Nothing > do
+  Filter out redundant negative facts (those that compare Just False to
+  the new solution)
+ let neg' = filter ((== PossiblyOverlap) . eqPmAltCon alt) neg
+ let pos' = (alt,args):pos
+ pure delta{ delta_tm_cs = TS (setEntrySDIE env x (VI ty pos' neg' cache n))}
+
+
+ * Enumerating inhabitation candidates
+
+  Information about a conlike that is relevant to coverage checking.
+ It is called an \"inhabitation candidate\" since it is a value which may
+ possibly inhabit some type, but only if its term constraints ('ic_tm_cs')
+ and type constraints ('ic_ty_cs') are permitting, and if all of its strict
+ argument types ('ic_strict_arg_tys') are inhabitable.
+ See @Note [Strict argument type constraints]@.
+data InhabitationCandidate =
+ InhabitationCandidate
+ { ic_tm_cs :: Bag TmCt
+ , ic_ty_cs :: Bag EvVar
+ , ic_strict_arg_tys :: [Type]
+ }
+
+instance Outputable InhabitationCandidate where
+ ppr (InhabitationCandidate tm_cs ty_cs strict_arg_tys) =
+ text "InhabitationCandidate" <+>
+ vcat [ text "ic_tm_cs =" <+> ppr tm_cs
+ , text "ic_ty_cs =" <+> ppr ty_cs
+ , text "ic_strict_arg_tys =" <+> ppr strict_arg_tys ]
+
+mkInhabitationCandidate :: Id > DataCon > DsM InhabitationCandidate
+ Precondition: idType x is a TyConApp, so that tyConAppArgs in here is safe.
+mkInhabitationCandidate x dc = do
+ let cl = RealDataCon dc
+ let tc_args = tyConAppArgs (idType x)
+ (arg_vars, ev_vars, strict_arg_tys, _ex_tyvars) < mkOneConFull tc_args cl
+ pure InhabitationCandidate
+ { ic_tm_cs = unitBag (TmVarCon x (PmAltConLike cl) arg_vars)
+ , ic_ty_cs = listToBag ev_vars
+ , ic_strict_arg_tys = strict_arg_tys
+ }
+
+  Generate all 'InhabitationCandidate's for a given type. The result is
+ either @'Left' ty@, if the type cannot be reduced to a closed algebraic type
+ (or if it's one trivially inhabited, like 'Int'), or @'Right' candidates@,
+ if it can. In this case, the candidates are the signature of the tycon, each
+ one accompanied by the term and type constraints it gives rise to.
+ See also Note [Checking EmptyCase Expressions]
+inhabitationCandidates :: Delta > Type
+ > DsM (Either Type (TyCon, Id, [InhabitationCandidate]))
+inhabitationCandidates MkDelta{ delta_ty_cs = ty_cs } ty = do
+ pmTopNormaliseType ty_cs ty >>= \case
+ NoChange _ > alts_to_check ty ty []
+ NormalisedByConstraints ty' > alts_to_check ty' ty' []
+ HadRedexes src_ty dcs core_ty > alts_to_check src_ty core_ty dcs
+ where
+  All these types are trivially inhabited
+ trivially_inhabited = [ charTyCon, doubleTyCon, floatTyCon
+ , intTyCon, wordTyCon, word8TyCon ]
+
+ build_newtype :: (Type, DataCon) > Id > DsM (Id, TmCt)
+ build_newtype (ty, dc) x = do
+  ty is the type of @dc x@. It's a @dataConTyCon dc@ application.
+ y < mkPmId ty
+ pure (y, TmVarCon y (PmAltConLike (RealDataCon dc)) [x])
+
+ build_newtypes :: Id > [(Type, DataCon)] > DsM (Id, [TmCt])
+ build_newtypes x = foldrM (\dc (x, cts) > go dc x cts) (x, [])
+ where
+ go dc x cts = second (:cts) <$> build_newtype dc x
+
+  Inhabitation candidates, using the result of pmTopNormaliseType
+ alts_to_check :: Type > Type > [(Type, DataCon)]
+ > DsM (Either Type (TyCon, Id, [InhabitationCandidate]))
+ alts_to_check src_ty core_ty dcs = case splitTyConApp_maybe core_ty of
+ Just (tc, _)
+  tc `elem` trivially_inhabited
+ > case dcs of
+ [] > return (Left src_ty)
+ (_:_) > do inner < mkPmId core_ty
+ (outer, new_tm_cts) < build_newtypes inner dcs
+ return $ Right (tc, outer, [InhabitationCandidate
+ { ic_tm_cs = listToBag new_tm_cts
+ , ic_ty_cs = emptyBag, ic_strict_arg_tys = [] }])
+
+  pmIsClosedType core_ty && not (isAbstractTyCon tc)
+  Don't consider abstract tycons since we don't know what their
+  constructors are, which makes the results of coverage checking
+  them extremely misleading.
+ > do
+ inner < mkPmId core_ty  it would be wrong to unify inner
+ alts < mapM (mkInhabitationCandidate inner) (tyConDataCons tc)
+ (outer, new_tm_cts) < build_newtypes inner dcs
+ let wrap_dcs alt = alt{ ic_tm_cs = listToBag new_tm_cts `unionBags` ic_tm_cs alt}
+ return $ Right (tc, outer, map wrap_dcs alts)
+  For other types conservatively assume that they are inhabited.
+ _other > return (Left src_ty)
+
+inhabitants :: Delta > Type > DsM (Either Type (Id, [Delta]))
+inhabitants delta ty = inhabitationCandidates delta ty >>= \case
+ Left ty' > pure (Left ty')
+ Right (_, va, candidates) > do
+ deltas < flip mapMaybeM candidates $
+ \InhabitationCandidate{ ic_tm_cs = tm_cs
+ , ic_ty_cs = ty_cs
+ , ic_strict_arg_tys = strict_arg_tys } > do
+ pmIsSatisfiable delta tm_cs ty_cs strict_arg_tys
+ pure (Right (va, deltas))
+
+
+ * Detecting vacuous types
+
+{ Note [Checking EmptyCase Expressions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Empty case expressions are strict on the scrutinee. That is, `case x of {}`
+will force argument `x`. Hence, `checkMatches` is not sufficient for checking
+empty cases, because it assumes that the match is not strict (which is true
+for all other cases, apart from EmptyCase). This gave rise to #10746. Instead,
+we do the following:
+
+1. We normalise the outermost type family redex, data family redex or newtype,
+ using pmTopNormaliseType (in types/FamInstEnv.hs). This computes 3
+ things:
+ (a) A normalised type src_ty, which is equal to the type of the scrutinee in
+ source Haskell (does not normalise newtypes or data families)
+ (b) The actual normalised type core_ty, which coincides with the result
+ topNormaliseType_maybe. This type is not necessarily equal to the input
+ type in source Haskell. And this is precicely the reason we compute (a)
+ and (c): the reasoning happens with the underlying types, but both the
+ patterns and types we print should respect newtypes and also show the
+ family type constructors and not the representation constructors.
+
+ (c) A list of all newtype data constructors dcs, each one corresponding to a
+ newtype rewrite performed in (b).
+
+ For an example see also Note [Type normalisation for EmptyCase]
+ in types/FamInstEnv.hs.
+
+2. Function Check.checkEmptyCase' performs the check:
+  If core_ty is not an algebraic type, then we cannot check for
+ inhabitation, so we emit (_ :: src_ty) as missing, conservatively assuming
+ that the type is inhabited.
+  If core_ty is an algebraic type, then we unfold the scrutinee to all
+ possible constructor patterns, using inhabitationCandidates, and then
+ check each one for constraint satisfiability, same as we do for normal
+ pattern match checking.
+}
+
+  A 'SatisfiabilityCheck' based on "NonVoid ty" constraints, e.g. Will
+ check if the @strict_arg_tys@ are actually all inhabited.
+ Returns the old 'Delta' if all the types are nonvoid according to 'Delta'.
+tysAreNonVoid :: RecTcChecker > [Type] > SatisfiabilityCheck
+tysAreNonVoid rec_env strict_arg_tys = SC $ \delta > do
+ all_non_void < checkAllNonVoid rec_env delta strict_arg_tys
+  Check if each strict argument type is inhabitable
+ pure $ if all_non_void
+ then Just delta
+ else Nothing
+
+  Implements two performance optimizations, as described in
+ @Note [Strict argument type constraints]@.
+checkAllNonVoid :: RecTcChecker > Delta > [Type] > DsM Bool
+checkAllNonVoid rec_ts amb_cs strict_arg_tys = do
+ let definitely_inhabited = definitelyInhabitedType (delta_ty_cs amb_cs)
+ tys_to_check < filterOutM definitely_inhabited strict_arg_tys
+ let rec_max_bound  tys_to_check `lengthExceeds` 1
+ = 1
+  otherwise
+ = defaultRecTcMaxBound
+ rec_ts' = setRecTcMaxBound rec_max_bound rec_ts
+ allM (nonVoid rec_ts' amb_cs) tys_to_check
+
+  Checks if a strict argument type of a conlike is inhabitable by a
+ terminating value (i.e, an 'InhabitationCandidate').
+ See @Note [Strict argument type constraints]@.
+nonVoid
+ :: RecTcChecker  ^ The per'TyCon' recursion depth limit.
+ > Delta  ^ The ambient term/type constraints (known to be
+  satisfiable).
+ > Type  ^ The strict argument type.
+ > DsM Bool  ^ 'True' if the strict argument type might be inhabited by
+  a terminating value (i.e., an 'InhabitationCandidate').
+  'False' if it is definitely uninhabitable by anything
+  (except bottom).
+nonVoid rec_ts amb_cs strict_arg_ty = do
+ mb_cands < inhabitationCandidates amb_cs strict_arg_ty
+ case mb_cands of
+ Right (tc, _, cands)
+  Just rec_ts' < checkRecTc rec_ts tc
+ > anyM (cand_is_inhabitable rec_ts' amb_cs) cands
+  A strict argument type is inhabitable by a terminating value if
+  at least one InhabitationCandidate is inhabitable.
+ _ > pure True
+  Either the type is trivially inhabited or we have exceeded the
+  recursion depth for some TyCon (so bail out and conservatively
+  claim the type is inhabited).
+ where
+  Checks if an InhabitationCandidate for a strict argument type:
+ 
+  (1) Has satisfiable term and type constraints.
+  (2) Has 'nonVoid' strict argument types (we bail out of this
+  check if recursion is detected).
+ 
+  See Note [Strict argument type constraints]
+ cand_is_inhabitable :: RecTcChecker > Delta
+ > InhabitationCandidate > DsM Bool
+ cand_is_inhabitable rec_ts amb_cs
+ (InhabitationCandidate{ ic_tm_cs = new_tm_cs
+ , ic_ty_cs = new_ty_cs
+ , ic_strict_arg_tys = new_strict_arg_tys }) =
+ fmap isJust $ runSatisfiabilityCheck amb_cs $ mconcat
+ [ tyIsSatisfiable False new_ty_cs
+ , tmIsSatisfiable new_tm_cs
+ , tysAreNonVoid rec_ts new_strict_arg_tys
+ ]
+
+  @'definitelyInhabitedType' ty@ returns 'True' if @ty@ has at least one
+ constructor @C@ such that:
+
+ 1. @C@ has no equality constraints.
+ 2. @C@ has no strict argument types.
+
+ See the @Note [Strict argument type constraints]@.
+definitelyInhabitedType :: Bag EvVar > Type > DsM Bool
+definitelyInhabitedType ty_cs ty = do
+ res < pmTopNormaliseType ty_cs ty
+ pure $ case res of
+ HadRedexes _ cons _ > any meets_criteria cons
+ _ > False
+ where
+ meets_criteria :: (Type, DataCon) > Bool
+ meets_criteria (_, con) =
+ null (dataConEqSpec con) &&  (1)
+ null (dataConImplBangs con)  (2)
+
+{ Note [Strict argument type constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the ConVar case of clause processing, each conlike K traditionally
+generates two different forms of constraints:
+
+* A term constraint (e.g., x ~ K y1 ... yn)
+* Type constraints from the conlike's context (e.g., if K has type
+ forall bs. Q => s1 .. sn > T tys, then Q would be its type constraints)
+
+As it turns out, these alone are not enough to detect a certain class of
+unreachable code. Consider the following example (adapted from #15305):
+
+ data K = K1  K2 !Void
+
+ f :: K > ()
+ f K1 = ()
+
+Even though `f` doesn't match on `K2`, `f` is exhaustive in its patterns. Why?
+Because it's impossible to construct a terminating value of type `K` using the
+`K2` constructor, and thus it's impossible for `f` to ever successfully match
+on `K2`.
+
+The reason is because `K2`'s field of type `Void` is //strict//. Because there
+are no terminating values of type `Void`, any attempt to construct something
+using `K2` will immediately loop infinitely or throw an exception due to the
+strictness annotation. (If the field were not strict, then `f` could match on,
+say, `K2 undefined` or `K2 (let x = x in x)`.)
+
+Since neither the term nor type constraints mentioned above take strict
+argument types into account, we make use of the `nonVoid` function to
+determine whether a strict type is inhabitable by a terminating value or not.
+
+`nonVoid ty` returns True when either:
+1. `ty` has at least one InhabitationCandidate for which both its term and type
+ constraints are satifiable, and `nonVoid` returns `True` for all of the
+ strict argument types in that InhabitationCandidate.
+2. We're unsure if it's inhabited by a terminating value.
+
+`nonVoid ty` returns False when `ty` is definitely uninhabited by anything
+(except bottom). Some examples:
+
+* `nonVoid Void` returns False, since Void has no InhabitationCandidates.
+ (This is what lets us discard the `K2` constructor in the earlier example.)
+* `nonVoid (Int :~: Int)` returns True, since it has an InhabitationCandidate
+ (through the Refl constructor), and its term constraint (x ~ Refl) and
+ type constraint (Int ~ Int) are satisfiable.
+* `nonVoid (Int :~: Bool)` returns False. Although it has an
+ InhabitationCandidate (by way of Refl), its type constraint (Int ~ Bool) is
+ not satisfiable.
+* Given the following definition of `MyVoid`:
+
+ data MyVoid = MkMyVoid !Void
+
+ `nonVoid MyVoid` returns False. The InhabitationCandidate for the MkMyVoid
+ constructor contains Void as a strict argument type, and since `nonVoid Void`
+ returns False, that InhabitationCandidate is discarded, leaving no others.
+
+* Performance considerations
+
+We must be careful when recursively calling `nonVoid` on the strict argument
+types of an InhabitationCandidate, because doing so naïvely can cause GHC to
+fall into an infinite loop. Consider the following example:
+
+ data Abyss = MkAbyss !Abyss
+
+ stareIntoTheAbyss :: Abyss > a
+ stareIntoTheAbyss x = case x of {}
+
+In principle, stareIntoTheAbyss is exhaustive, since there is no way to
+construct a terminating value using MkAbyss. However, both the term and type
+constraints for MkAbyss are satisfiable, so the only way one could determine
+that MkAbyss is unreachable is to check if `nonVoid Abyss` returns False.
+There is only one InhabitationCandidate for Abyss—MkAbyss—and both its term
+and type constraints are satisfiable, so we'd need to check if `nonVoid Abyss`
+returns False... and now we've entered an infinite loop!
+
+To avoid this sort of conundrum, `nonVoid` uses a simple test to detect the
+presence of recursive types (through `checkRecTc`), and if recursion is
+detected, we bail out and conservatively assume that the type is inhabited by
+some terminating value. This avoids infinite loops at the expense of making
+the coverage checker incomplete with respect to functions like
+stareIntoTheAbyss above. Then again, the same problem occurs with recursive
+newtypes, like in the following code:
+
+ newtype Chasm = MkChasm Chasm
+
+ gazeIntoTheChasm :: Chasm > a
+ gazeIntoTheChasm x = case x of {}  Erroneously warned as nonexhaustive
+
+So this limitation is somewhat understandable.
+
+Note that even with this recursion detection, there is still a possibility that
+`nonVoid` can run in exponential time. Consider the following data type:
+
+ data T = MkT !T !T !T
+
+If we call `nonVoid` on each of its fields, that will require us to once again
+check if `MkT` is inhabitable in each of those three fields, which in turn will
+require us to check if `MkT` is inhabitable again... As you can see, the
+branching factor adds up quickly, and if the recursion depth limit is, say,
+100, then `nonVoid T` will effectively take forever.
+
+To mitigate this, we check the branching factor every time we are about to call
+`nonVoid` on a list of strict argument types. If the branching factor exceeds 1
+(i.e., if there is potential for exponential runtime), then we limit the
+maximum recursion depth to 1 to mitigate the problem. If the branching factor
+is exactly 1 (i.e., we have a linear chain instead of a tree), then it's okay
+to stick with a larger maximum recursion depth.
+
+Another microoptimization applies to data types like this one:
+
+ data S a = ![a] !T
+
+Even though there is a strict field of type [a], it's quite silly to call
+nonVoid on it, since it's "obvious" that it is inhabitable. To make this
+intuition formal, we say that a type is definitely inhabitable (DI) if:
+
+ * It has at least one constructor C such that:
+ 1. C has no equality constraints (since they might be unsatisfiable)
+ 2. C has no strict argument types (since they might be uninhabitable)
+
+It's relatively cheap to check if a type is DI, so before we call `nonVoid`
+on a list of strict argument types, we filter out all of the DI ones.
+}
+
+
+ * Providing positive evidence for a Delta
+
+  @provideEvidenceForEquation vs n delta@ returns a list of
+ at most @n@ (but perhaps empty) refinements of @delta@ that instantiate
+ @vs@ to compatible constructor applications or wildcards.
+ Negative information is only retained if literals are involved or when
+ for recursive GADTs.
+provideEvidenceForEquation :: [Id] > Int > Delta > DsM [Delta]
+provideEvidenceForEquation = go init_ts
+ where
+  Choosing 1 here will not be enough for RedBlack, but any other bound
+  might potentially lead to combinatorial explosion, so we are extremely
+  cautious and pick 2 here.
+ init_ts = setRecTcMaxBound 2 initRecTc
+ go _ _ 0 _ = pure []
+ go _ [] _ delta = pure [delta]
+ go rec_ts (x:xs) n delta = do
+ VI ty pos neg pm _ < initLookupVarInfo delta x
+ case pos of
+ _:_ > do
+  All solutions must be valid at once. Try to find candidates for their
+  fields. Example:
+  f x@(Just _) True = case x of SomePatSyn _ > ()
+  after this clause, we want to report that
+  * @f Nothing _@ is uncovered
+  * @f x False@ is uncovered
+  where @x@ will have two possibly compatible solutions, @Just y@ for
+  some @y@ and @SomePatSyn z@ for some @z@. We must find evidence for @y@
+  and @z@ that is valid at the same time. These constitute arg_vas below.
+ let arg_vas = concatMap (\(_cl, args) > args) pos
+ go rec_ts (arg_vas ++ xs) n delta
+ []
+  First the simple case where we don't need to query the oracles
+  isVanillaDataType ty
+  So no type info unleashed in pattern match
+ , null neg
+  No termlevel info either
+  notNull [ l  PmAltLit l < neg ]
+  ... or there are literals involved, in which case we don't want
+  to split on possible constructors
+ > go rec_ts xs n delta
+ [] > do
+  We have to pick one of the available constructors and try it
+  It's important that each of the ConLikeSets in pm is still inhabited,
+  so that it doesn't matter from which we pick.
+  I think we implicitly uphold that invariant, but not too sure
+ case getUnmatchedConstructor pm of
+ Unsatisfiable > pure []
+  No COMPLETE sets available, so we can assume it's inhabited
+ PossiblySatisfiable > go rec_ts xs n delta
+ Satisfiable cl
+  Just rec_ts' < checkRecTc rec_ts (fst (splitTyConApp ty))
+ > split_at_con rec_ts' delta n x xs cl
+  otherwise
+  We ran out of fuel; just conservatively assume that this is
+  inhabited.
+ > go rec_ts xs n delta
+
+   @split_at_con _ delta _ x _ con@ splits the given delta into two
+  models: One where we assume x is con and one where we assume it can't be
+  con. Really similar to the ConVar case in Check, only that we don't
+  really have a pattern driving the split.
+ split_at_con
+ :: RecTcChecker  ^ Detects when we split the same TyCon too often
+ > Delta  ^ The model we like to refine to the split
+ > Int  ^ The number of equations still to produce
+ > Id > [Id]  ^ Head and tail of the value abstractions
+ > ConLike  ^ The ConLike over which to split
+ > DsM [Delta]
+ split_at_con rec_ts delta n x xs cl = do
+  This will be really similar to the ConVar case
+ let (_,ex_tvs,_,_,_,_,_) = conLikeFullSig cl
+  we might need to freshen ex_tvs. Not sure
+  We may need to reduce type famlies/synonyms in x's type first
+ res < pmTopNormaliseType (delta_ty_cs delta) (idType x)
+ let res_ty = normalisedSourceType res
+ env < dsGetFamInstEnvs
+ case guessConLikeUnivTyArgsFromResTy env res_ty cl of
+ Nothing > pure [delta]  We can't split this one, so assume it's inhabited
+ Just arg_tys > do
+ ev_pos < refineToAltCon delta x (PmAltConLike cl) arg_tys ex_tvs >>= \case
+ Nothing > pure []
+ Just (delta', arg_vas) >
+ go rec_ts (arg_vas ++ xs) n delta'
+
+  Only n' more equations to go...
+ let n' = n  length ev_pos
+ ev_neg < addRefutableAltCon delta x (PmAltConLike cl) >>= \case
+ Nothing > pure []
+ Just delta' > go rec_ts (x:xs) n' delta'
+
+  Actually there was no need to split if we see that both branches
+  were inhabited and we had no negative information on the variable!
+  So only refine delta if we find that one branch was indeed
+  uninhabited.
+ let neg = lookupRefuts delta x
+ case (ev_pos, ev_neg) of
+ ([], _) > pure ev_neg
+ (_, []) > pure ev_pos
+ _  null neg > pure [delta]
+  otherwise > pure (ev_pos ++ ev_neg)
+
+  Checks if every data con of the type 'isVanillaDataCon'.
+isVanillaDataType :: Type > Bool
+isVanillaDataType ty = fromMaybe False $ do
+ (tc, _) < splitTyConApp_maybe ty
+ dcs < tyConDataCons_maybe tc
+ pure (all isVanillaDataCon dcs)
+
+ Most of our actions thread around a delta from one computation to the next,
+ thereby potentially failing. This is expressed in the following Monad:
+ type PmM a = StateT Delta (MaybeT DsM) a
+
+  Records that a variable @x@ is equal to a 'CoreExpr' @e@.
+addVarCoreCt :: Delta > Id > CoreExpr > DsM (Maybe Delta)
+addVarCoreCt delta x e = runMaybeT (execStateT (core_expr x e) delta)
+ where
+   Takes apart a 'CoreExpr' and tries to extract as much information about
+  literals and constructor applications as possible.
+ core_expr :: Id > CoreExpr > StateT Delta (MaybeT DsM) ()
+  TODO: Handle newtypes properly, by wrapping the expression in a DataCon
+ core_expr x (Cast e _co) = core_expr x e
+ core_expr x (Tick _t e) = core_expr x e
+ core_expr x e
+  Just (pmLitAsStringLit > Just s) < coreExprAsPmLit e
+ , expr_ty `eqType` stringTy
+  See Note [Representation of Strings in TmState]
+ = case unpackFS s of
+  We need this special case to break a loop with coreExprAsPmLit
+  Otherwise we alternate endlessly between [] and ""
+ [] > data_con_app x nilDataCon []
+ s' > core_expr x (mkListExpr charTy (map mkCharExpr s'))
+  Just lit < coreExprAsPmLit e
+ = pm_lit x lit
+  Just (_in_scope, _empty_floats@[], dc, _arg_tys, args)
+ < exprIsConApp_maybe empty_in_scope e
+ = do { arg_ids < traverse bind_expr args
+ ; data_con_app x dc arg_ids }
+  TODO: Think about how to recognize PatSyns
+  Var y < e, not (isDataConWorkId x)
+ = modifyT (\delta > addVarVarCt delta (x, y))
+  otherwise
+  TODO: Use a CoreMap to identify the CoreExpr with a unique representant
+ = pure ()
+ where
+ empty_in_scope = (emptyInScopeSet, const NoUnfolding)
+ expr_ty = exprType e
+
+ bind_expr :: CoreExpr > StateT Delta (MaybeT DsM) Id
+ bind_expr e = do
+ x < lift (lift (mkPmId (exprType e)))
+ core_expr x e
+ pure x
+
+ data_con_app :: Id > DataCon > [Id] > StateT Delta (MaybeT DsM) ()
+ data_con_app x dc args = pm_alt_con_app x (PmAltConLike (RealDataCon dc)) args
+
+ pm_lit :: Id > PmLit > StateT Delta (MaybeT DsM) ()
+ pm_lit x lit = pm_alt_con_app x (PmAltLit lit) []
+
+   Adds the given constructor application as a solution for @x@.
+ pm_alt_con_app :: Id > PmAltCon > [Id] > StateT Delta (MaybeT DsM) ()
+ pm_alt_con_app x con args = modifyT $ \delta > addVarConCt delta x con args
+
+  Like 'modify', but with an effectful modifier action
+modifyT :: Monad m => (s > m s) > StateT s m ()
+modifyT f = StateT $ fmap ((,) ()) . f
diff git a/compiler/deSugar/PmOracle.hsboot b/compiler/deSugar/PmOracle.hsboot
new file mode 100644
index 0000000000000000000000000000000000000000..e099fe78a64b4eab7f794240ce5209798d6c11ae
 /dev/null
+++ b/compiler/deSugar/PmOracle.hsboot
@@ 0,0 +1,7 @@
+module PmOracle where
+
+import GhcPrelude ()
+
+data Delta
+
+initDelta :: Delta
diff git a/compiler/deSugar/PmPpr.hs b/compiler/deSugar/PmPpr.hs
index 06b60a6806f7f17a713bb68a45e9b67b85bed7f1..bee38ed46b5a56183348d411dbcc089e99fc377c 100644
 a/compiler/deSugar/PmPpr.hs
+++ b/compiler/deSugar/PmPpr.hs
@@ 1,4 +1,4 @@
{# LANGUAGE CPP #}
+{# LANGUAGE CPP, ViewPatterns #}
  Provides factilities for prettyprinting 'PmExpr's in a way approriate for
 user facing pattern match warnings.
@@ 10,20 +10,20 @@ module PmPpr (
import GhcPrelude
import Name
import NameEnv
import NameSet
+import Id
+import VarEnv
import UniqDFM
import UniqSet
import ConLike
import DataCon
import TysWiredIn
import Outputable
import Control.Monad.Trans.State.Strict
import Maybes
+import Control.Monad.Trans.RWS.CPS
import Util
+import Maybes
+import Data.List.NonEmpty (NonEmpty, nonEmpty, toList)
import TmOracle
+import PmExpr
+import PmOracle
  Prettyprint the guts of an uncovered value vector abstraction, i.e., its
 components and refutable shapes associated to any mentioned variables.
@@ 35,22 +35,31 @@ import TmOracle
 where p is not one of {3, 4}
 q is not one of {0, 5}
 @
pprUncovered :: ([PmExpr], PmRefutEnv) > SDoc
pprUncovered (expr_vec, refuts)
  null cs = fsep vec  there are no literal constraints
  otherwise = hang (fsep vec) 4 $
 text "where" <+> vcat (map pprRefutableShapes cs)
+
+ When the set of refutable shapes contains more than 3 elements, the
+ additional elements are indicated by "...".
+pprUncovered :: Delta > [Id] > SDoc
+pprUncovered delta vas
+  isNullUDFM refuts = fsep vec  there are no refutations
+  otherwise = hang (fsep vec) 4 $
+ text "where" <+> vcat (map (pprRefutableShapes . snd) (udfmToList refuts))
where
 sdoc_vec = mapM pprPmExprWithParens expr_vec
 (vec,cs) = runPmPpr sdoc_vec (prettifyRefuts refuts)
+ ppr_action = mapM (pprPmExprVar 2) vas
+ (vec, renamings) = runPmPpr delta ppr_action
+ refuts = prettifyRefuts delta renamings
  Output refutable shapes of a variable in the form of @var is not one of {2,
 Nothing, 3}@.
+ Nothing, 3}@. Will never print more than 3 refutable shapes, the tail is
+ indicated by an ellipsis.
pprRefutableShapes :: (SDoc,[PmAltCon]) > SDoc
pprRefutableShapes (var, alts)
 = var <+> text "is not one of" <+> braces (pprWithCommas ppr_alt alts)
+ = var <+> text "is not one of" <+> format_alts alts
where
 ppr_alt (PmAltLit lit) = ppr lit
+ format_alts = braces . fsep . punctuate comma . shorten . map ppr_alt
+ shorten (a:b:c:_:_) = a:b:c:[text "..."]
+ shorten xs = xs
+ ppr_alt (PmAltConLike cl) = ppr cl
+ ppr_alt (PmAltLit lit) = ppr lit
{ 1. Literals
~~~~~~~~~~~~~~
@@ 78,114 +87,115 @@ substitution to the vectors before printing them out (see function `pprOne' in
Check.hs) to be more precise.
}
  A 'PmRefutEnv' with pretty names for the occuring variables.
type PrettyPmRefutEnv = DNameEnv (SDoc, [PmAltCon])

  Assigns pretty names to constraint variables in the domain of the given
 'PmRefutEnv'.
prettifyRefuts :: PmRefutEnv > PrettyPmRefutEnv
prettifyRefuts = listToUDFM . zipWith rename nameList . udfmToList
+  Extract and assigns pretty names to constraint variables with refutable
+ shapes.
+prettifyRefuts :: Delta > DIdEnv SDoc > DIdEnv (SDoc, [PmAltCon])
+prettifyRefuts delta = listToUDFM . map attach_refuts . udfmToList
where
 rename new (old, lits) = (old, (new, lits))
  Try nice names p,q,r,s,t before using the (ugly) t_i
 nameList :: [SDoc]
 nameList = map text ["p","q","r","s","t"] ++
 [ text ('t':show u)  u < [(0 :: Int)..] ]

type PmPprM a = State (PrettyPmRefutEnv, NameSet) a
 (the first part of the state is read only. make it a reader?)

runPmPpr :: PmPprM a > PrettyPmRefutEnv > (a, [(SDoc,[PmAltCon])])
runPmPpr m lit_env = (result, mapMaybe is_used (udfmToList lit_env))
 where
 (result, (_lit_env, used)) = runState m (lit_env, emptyNameSet)

 is_used (k,v)
  elemUniqSet_Directly k used = Just v
  otherwise = Nothing

addUsed :: Name > PmPprM ()
addUsed x = modify (\(negated, used) > (negated, extendNameSet used x))

checkNegation :: Name > PmPprM (Maybe SDoc)  the clean name if it is negated
checkNegation x = do
 negated < gets fst
 return $ case lookupDNameEnv negated x of
 Just (new, _) > Just new
 Nothing > Nothing

  Pretty print a pmexpr, but remember to prettify the names of the variables
+ attach_refuts (u, sdoc) = (u, (sdoc, lookupRefuts delta u))
+
+
+type PmPprM a = RWS Delta () (DIdEnv SDoc, [SDoc]) a
+
+ Try nice names p,q,r,s,t before using the (ugly) t_i
+nameList :: [SDoc]
+nameList = map text ["p","q","r","s","t"] ++
+ [ text ('t':show u)  u < [(0 :: Int)..] ]
+
+runPmPpr :: Delta > PmPprM a > (a, DIdEnv SDoc)
+runPmPpr delta m = case runRWS m delta (emptyDVarEnv, nameList) of
+ (a, (renamings, _), _) > (a, renamings)
+
+  Allocates a new, clean name for the given 'Id' if it doesn't already have
+ one.
+getCleanName :: Id > PmPprM SDoc
+getCleanName x = do
+ (renamings, name_supply) < get
+ let (clean_name:name_supply') = name_supply
+ case lookupDVarEnv renamings x of
+ Just nm > pure nm
+ Nothing > do
+ put (extendDVarEnv renamings x clean_name, name_supply')
+ pure clean_name
+
+checkRefuts :: Id > PmPprM (Maybe SDoc)  the clean name if it has negative info attached
+checkRefuts x = do
+ delta < ask
+ case lookupRefuts delta x of
+ [] > pure Nothing  Will just be a wildcard later on
+ _ > Just <$> getCleanName x
+
+  Pretty print a variable, but remember to prettify the names of the variables
 that refer to negliterals. The ones that cannot be shown are printed as
 underscores.
pprPmExpr :: PmExpr > PmPprM SDoc
pprPmExpr (PmExprVar x) = do
 mb_name < checkNegation x
 case mb_name of
 Just name > addUsed x >> return name
 Nothing > return underscore
pprPmExpr (PmExprCon con args) = pprPmExprCon con args
pprPmExpr (PmExprLit l) = return (ppr l)
pprPmExpr (PmExprOther _) = return underscore  don't show

needsParens :: PmExpr > Bool
needsParens (PmExprVar {}) = False
needsParens (PmExprLit l) = isNegatedPmLit l
needsParens (PmExprOther {}) = False  will become a wildcard
needsParens (PmExprCon (RealDataCon c) es)
  isTupleDataCon c
  isConsDataCon c  null es = False
  otherwise = True
needsParens (PmExprCon (PatSynCon _) es) = not (null es)

pprPmExprWithParens :: PmExpr > PmPprM SDoc
pprPmExprWithParens expr
  needsParens expr = parens <$> pprPmExpr expr
  otherwise = pprPmExpr expr

pprPmExprCon :: ConLike > [PmExpr] > PmPprM SDoc
pprPmExprCon (RealDataCon con) args
  isTupleDataCon con = mkTuple <$> mapM pprPmExpr args
  isConsDataCon con = pretty_list
 where
 mkTuple :: [SDoc] > SDoc
 mkTuple = parens . fsep . punctuate comma

  lazily, to be used in the list case only
 pretty_list :: PmPprM SDoc
 pretty_list = case isNilPmExpr (last list) of
 True > brackets . fsep . punctuate comma <$> mapM pprPmExpr (init list)
 False > parens . hcat . punctuate colon <$> mapM pprPmExpr list

 list = list_elements args

 list_elements [x,y]
  PmExprCon c es < y, RealDataCon nilDataCon == c
 = ASSERT(null es) [x,y]
  PmExprCon c es < y, RealDataCon consDataCon == c
 = x : list_elements es
  otherwise = [x,y]
 list_elements list = pprPanic "list_elements:" (ppr list)
pprPmExprCon cl args
+pprPmExprVar :: Int > Id > PmPprM SDoc
+pprPmExprVar prec x = do
+ delta < ask
+ case lookupSolution delta x of
+ Just (alt, args) > pprPmExprCon prec alt args
+ Nothing > fromMaybe underscore <$> checkRefuts x
+
+pprPmExprCon :: Int > PmAltCon > [Id] > PmPprM SDoc
+pprPmExprCon _prec (PmAltLit l) _ = pure (ppr l)
+pprPmExprCon prec (PmAltConLike cl) args = do
+ delta < ask
+ pprConLike delta prec cl args
+
+pprConLike :: Delta > Int > ConLike > [Id] > PmPprM SDoc
+pprConLike delta _prec cl args
+  Just pm_expr_list < pmExprAsList delta (PmAltConLike cl) args
+ = case pm_expr_list of
+ NilTerminated list >
+ brackets . fsep . punctuate comma <$> mapM (pprPmExprVar 0) list
+ WcVarTerminated pref x >
+ parens . fcat . punctuate colon <$> mapM (pprPmExprVar 0) (toList pref ++ [x])
+pprConLike _delta _prec (RealDataCon con) args
+  isUnboxedTupleCon con
+ , let hash_parens doc = text "(#" <+> doc <+> text "#)"
+ = hash_parens . fsep . punctuate comma <$> mapM (pprPmExprVar 0) args
+  isTupleDataCon con
+ = parens . fsep . punctuate comma <$> mapM (pprPmExprVar 0) args
+pprConLike _delta prec cl args
 conLikeIsInfix cl = case args of
 [x, y] > do x' < pprPmExprWithParens x
 y' < pprPmExprWithParens y
 return (x' <+> ppr cl <+> y')
+ [x, y] > do x' < pprPmExprVar 1 x
+ y' < pprPmExprVar 1 y
+ return (cparen (prec > 0) (x' <+> ppr cl <+> y'))
 can it be infix but have more than two arguments?
list > pprPanic "pprPmExprCon:" (ppr list)
 null args = return (ppr cl)
  otherwise = do args' < mapM pprPmExprWithParens args
 return (fsep (ppr cl : args'))

  Check whether a literal is negated
isNegatedPmLit :: PmLit > Bool
isNegatedPmLit (PmOLit b _) = b
isNegatedPmLit _other_lit = False

  Check whether a PmExpr is syntactically e
isNilPmExpr :: PmExpr > Bool
isNilPmExpr (PmExprCon c _) = c == RealDataCon nilDataCon
isNilPmExpr _other_expr = False

  Check if a DataCon is (:).
isConsDataCon :: DataCon > Bool
isConsDataCon con = consDataCon == con
+  otherwise = do args' < mapM (pprPmExprVar 2) args
+ return (cparen (prec > 1) (fsep (ppr cl : args')))
+
+  The result of 'pmExprAsList'.
+data PmExprList
+ = NilTerminated [Id]
+  WcVarTerminated (NonEmpty Id) Id
+
+  Extract a list of 'PmExpr's out of a sequence of cons cells, optionally
+ terminated by a wildcard variable instead of @[]@. Some examples:
+
+ * @pmExprAsList (1:2:[]) == Just ('NilTerminated' [1,2])@, a regular,
+ @[]@terminated list. Should be prettyprinted as @[1,2]@.
+ * @pmExprAsList (1:2:x) == Just ('WcVarTerminated' [1,2] x)@, a list prefix
+ ending in a wildcard variable x (of list type). Should be prettyprinted as
+ (1:2:_).
+ * @pmExprAsList [] == Just ('NilTerminated' [])@
+pmExprAsList :: Delta > PmAltCon > [Id] > Maybe PmExprList
+pmExprAsList delta = go_con []
+ where
+ go_var rev_pref x
+  Just (alt, args) < lookupSolution delta x
+ = go_con rev_pref alt args
+ go_var rev_pref x
+  Just pref < nonEmpty (reverse rev_pref)
+ = Just (WcVarTerminated pref x)
+ go_var _ _
+ = Nothing
+
+ go_con rev_pref (PmAltConLike (RealDataCon c)) es
+  c == nilDataCon
+ = ASSERT( null es ) Just (NilTerminated (reverse rev_pref))
+  c == consDataCon
+ = ASSERT( length es == 2 ) go_var (es !! 0 : rev_pref) (es !! 1)
+ go_con _ _ _
+ = Nothing
diff git a/compiler/deSugar/TmOracle.hs b/compiler/deSugar/TmOracle.hs
deleted file mode 100644
index db3ce6d770a9c503aff909604e82c0a3fa2b4e00..0000000000000000000000000000000000000000
 a/compiler/deSugar/TmOracle.hs
+++ /dev/null
@@ 1,362 +0,0 @@
{
Author: George Karachalias
}

{# LANGUAGE CPP, MultiWayIf #}

  The term equality oracle. The main export of the module are the functions
 'tmOracle', 'solveOneEq' and 'addSolveRefutableAltCon'.

 If you are looking for an oracle that can solve typelevel constraints, look
 at 'TcSimplify.tcCheckSatisfiability'.
module TmOracle (

  reexported from PmExpr
 PmExpr(..), PmLit(..), PmAltCon(..), TmVarCt(..), TmVarCtEnv,
 PmRefutEnv, eqPmLit, isNotPmExprOther, lhsExprToPmExpr, hsExprToPmExpr,

  the term oracle
 tmOracle, TmState, initialTmState, wrapUpTmState, solveOneEq,
 extendSubst, canDiverge, isRigid,
 addSolveRefutableAltCon, lookupRefutableAltCons,

  misc.
 exprDeepLookup, pmLitType
 ) where

#include "HsVersions.h"

import GhcPrelude

import PmExpr

import Util
import Id
import Name
import Type
import HsLit
import TcHsSyn
import MonadUtils
import ListSetOps (insertNoDup, unionLists)
import Maybes
import Outputable
import NameEnv
import UniqFM
import UniqDFM

{
%************************************************************************
%* *
 The term equality oracle
%* *
%************************************************************************
}

  Pretty much a @['TmVarCt']@ association list where the domain is 'Name'
 instead of 'Id'. This is the type of 'tm_pos', where we store solutions for
 rigid pattern match variables.
type TmVarCtEnv = NameEnv PmExpr

  An environment assigning shapes to variables that immediately lead to a
 refutation. So, if this maps @x :> [3]@, then trying to solve a 'TmVarCt'
 like @x ~ 3@ immediately leads to a contradiction.
 Determinism is important since we use this for warning messages in
 'PmPpr.pprUncovered'. We don't do the same for 'TmVarCtEnv', so that is a plain
 'NameEnv'.

 See also Note [Refutable shapes] in TmOracle.
type PmRefutEnv = DNameEnv [PmAltCon]

  The state of the term oracle. Tracks all termlevel facts of the form "x is
 @True@" ('tm_pos') and "x is not @5@" ('tm_neg').

 Subject to Note [The Pos/Neg invariant].
data TmState = TmS
 { tm_pos :: !TmVarCtEnv
  ^ A substitution with solutions we extend with every step and return as a
  result. The substitution is in /triangular form/: It might map @x@ to @y@
  where @y@ itself occurs in the domain of 'tm_pos', rendering lookup
  nonidempotent. This means that 'varDeepLookup' potentially has to walk
  along a chain of vartovar mappings until we find the solution but has the
  advantage that when we update the solution for @y@ above, we automatically
  update the solution for @x@ in a unionfindlike fashion.
  Invariant: Only maps to other variables ('PmExprVar') or to WHNFs
  ('PmExprLit', 'PmExprCon'). Ergo, never maps to a 'PmExprOther'.
 , tm_neg :: !PmRefutEnv
  ^ Maps each variable @x@ to a list of 'PmAltCon's that @x@ definitely
  cannot match. Example, @x :> [3, 4]@ means that @x@ cannot match a literal
  3 or 4. Should we later solve @x@ to a variable @y@
  ('extendSubstAndSolve'), we merge the refutable shapes of @x@ into those of
  @y@. See also Note [The Pos/Neg invariant].
 }

{ Note [The Pos/Neg invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Invariant: In any 'TmState', The domains of 'tm_pos' and 'tm_neg' are disjoint.

For example, it would make no sense to say both
 tm_pos = [...x :> 3 ...]
 tm_neg = [...x :> [4,42]... ]
The positive information is strictly more informative than the negative.

Suppose we are adding the (positive) fact @x :> e@ to 'tm_pos'. Then we must
delete any binding for @x@ from 'tm_neg', to uphold the invariant.

But there is more! Suppose we are adding @x :> y@ to 'tm_pos', and 'tm_neg'
contains @x :> cs, y :> ds@. Then we want to update 'tm_neg' to
@y :> (cs ++ ds)@, to make use of the negative information we have about @x@.
}

instance Outputable TmState where
 ppr state = braces (fsep (punctuate comma (pos ++ neg)))
 where
 pos = map pos_eq (nonDetUFMToList (tm_pos state))
 neg = map neg_eq (udfmToList (tm_neg state))
 pos_eq (l, r) = ppr l <+> char '~' <+> ppr r
 neg_eq (l, r) = ppr l <+> char '≁' <+> ppr r

  Initial state of the oracle.
initialTmState :: TmState
initialTmState = TmS emptyNameEnv emptyDNameEnv

  Wrap up the term oracle's state once solving is complete. Return the
 flattened 'tm_pos' and 'tm_neg'.
wrapUpTmState :: TmState > (TmVarCtEnv, PmRefutEnv)
wrapUpTmState solver_state
 = (flattenTmVarCtEnv (tm_pos solver_state), tm_neg solver_state)

  Flatten the triangular subsitution.
flattenTmVarCtEnv :: TmVarCtEnv > TmVarCtEnv
flattenTmVarCtEnv env = mapNameEnv (exprDeepLookup env) env

  Check whether a constraint (x ~ BOT) can succeed,
 given the resulting state of the term oracle.
canDiverge :: Name > TmState > Bool
canDiverge x TmS{ tm_pos = pos, tm_neg = neg }
  If the variable seems not evaluated, there is a possibility for
  constraint x ~ BOT to be satisfiable. That's the case when we haven't found
  a solution (i.e. some equivalent literal or constructor) for it yet.
  (_, PmExprVar y) < varDeepLookup pos x  seems not forced
  Even if we don't have a solution yet, it might be involved in a negative
  constraint, in which case we must already have evaluated it earlier.
 , Nothing < lookupDNameEnv neg y
 = True
  Variable x is already in WHNF or we know some refutable shape, so the
  constraint is nonsatisfiable
  otherwise = False

  Check whether the equality @x ~ e@ leads to a refutation. Make sure that
 @x@ and @e@ are completely substituted before!
isRefutable :: Name > PmExpr > PmRefutEnv > Bool
isRefutable x e env
 = fromMaybe False $ elem <$> exprToAlt e <*> lookupDNameEnv env x

  Solve an equality (toplevel).
solveOneEq :: TmState > TmVarCt > Maybe TmState
solveOneEq solver_env (TVC x e) = unify solver_env (PmExprVar (idName x), e)

exprToAlt :: PmExpr > Maybe PmAltCon
exprToAlt (PmExprLit l) = Just (PmAltLit l)
exprToAlt _ = Nothing

  Record that a particular 'Id' can't take the shape of a 'PmAltCon' in the
 'TmState' and return @Nothing@ if that leads to a contradiction.
addSolveRefutableAltCon :: TmState > Id > PmAltCon > Maybe TmState
addSolveRefutableAltCon original@TmS{ tm_pos = pos, tm_neg = neg } x nalt
 = case exprToAlt e of
  We have to take care to preserve Note [The Pos/Neg invariant]
 Nothing > Just extended  Not solved yet
 Just alt  We have a solution
  alt == nalt > Nothing  ... which is contradictory
  otherwise > Just original  ... which is compatible, rendering the
 where  refutation redundant
 (y, e) = varDeepLookup pos (idName x)
 extended = original { tm_neg = neg' }
 neg' = alterDNameEnv (delNulls (insertNoDup nalt)) neg y

  When updating 'tm_neg', we want to delete any 'null' entries. This adapter
 intends to provide a suitable interface for 'alterDNameEnv'.
delNulls :: ([a] > [a]) > Maybe [a] > Maybe [a]
delNulls f mb_entry
  ret@(_:_) < f (fromMaybe [] mb_entry) = Just ret
  otherwise = Nothing

  Return all 'PmAltCon' shapes that are impossible for 'Id' to take, i.e.
 would immediately lead to a refutation by the term oracle.
lookupRefutableAltCons :: Id > TmState > [PmAltCon]
lookupRefutableAltCons x TmS { tm_neg = neg }
 = fromMaybe [] (lookupDNameEnv neg (idName x))

  Is the given variable /rigid/ (i.e., we have a solution for it) or
 /flexible/ (i.e., no solution)? Returns the solution if /rigid/. A
 semantically helpful alias for 'lookupNameEnv'.
isRigid :: TmState > Name > Maybe PmExpr
isRigid TmS{ tm_pos = pos } x = lookupNameEnv pos x

  @isFlexible tms = isNothing . 'isRigid' tms@
isFlexible :: TmState > Name > Bool
isFlexible tms = isNothing . isRigid tms

  Try to unify two 'PmExpr's and record the gained knowledge in the
 'TmState'.

 Returns @Nothing@ when there's a contradiction. Returns @Just tms@
 when the constraint was compatible with prior facts, in which case @tms@ has
 integrated the knowledge from the equality constraint.
unify :: TmState > (PmExpr, PmExpr) > Maybe TmState
unify tms eq@(e1, e2) = case eq of
  We cannot do a thing about these cases
 (PmExprOther _,_) > boring
 (_,PmExprOther _) > boring

 (PmExprLit l1, PmExprLit l2) > case eqPmLit l1 l2 of
  See Note [Undecidable Equality for Overloaded Literals]
 True > boring
 False > unsat

 (PmExprCon c1 ts1, PmExprCon c2 ts2)
  c1 == c2 > foldlM unify tms (zip ts1 ts2)
  otherwise > unsat

 (PmExprVar x, PmExprVar y)
  x == y > boring

  It's important to handle both rigid cases first, otherwise we get cyclic
  substitutions. Cf. 'extendSubstAndSolve' and
  @testsuite/tests/pmcheck/should_compile/CyclicSubst.hs@.
 (PmExprVar x, _)
  Just e1' < isRigid tms x > unify tms (e1', e2)
 (_, PmExprVar y)
  Just e2' < isRigid tms y > unify tms (e1, e2')
 (PmExprVar x, PmExprVar y) > Just (equate x y tms)
 (PmExprVar x, _) > trySolve x e2 tms
 (_, PmExprVar y) > trySolve y e1 tms

 _ > WARN( True, text "unify: Catch all" <+> ppr eq)
 boring  I HATE CATCHALLS
 where
 boring = Just tms
 unsat = Nothing

  Merges the equivalence classes of @x@ and @y@ by extending the substitution
 with @x :> y@.
 Preconditions: @x /= y@ and both @x@ and @y@ are flexible (cf.
 'isFlexible'/'isRigid').
equate :: Name > Name > TmState > TmState
equate x y tms@TmS{ tm_pos = pos, tm_neg = neg }
 = ASSERT( x /= y )
 ASSERT( isFlexible tms x )
 ASSERT( isFlexible tms y )
 tms'
 where
 pos' = extendNameEnv pos x (PmExprVar y)
  Be careful to uphold Note [The Pos/Neg invariant] by merging the refuts
  of x into those of y
 nalts = fromMaybe [] (lookupDNameEnv neg x)
 neg' = alterDNameEnv (delNulls (unionLists nalts)) neg y
 `delFromDNameEnv` x
 tms' = TmS { tm_pos = pos', tm_neg = neg' }

  Extend the substitution with a mapping @x: > e@ if compatible with
 refutable shapes of @x@ and its solution, reject (@Nothing@) otherwise.

 Precondition: @x@ is flexible (cf. 'isFlexible'/'isRigid').
 Precondition: @e@ is a 'PmExprCon' or 'PmExprLit'
trySolve:: Name > PmExpr > TmState > Maybe TmState
trySolve x e _tms@TmS{ tm_pos = pos, tm_neg = neg }
  ASSERT( isFlexible _tms x )
 ASSERT( _is_whnf e )
 isRefutable x e neg
 = Nothing
  otherwise
 = Just (TmS (extendNameEnv pos x e) (delFromDNameEnv neg x))
 where
 _is_whnf PmExprCon{} = True
 _is_whnf PmExprLit{} = True
 _is_whnf _ = False

  When we know that a variable is fresh, we do not actually have to
 check whether anything changes, we know that nothing does. Hence,
 @extendSubst@ simply extends the substitution, unlike what
 'extendSubstAndSolve' does.
extendSubst :: Id > PmExpr > TmState > TmState
extendSubst y e solver_state@TmS{ tm_pos = pos }
  isNotPmExprOther simpl_e
 = solver_state { tm_pos = extendNameEnv pos x simpl_e }
  otherwise = solver_state
 where
 x = idName y
 simpl_e = exprDeepLookup pos e

  Apply an (unflattened) substitution to a variable and return its
 representative in the triangular substitution @env@ and the completely
 substituted expression. The latter may just be the representative wrapped
 with 'PmExprVar' if we haven't found a solution for it yet.
varDeepLookup :: TmVarCtEnv > Name > (Name, PmExpr)
varDeepLookup env x = case lookupNameEnv env x of
 Just (PmExprVar y) > varDeepLookup env y
 Just e > (x, exprDeepLookup env e)  go deeper
 Nothing > (x, PmExprVar x)  terminal
{# INLINE varDeepLookup #}

  Apply an (unflattened) substitution to an expression.
exprDeepLookup :: TmVarCtEnv > PmExpr > PmExpr
exprDeepLookup env (PmExprVar x) = snd (varDeepLookup env x)
exprDeepLookup env (PmExprCon c es) = PmExprCon c (map (exprDeepLookup env) es)
exprDeepLookup _ other_expr = other_expr  PmExprLit, PmExprOther

  External interface to the term oracle.
tmOracle :: TmState > [TmVarCt] > Maybe TmState
tmOracle tm_state eqs = foldlM solveOneEq tm_state eqs

  Type of a PmLit
pmLitType :: PmLit > Type  should be in PmExpr but gives cyclic imports :(
pmLitType (PmSLit lit) = hsLitType lit
pmLitType (PmOLit _ lit) = overLitType lit

{ Note [Refutable shapes]
~~~~~~~~~~~~~~~~~~~~~~~~~~

Consider a pattern match like

 foo x
  0 < x = 42
  0 < x = 43
  1 < x = 44
  otherwise = 45

This will result in the following initial matching problem:

 PatVec: x (0 < x)
 ValVec: $tm_y

Where the first line is the pattern vector and the second line is the value
vector abstraction. When we handle the first pattern guard in Check, it will be
desugared to a match of the form

 PatVec: x 0
 ValVec: $tm_y x

In LitVar, this will split the value vector abstraction for `x` into a positive
`PmLit 0` and a negative `PmLit x [0]` value abstraction. While the former is
immediately matched against the pattern vector, the latter (vector value
abstraction `~[0] $tm_y`) is completely uncovered by the clause.

`pmcheck` proceeds by *discarding* the the value vector abstraction involving
the guard to accomodate for the desugaring. But this also discards the valuable
information that `x` certainly is not the literal 0! Consequently, we wouldn't
be able to report the second clause as redundant.

That's a typical example of why we need the term oracle, and in this specific
case, the ability to encode that `x` certainly is not the literal 0. Now the
term oracle can immediately refute the constraint `x ~ 0` generated by the
second clause and report the clause as redundant. After the third clause, the
set of such *refutable* literals is again extended to `[0, 1]`.

In general, we want to store a set of refutable shapes (`PmAltCon`) for each
variable. That's the purpose of the `PmRefutEnv`. `addSolveRefutableAltCon` will
add such a refutable mapping to the `PmRefutEnv` in the term oracles state and
check if causes any immediate contradiction. Whenever we record a solution in
the substitution via `extendSubstAndSolve`, the refutable environment is checked
for any matching refutable `PmAltCon`.
}
diff git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index 7946e233c4759deaad2a053b2b8e831184942467..884006b48799c87e1ad58ebae2be2b82d2bc5579 100644
 a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ 336,7 +336,7 @@ Library
PprCore
PmExpr
PmPpr
 TmOracle
+ PmOracle
Check
Coverage
Desugar
@@ 572,7 +572,6 @@ Library
IOEnv
Json
ListSetOps
 ListT
Maybes
MonadUtils
OrdList
diff git a/compiler/typecheck/TcRnTypes.hs b/compiler/typecheck/TcRnTypes.hs
index 5d56d33d53332ba32afa3cb6987d1088a972647e..0a2db75a2d7c457c96ef2e13947d079208eefe5d 100644
 a/compiler/typecheck/TcRnTypes.hs
+++ b/compiler/typecheck/TcRnTypes.hs
@@ 167,7 +167,7 @@ import TcType
import Annotations
import InstEnv
import FamInstEnv
import PmExpr
+import {# SOURCE #} PmOracle (Delta)
import IOEnv
import RdrName
import Name
@@ 390,10 +390,9 @@ data DsLclEnv = DsLclEnv {
dsl_loc :: RealSrcSpan,  To put in patternmatching error msgs
 See Note [Note [Type and Term Equality Propagation] in Check.hs
  These two fields are augmented as we walk inwards,
  through each patttern match in turn
 dsl_dicts :: Bag EvVar,  Constraints from GADT patternmatching
 dsl_tm_cs :: Bag TmVarCt,  Constraints form termlevel pattern matching
+  The oracle state Delta is augmented as we walk inwards,
+  through each pattern match in turn
+ dsl_delta :: Delta,
dsl_pm_iter :: IORef Int  Number of iterations for pmcheck so far
 We fail if this gets too big
diff git a/compiler/typecheck/TcSimplify.hs b/compiler/typecheck/TcSimplify.hs
index 48c730566957c6fd2f9d6fe192ffeae59131716f..18f2c505d91f3bdae41a18d91a11a890bab989bf 100644
 a/compiler/typecheck/TcSimplify.hs
+++ b/compiler/typecheck/TcSimplify.hs
@@ 688,6 +688,8 @@ constraints or to decide if a particular set of constraints is satisfiable,
the purpose of tcNormalise is to take a type, plus some local constraints, and
normalise the type as much as possible with respect to those constraints.
+It does *not* reduce type or data family applications or look through newtypes.
+
Why is this useful? As one example, when coveragechecking an EmptyCase
expression, it's possible that the type of the scrutinee will only reduce
if some local equalities are solved for. See "Wrinkle: Local equalities"
diff git a/compiler/utils/Binary.hs b/compiler/utils/Binary.hs
index 7a3b7a1472300fec9cbb146360dc8ea571ce86cd..164549fe58c961aa5903216232da8490768f729b 100644
 a/compiler/utils/Binary.hs
+++ b/compiler/utils/Binary.hs
@@ 935,7 +935,6 @@ putTypeRep bh (Fun arg res) = do
put_ bh (3 :: Word8)
putTypeRep bh arg
putTypeRep bh res
putTypeRep _ _ = fail "Binary.putTypeRep: Impossible"
getSomeTypeRep :: BinHandle > IO SomeTypeRep
getSomeTypeRep bh = do
diff git a/compiler/utils/ListSetOps.hs b/compiler/utils/ListSetOps.hs
index 8078d5603e081a22bf834696e189c649ea1705c2..a8b717df006295d3c9d55b2ad3e07a18d602bb92 100644
 a/compiler/utils/ListSetOps.hs
+++ b/compiler/utils/ListSetOps.hs
@@ 14,7 +14,7 @@ module ListSetOps (
Assoc, assoc, assocMaybe, assocUsing, assocDefault, assocDefaultUsing,
 Duplicate handling
 hasNoDups, removeDups, findDupsEq, insertNoDup,
+ hasNoDups, removeDups, findDupsEq,
equivClasses,
 Indexing
@@ 178,10 +178,3 @@ findDupsEq _ [] = []
findDupsEq eq (x:xs)  null eq_xs = findDupsEq eq xs
 otherwise = (x : eq_xs) : findDupsEq eq neq_xs
where (eq_xs, neq_xs) = partition (eq x) xs

  \( O(n) \). @'insertNoDup' x xs@ treats @xs@ as a set, inserting @x@ only
 when an equal element couldn't be found in @xs@.
insertNoDup :: (Eq a) => a > [a] > [a]
insertNoDup x set
  elem x set = set
  otherwise = x:set
diff git a/compiler/utils/ListT.hs b/compiler/utils/ListT.hs
deleted file mode 100644
index 66e52ed9f4761837710ee2415990ae1772ccf584..0000000000000000000000000000000000000000
 a/compiler/utils/ListT.hs
+++ /dev/null
@@ 1,79 +0,0 @@
{# LANGUAGE CPP #}
{# LANGUAGE DeriveFunctor #}
{# LANGUAGE UndecidableInstances #}
{# LANGUAGE Rank2Types #}
{# LANGUAGE FlexibleInstances #}
{# LANGUAGE MultiParamTypeClasses #}


 
 Module : Control.Monad.Logic
 Copyright : (c) Dan Doel
 License : BSD3

 Maintainer : dan.doel@gmail.com
 Stability : experimental
 Portability : nonportable (multiparameter type classes)

 A backtracking, logic programming monad.

 Adapted from the paper
 /Backtracking, Interleaving, and Terminating
 Monad Transformers/, by
 Oleg Kiselyov, Chungchieh Shan, Daniel P. Friedman, Amr Sabry
 ().


module ListT (
 ListT(..),
 runListT,
 select,
 fold
 ) where

import GhcPrelude

import Control.Applicative

import Control.Monad
import Control.Monad.Fail as MonadFail


  A monad transformer for performing backtracking computations
 layered over another monad 'm'
newtype ListT m a =
 ListT { unListT :: forall r. (a > m r > m r) > m r > m r }
 deriving (Functor)

select :: Monad m => [a] > ListT m a
select xs = foldr (<>) mzero (map pure xs)

fold :: ListT m a > (a > m r > m r) > m r > m r
fold = runListT


  Runs a ListT computation with the specified initial success and
 failure continuations.
runListT :: ListT m a > (a > m r > m r) > m r > m r
runListT = unListT

instance Applicative (ListT f) where
 pure a = ListT $ \sk fk > sk a fk
 f <*> a = ListT $ \sk fk > unListT f (\g fk' > unListT a (sk . g) fk') fk

instance Alternative (ListT f) where
 empty = ListT $ \_ fk > fk
 f1 <> f2 = ListT $ \sk fk > unListT f1 sk (unListT f2 sk fk)

instance Monad (ListT m) where
 m >>= f = ListT $ \sk fk > unListT m (\a fk' > unListT (f a) sk fk') fk
#if !MIN_VERSION_base(4,13,0)
 fail = MonadFail.fail
#endif

instance MonadFail.MonadFail (ListT m) where
 fail _ = ListT $ \_ fk > fk

instance MonadPlus (ListT m) where
 mzero = ListT $ \_ fk > fk
 m1 `mplus` m2 = ListT $ \sk fk > unListT m1 sk (unListT m2 sk fk)
diff git a/compiler/utils/Outputable.hs b/compiler/utils/Outputable.hs
index a5306faaa4684dc018f1d6d4914794bb67901c43..cd3e2a5f5b5c55e5cfead394ed01facf2ec922f7 100644
 a/compiler/utils/Outputable.hs
+++ b/compiler/utils/Outputable.hs
@@ 1205,7 +1205,7 @@ pprTraceM str doc = pprTrace str doc (pure ())
  @pprTraceWith desc f x@ is equivalent to @pprTrace desc (f x) x@.
 This allows you to print details from the returned value as well as from
 ambient variables.
pprTraceWith :: Outputable a => String > (a > SDoc) > a > a
+pprTraceWith :: String > (a > SDoc) > a > a
pprTraceWith desc f x = pprTrace desc (f x) x
  @pprTraceIt desc x@ is equivalent to @pprTrace desc (ppr x) x@
diff git a/compiler/utils/Util.hs b/compiler/utils/Util.hs
index 51812cca75cab95932216299d24d5d7502f33b97..d6cea5ca8d7e117cfbe4bf977fc24c5a33fff71c 100644
 a/compiler/utils/Util.hs
+++ b/compiler/utils/Util.hs
@@ 14,6 +14,9 @@ module Util (
ghciTablesNextToCode,
isWindowsHost, isDarwinHost,
+  * Miscellaneous higherorder functions
+ applyWhen, nTimes,
+
 * General list processing
zipEqual, zipWithEqual, zipWith3Equal, zipWith4Equal,
zipLazy, stretchZipWith, zipWithAndUnzip, zipAndUnzip,
@@ 57,9 +60,6 @@ module Util (
takeList, dropList, splitAtList, split,
dropTail, capitalise,
  * For loop
 nTimes,

 * Sorting
sortWith, minWith, nubSort, ordNub,
@@ 222,12 +222,17 @@ isDarwinHost = False
{
************************************************************************
* *
\subsection{A for loop}
+\subsection{Miscellaneous higherorder functions}
* *
************************************************************************
}
  Compose a function with itself n times. (nth rather than twice)
+  Apply a function iff some condition is met.
+applyWhen :: Bool > (a > a) > a > a
+applyWhen True f x = f x
+applyWhen _ _ x = x
+
+  A for loop: Compose a function with itself n times. (nth rather than twice)
nTimes :: Int > (a > a) > (a > a)
nTimes 0 _ = id
nTimes 1 f = f
diff git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 3c951466c94f6fe78deb8060d058a8cdeeaa94e7..5cf3c4b9ccd9f74ac332980154029009c2ac5720 100644
 a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ 15541,49 +15541,6 @@ the user must provide a type signature. ::
foo :: [a] > Int
foo T = 5
.. _multiplecompletepragmas:

Disambiguating between multiple ``COMPLETE`` pragmas


What should happen if there are multiple ``COMPLETE`` sets that apply to a
single set of patterns? Consider this example: ::

 data T = MkT1  MkT2  MkT2Internal
 {# COMPLETE MkT1, MkT2 #}
 {# COMPLETE MkT1, MkT2Internal #}

 f :: T > Bool
 f MkT1 = True
 f MkT2 = False

Which ``COMPLETE`` pragma should be used when checking the coverage of the
patterns in ``f``? If we pick the ``COMPLETE`` set that covers ``MkT1`` and
``MkT2``, then ``f`` is exhaustive, but if we pick the other ``COMPLETE`` set
that covers ``MkT1`` and ``MkT2Internal``, then ``f`` is *not* exhaustive,
since it fails to match ``MkT2Internal``. An intuitive way to solve this
dilemma is to recognize that picking the former ``COMPLETE`` set produces the
fewest number of uncovered pattern clauses, and thus is the better choice.

GHC disambiguates between multiple ``COMPLETE`` sets based on this rationale.
To make things more formal, when the patternmatch checker requests a set of
constructors for some data type constructor ``T``, the checker returns:

* The original set of data constructors for ``T``
* Any ``COMPLETE`` sets of type ``T``

GHC then checks for pattern coverage using each of these sets. If any of these
sets passes the pattern coverage checker with no warnings, then we are done. If
each set produces at least one warning, then GHC must pick one of the sets of
warnings depending on how good the results are. The results are prioritized in
this order:

1. Fewest uncovered clauses
2. Fewest redundant clauses
3. Fewest inaccessible clauses
4. Whether the match comes from the original set of data constructors or from a
 ``COMPLETE`` pragma (prioritizing the former over the latter)

.. _overlappragma:
``OVERLAPPING``, ``OVERLAPPABLE``, ``OVERLAPS``, and ``INCOHERENT`` pragmas
diff git a/libraries/binary b/libraries/binary
index e26b50021c9fdc3d82cd6ed2ee2edb819c7d048b..dfaf780596328c9184758452b78288e8f405fcc1 160000
 a/libraries/binary
+++ b/libraries/binary
@@ 1 +1 @@
Subproject commit e26b50021c9fdc3d82cd6ed2ee2edb819c7d048b
+Subproject commit dfaf780596328c9184758452b78288e8f405fcc1
diff git a/testsuite/tests/deSugar/should_compile/T14546d.hs b/testsuite/tests/deSugar/should_compile/T14546d.hs
new file mode 100644
index 0000000000000000000000000000000000000000..099e64727a996fb24ac6eb9dc4a0dbac2220e9cf
 /dev/null
+++ b/testsuite/tests/deSugar/should_compile/T14546d.hs
@@ 0,0 +1,8 @@
+data D = D deriving Eq
+
+instance Num D where
+ fromInteger _ = D
+
+main = do
+ case 3 :: D of
+ 1 > putStrLn "A"
diff git a/testsuite/tests/deSugar/should_compile/T14546d.stderr b/testsuite/tests/deSugar/should_compile/T14546d.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..db5b9ca2856750aa6588202ffed7b42bb8754ca9
 /dev/null
+++ b/testsuite/tests/deSugar/should_compile/T14546d.stderr
@@ 0,0 +1,9 @@
+
+T14546d.hs:3:10: warning: [Wmissingmethods (in Wdefault)]
+ • No explicit implementation for
+ ‘+’, ‘*’, ‘abs’, ‘signum’, and (either ‘negate’ or ‘’)
+ • In the instance declaration for ‘Num D’
+
+T14546d.hs:7:5: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In a case alternative: Patterns not matched: 3
diff git a/testsuite/tests/deSugar/should_compile/T14773b.stderr b/testsuite/tests/deSugar/should_compile/T14773b.stderr
index 557b10b8f07a820ca4a6d2cd5064994d93f3b397..b204b4f485b42c87d4932420a50ac086027f2a75 100644
 a/testsuite/tests/deSugar/should_compile/T14773b.stderr
+++ b/testsuite/tests/deSugar/should_compile/T14773b.stderr
@@ 1,4 +1,8 @@
+T14773b.hs:4:10: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a pattern binding guards: = ...
+
T14773b.hs:4:10: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In a pattern binding guards:
diff git a/testsuite/tests/deSugar/should_compile/all.T b/testsuite/tests/deSugar/should_compile/all.T
index 5369b3f47316678213f27af1691f219792514d2a..21cf1b058d5d596116c2199f71fe981757975f3f 100644
 a/testsuite/tests/deSugar/should_compile/all.T
+++ b/testsuite/tests/deSugar/should_compile/all.T
@@ 102,6 +102,7 @@ test('T14135', normal, compile, [''])
test('T14546a', normal, compile, ['Wincompletepatterns'])
test('T14546b', normal, compile, ['Wincompletepatterns'])
test('T14546c', normal, compile, ['Wincompletepatterns'])
+test('T14546d', normal, compile, ['Wincompletepatterns'])
test('T14547', normal, compile, ['Wincompletepatterns'])
test('T14773a', normal, compile, ['Wincompletepatterns'])
test('T14773b', normal, compile, ['Wincompletepatterns'])
diff git a/testsuite/tests/dependent/should_compile/KindEqualities.stderr b/testsuite/tests/dependent/should_compile/KindEqualities.stderr
index ad9562eae8501243ab5ed11d4f95463beb54d910..684c1380aa9c20a1e1320b1efb16803f388c3657 100644
 a/testsuite/tests/dependent/should_compile/KindEqualities.stderr
+++ b/testsuite/tests/dependent/should_compile/KindEqualities.stderr
@@ 2,4 +2,5 @@
KindEqualities.hs:25:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘zero’:
 Patterns not matched: (TyApp (TyApp _ _) _)
+ Patterns not matched:
+ (TyApp (TyApp p _) _) where p is not one of {TyInt}
diff git a/testsuite/tests/driver/T8101.stderr b/testsuite/tests/driver/T8101.stderr
index 9f5736044826dbf941fa78a27a77e73d4a34aa55..a486f965e3ecbf0a17ea5a5ce3f60cacb19cdada 100644
 a/testsuite/tests/driver/T8101.stderr
+++ b/testsuite/tests/driver/T8101.stderr
@@ 1,3 +1,4 @@
+
T8101.hs:7:9: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In a case alternative:
diff git a/testsuite/tests/haddock/should_compile_noflag_haddock/haddockSimplUtilsBug.stderr b/testsuite/tests/haddock/should_compile_noflag_haddock/haddockSimplUtilsBug.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..2d3393b249886304df279439a14890a4e56908d1
 /dev/null
+++ b/testsuite/tests/haddock/should_compile_noflag_haddock/haddockSimplUtilsBug.stderr
@@ 0,0 +1,4 @@
+
+haddockSimplUtilsBug.hs:28:7: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a case alternative: _ > ...
diff git a/testsuite/tests/parser/should_compile/read044.stderr b/testsuite/tests/parser/should_compile/read044.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..d459248082ad271c3a3027d3d448c6a782626fca
 /dev/null
+++ b/testsuite/tests/parser/should_compile/read044.stderr
@@ 0,0 +1,4 @@
+
+read044.hs:5:13: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a case alternative: False > ...
diff git a/testsuite/tests/patsyn/should_run/all.T b/testsuite/tests/patsyn/should_run/all.T
index 57bc41fc73c44515d73d496ff49d6639293352b7..90f577174ec948790e9f7cd853047c9d91a709c9 100644
 a/testsuite/tests/patsyn/should_run/all.T
+++ b/testsuite/tests/patsyn/should_run/all.T
@@ 13,7 +13,7 @@ test('unboxedwrapper', normal, compile_and_run, [''])
test('recordsrun', normal, compile_and_run, [''])
test('ghci', just_ghci, ghci_script, ['ghci.script'])
test('T11985', just_ghci, ghci_script, ['T11985.script'])
test('T11224', normal, compile_and_run, [''])
+test('T11224', normal, compile_and_run, ['Wincompletepatterns Woverlappingpatterns'])
# we omit profasm/profthreaded because it doesn't bring much to the table but
# introduces its share of complexity, as the test as it is fails with
# profasm:
diff git a/testsuite/tests/pmcheck/complete_sigs/T13021.hs b/testsuite/tests/pmcheck/complete_sigs/T13021.hs
new file mode 100644
index 0000000000000000000000000000000000000000..cb2a725a982b91c7de5ab555661d8c76b0579005
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13021.hs
@@ 0,0 +1,11 @@
+{# OPTIONS_GHC Woverlappingpatterns Wincompletepatterns #}
+
+module Lib where
+
+data T = A  B  C
+
+{# COMPLETE B #}
+
+foo :: T > ()
+foo A = ()
+foo B = ()
diff git a/testsuite/tests/pmcheck/complete_sigs/T13363a.hs b/testsuite/tests/pmcheck/complete_sigs/T13363a.hs
new file mode 100644
index 0000000000000000000000000000000000000000..1d614b7d05da1ee90ee81134f8dc4bb2a6421435
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13363a.hs
@@ 0,0 +1,16 @@
+{# OPTIONS_GHC Wincompletepatterns Woverlappingpatterns #}
+{# LANGUAGE PatternSynonyms #}
+
+module Lib where
+
+data Boolean = F  T
+ deriving Eq
+
+pattern TooGoodToBeTrue :: Boolean
+pattern TooGoodToBeTrue = T
+{# COMPLETE F, TooGoodToBeTrue #}
+
+catchAll :: Boolean > Int
+catchAll F = 0
+catchAll TooGoodToBeTrue = 1
+catchAll _ = error "impossible"
diff git a/testsuite/tests/pmcheck/complete_sigs/T13363a.stderr b/testsuite/tests/pmcheck/complete_sigs/T13363a.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..a91b02806a29854adaf27712fb2f328ed3e5d242
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13363a.stderr
@@ 0,0 +1,4 @@
+
+T13363a.hs:16:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘catchAll’: catchAll _ = ...
diff git a/testsuite/tests/pmcheck/complete_sigs/T13363b.hs b/testsuite/tests/pmcheck/complete_sigs/T13363b.hs
new file mode 100644
index 0000000000000000000000000000000000000000..0ef350e491a5cb6fa3149d50c5b38289d66c5c2c
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13363b.hs
@@ 0,0 +1,17 @@
+{# OPTIONS_GHC Wincompletepatterns Woverlappingpatterns #}
+{# LANGUAGE PatternSynonyms #}
+
+module Lib where
+
+data T = A  B  C
+ deriving Eq
+
+pattern BC :: T
+pattern BC = C
+
+{# COMPLETE A, BC #}
+
+f A = 1
+f B = 2
+f BC = 3
+f _ = error "impossible"
diff git a/testsuite/tests/pmcheck/complete_sigs/T13363b.stderr b/testsuite/tests/pmcheck/complete_sigs/T13363b.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..541c084be16f87630fa198b487c5f41774b2b0b2
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13363b.stderr
@@ 0,0 +1,4 @@
+
+T13363b.hs:17:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘f’: f _ = ...
diff git a/testsuite/tests/pmcheck/complete_sigs/T13717.hs b/testsuite/tests/pmcheck/complete_sigs/T13717.hs
new file mode 100644
index 0000000000000000000000000000000000000000..e9460371c86357bd5ae32d02e4e87924b3ec4f30
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13717.hs
@@ 0,0 +1,41 @@
+{# OPTIONS_GHC Wincompletepatterns #}
+{# LANGUAGE DataKinds #}
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ScopedTypeVariables #}
+{# LANGUAGE GADTs #}
+{# LANGUAGE KindSignatures #}
+{# LANGUAGE ViewPatterns #}
+{# LANGUAGE Trustworthy #}
+{# LANGUAGE EmptyCase #}
+
+module Fin (Nat (..), Fin (FZ, FS)) where
+import Numeric.Natural
+import Unsafe.Coerce
+
+data Nat = Z  S Nat
+
+ Fin *must* be exported abstractly (or placed in an Unsafe
+ module) to maintain type safety.
+newtype Fin (n :: Nat) = Fin Natural
+
+data FinView n where
+ VZ :: FinView ('S n)
+ VS :: !(Fin n) > FinView ('S n)
+
+viewFin :: Fin n > FinView n
+viewFin (Fin 0) = unsafeCoerce VZ
+viewFin (Fin n) = unsafeCoerce (VS (Fin (n  1)))
+
+pattern FZ :: () => n ~ 'S m => Fin n
+pattern FZ < (viewFin > VZ) where
+ FZ = Fin 0
+
+pattern FS :: () => n ~ 'S m => Fin m > Fin n
+pattern FS m < (viewFin > VS m) where
+ FS (Fin m) = Fin (1 + m)
+
+{# COMPLETE FZ, FS #}
+
+finZAbsurd :: Fin 'Z > a
+finZAbsurd x = case x of
+
diff git a/testsuite/tests/pmcheck/complete_sigs/T13964.hs b/testsuite/tests/pmcheck/complete_sigs/T13964.hs
new file mode 100644
index 0000000000000000000000000000000000000000..36a87a9a251034e21090e2d048a4bb5f2802d7db
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13964.hs
@@ 0,0 +1,20 @@
+{# OPTIONS_GHC Wincompletepatterns #}
+
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ViewPatterns #}
+
+module Bug (Boolean(F, TooGoodToBeTrue), catchAll) where
+
+data Boolean = F  T
+ deriving Eq
+
+pattern TooGoodToBeTrue :: Boolean
+pattern TooGoodToBeTrue < ((== T) > True)
+ where
+ TooGoodToBeTrue = T
+{# COMPLETE F, TooGoodToBeTrue #}
+
+catchAll :: Boolean > Int
+catchAll F = 0
+ catchAll TooGoodToBeTrue = 1
+
diff git a/testsuite/tests/pmcheck/complete_sigs/T13964.stderr b/testsuite/tests/pmcheck/complete_sigs/T13964.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..606756a783974b05ee6d8d578815e97866c9ab45
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13964.stderr
@@ 0,0 +1,4 @@
+
+T13964.hs:18:1: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In an equation for ‘catchAll’: Patterns not matched: T
diff git a/testsuite/tests/pmcheck/complete_sigs/T13965.hs b/testsuite/tests/pmcheck/complete_sigs/T13965.hs
new file mode 100644
index 0000000000000000000000000000000000000000..ac18dad115299c1bd119e1746369277cf5bedbf5
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13965.hs
@@ 0,0 +1,20 @@
+{# OPTIONS_GHC Woverlappingpatterns #}
+
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ViewPatterns #}
+module Bug (Boolean(F, TooGoodToBeTrue), catchAll) where
+
+data Boolean = F  T
+ deriving Eq
+
+pattern TooGoodToBeTrue :: Boolean
+pattern TooGoodToBeTrue < ((== T) > True)
+ where
+ TooGoodToBeTrue = T
+{# COMPLETE F, TooGoodToBeTrue #}
+
+catchAll :: Boolean > Int
+catchAll F = 0
+catchAll TooGoodToBeTrue = 1
+catchAll F = 2
+
diff git a/testsuite/tests/pmcheck/complete_sigs/T13965.stderr b/testsuite/tests/pmcheck/complete_sigs/T13965.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..78aaa9490a5607c8cce5f5b5211188a0c77c463e
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T13965.stderr
@@ 0,0 +1,4 @@
+
+T13965.hs:19:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘catchAll’: catchAll F = ...
diff git a/testsuite/tests/pmcheck/complete_sigs/T14059b.hs b/testsuite/tests/pmcheck/complete_sigs/T14059b.hs
new file mode 100644
index 0000000000000000000000000000000000000000..52f3593ecdc01cda81239ffd6366091e2bea7186
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T14059b.hs
@@ 0,0 +1,27 @@
+{# LANGUAGE GADTs #}
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ScopedTypeVariables #}
+{# LANGUAGE TypeFamilies #}
+{# LANGUAGE TypeInType #}
+{# LANGUAGE TypeOperators #}
+{# OPTIONS_GHC Wincompletepatterns #}
+module Bug where
+
+data family Sing (a :: k)
+
+data instance Sing (z :: Bool) where
+ SFalse :: Sing False
+ STrue :: Sing True
+
+pattern STooGoodToBeTrue :: forall (z :: Bool). ()
+ => z ~ True
+ => Sing z
+pattern STooGoodToBeTrue = STrue
+{# COMPLETE SFalse, STooGoodToBeTrue #}
+
+wibble :: Sing (z :: Bool) > Bool
+wibble STrue = True
+
+wobble :: Sing (z :: Bool) > Bool
+wobble STooGoodToBeTrue = True
+
diff git a/testsuite/tests/pmcheck/complete_sigs/T14059b.stderr b/testsuite/tests/pmcheck/complete_sigs/T14059b.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..8c8569c6380cc9aec9c3319990c2cc6e21687bfa
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T14059b.stderr
@@ 0,0 +1,10 @@
+
+T14059b.hs:23:1: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In an equation for ‘wibble’:
+ Patterns not matched: p where p is not one of {STrue}
+
+T14059b.hs:26:1: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In an equation for ‘wobble’:
+ Patterns not matched: p where p is not one of {STooGoodToBeTrue}
diff git a/testsuite/tests/pmcheck/complete_sigs/T14851.hs b/testsuite/tests/pmcheck/complete_sigs/T14851.hs
new file mode 100644
index 0000000000000000000000000000000000000000..d5baac58a26cf907f135759f6e681305cafde6e0
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T14851.hs
@@ 0,0 +1,21 @@
+{# OPTIONS_GHC Woverlappingpatterns #}
+
+{# LANGUAGE PatternSynonyms, ViewPatterns #}
+
+module Bug where
+
+import Type.Reflection
+
+pattern X arg < (checkFun > arg)
+
+checkFun :: TypeRep fun > a
+checkFun = undefined
+
+f x = case (x, True) of
+ (X _, _) > 5
+ _ > 6
+
+g x = case x of
+ (X _) > 5
+ _ > 6
+
diff git a/testsuite/tests/pmcheck/complete_sigs/T17149.hs b/testsuite/tests/pmcheck/complete_sigs/T17149.hs
new file mode 100644
index 0000000000000000000000000000000000000000..ba68154f3c554a7d73f5060146105eb3f3d70f72
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T17149.hs
@@ 0,0 +1,21 @@
+{# OPTIONS_GHC Wincompletepatterns #}
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ViewPatterns #}
+{# LANGUAGE MultiParamTypeClasses #}
+
+module Bug where
+
+class Member a b where
+ prj :: b > Maybe a
+
+pattern P :: Member a b => a > b
+pattern P a < (prj > Just a)
+
+{# COMPLETE P :: Bool #}
+
+  Trying to instantiate P with 0 type arguments doesn't work, it takes 2.
+ This seemingly unrelated fact, only relevant through the COMPLETE set, may
+ not lead the compiler to crash or do shady stuff.
+fun :: Bool > ()
+fun True = ()
+fun _ = ()
diff git a/testsuite/tests/pmcheck/complete_sigs/all.T b/testsuite/tests/pmcheck/complete_sigs/all.T
index d58c182f8eda6236a6323bae7a44683007a43cfa..7e70f3aa106965e77d1d771681dc34266de7118a 100644
 a/testsuite/tests/pmcheck/complete_sigs/all.T
+++ b/testsuite/tests/pmcheck/complete_sigs/all.T
@@ 13,5 +13,14 @@ test('completesig12', normal, compile, [''])
test('completesig13', normal, compile, [''])
test('completesig14', normal, compile, [''])
test('completesig15', normal, compile_fail, [''])
+test('T13021', normal, compile, [''])
+test('T13363a', normal, compile, [''])
+test('T13363b', normal, compile, [''])
+test('T13717', expect_broken('13717'), compile, [''])
+test('T13964', normal, compile, [''])
+test('T13965', normal, compile, [''])
test('T14059a', normal, compile, [''])
test('T14253', expect_broken(14253), compile, [''])
+test('T14059b', expect_broken('14059'), compile, [''])
+test('T14253', normal, compile, [''])
+test('T14851', normal, compile, [''])
+test('T17149', normal, compile, [''])
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig01.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig01.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..63cff81b5bd0313b50f2211b16f76a96e757a8a3
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig01.stderr
@@ 0,0 +1,4 @@
+
+completesig01.hs:20:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘b’: b C = ...
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig02.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig02.stderr
index 25b24fd8364c5168b299f76e719697608c1f784f..6da127a6741221493b0506fef36561e120c6eab2 100644
 a/testsuite/tests/pmcheck/complete_sigs/completesig02.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig02.stderr
@@ 1,4 +1,4 @@
completesig02.hs:10:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
 In an equation for ‘a’: Patterns not matched: _
+ In an equation for ‘a’: Patterns not matched: ()
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
index 50bc9bfebf411f6ef3c42fb17f1efe202b06fda1..66a7604ca947b1c8d4036a25b81a3b2ab07f1278 100644
 a/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
@@ 18,8 +18,8 @@ completesig06.hs:23:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘m4’:
Patterns not matched:
 B D
A D
+ B D
completesig06.hs:29:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig07.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig07.stderr
index bf5edb92056695742930f26c7a2cd3d2182b2ded..b8e56892c9dd9cc08f7296c9136e7fa0c847646b 100644
 a/testsuite/tests/pmcheck/complete_sigs/completesig07.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig07.stderr
@@ 1,4 +1,12 @@
+completesig07.hs:14:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m1’: m1 A = ...
+
+completesig07.hs:20:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m2’: m2 A D = ...
+
completesig07.hs:23:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘m3’:
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig08.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig08.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..2c7d6658e74419a94e14b7486d6112ed4482ca68
 /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig08.stderr
@@ 0,0 +1,20 @@
+
+completesig08.hs:15:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m1’: m1 C = ...
+
+completesig08.hs:20:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m2’: m2 C D = ...
+
+completesig08.hs:25:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m3’: m3 A E = ...
+
+completesig08.hs:26:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m3’: m3 A F = ...
+
+completesig08.hs:30:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m4’: m4 E = ...
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig10.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig10.stderr
index 3d97bb47d79a04eb1bd05ca8460fcc130aca8c3f..36b367068c8928940c5cd60db387c0ad6b44d709 100644
 a/testsuite/tests/pmcheck/complete_sigs/completesig10.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig10.stderr
@@ 1,4 +1,12 @@
+completesig10.hs:15:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m1’: m1 C = ...
+
+completesig10.hs:16:1: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In an equation for ‘m1’: m1 D = ...
+
completesig10.hs:20:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘m2’: Patterns not matched: A
diff git a/testsuite/tests/pmcheck/complete_sigs/completesig11.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig11.stderr
index 810707100859568fc7d6def960cff05c1cc3f2ef..d7bf2f3079aeeda3cb997a4bde2022f00e09a366 100644
 a/testsuite/tests/pmcheck/complete_sigs/completesig11.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig11.stderr
@@ 1,4 +1,4 @@
completesig11.hs:14:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
 In an equation for ‘m1’: Patterns not matched: BS
+ In an equation for ‘m1’: Patterns not matched: B
diff git a/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.hs b/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.hs
new file mode 100644
index 0000000000000000000000000000000000000000..0933baae96363f82663aa0d659b66974902217c0
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.hs
@@ 0,0 +1,8 @@
+  Ideally, we want the pattern match in `f` to be discovered as exhaustive,
+ with a redundant match on the second clause.
+module Lib where
+
+f :: ()
+f = case (True, False) of
+ (True, False) > ()
+ (True, True) > ()
diff git a/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.stderr b/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..1f093239568571bbdd932f837b352121e0ebdb73
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/CaseOfKnownCon.stderr
@@ 0,0 +1,4 @@
+
+CaseOfKnownCon.hs:8:3: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a case alternative: (True, True) > ...
diff git a/testsuite/tests/pmcheck/should_compile/CyclicSubst.hs b/testsuite/tests/pmcheck/should_compile/CyclicSubst.hs
index fde022c5cb5d4e74f78feebd0c0f9b5727cbdc35..3f37b66b54afdf0fe5d1f0d63b91ecc3984427ae 100644
 a/testsuite/tests/pmcheck/should_compile/CyclicSubst.hs
+++ b/testsuite/tests/pmcheck/should_compile/CyclicSubst.hs
@@ 1,5 +1,5 @@
  The following match demonstrates why we need to detect cyclic solutions
 when extending 'TmOracle.tm_pos'.
+ when extending 'PmOracle.tm_pos'.

 TLDR; solving @x :> y@ where @x@ is the representative of @y@'s equivalence
 class can easily lead to a cycle in the substitution.
diff git a/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
index 53be507400dd30ec7238610f36abfb3624fadb8c..8cedcddaf5388e174244069f6251bc9a90251e2e 100644
 a/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
@@ 1,3 +1,4 @@
+
EmptyCase005.hs:24:8: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In a case alternative: Patterns not matched: (Void3 _)
diff git a/testsuite/tests/pmcheck/should_compile/PmExprVars.hs b/testsuite/tests/pmcheck/should_compile/PmExprVars.hs
index 7b17cd5b664cdd07672c095e6411b3c5b0005cb5..fc95d432466ad657ec195bd7991c8834b60e5653 100644
 a/testsuite/tests/pmcheck/should_compile/PmExprVars.hs
+++ b/testsuite/tests/pmcheck/should_compile/PmExprVars.hs
@@ 1,20 +1,5 @@
module PmExprVars where
  Demonstrates why we can't lower constructors as flexible meta variables.
 If we did, we'd get a warning that cases 1 and 2 were redundant, implying
 cases 0 and 3 are not. Arguably this might be better than not warning at
 all, but it's very surprising having to supply the third case but not the
 first two cases. And it's probably buggy somwhere else. Delete this when we
 detect that all but the last case is redundant.
consAreRigid :: Int
consAreRigid = case False of
 False > case False of
 False > 0
 True > 1
 True > case False of
 False > 2
 True > 3

data D a = A  B
class C a where
diff git a/testsuite/tests/pmcheck/should_compile/T11336b.hs b/testsuite/tests/pmcheck/should_compile/T11336b.hs
new file mode 100644
index 0000000000000000000000000000000000000000..37734eabb6a042ec8b32f06740cb4d334677ce69
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T11336b.hs
@@ 0,0 +1,26 @@
+{# LANGUAGE PatternSynonyms #}
+{# LANGUAGE ViewPatterns #}
+
+  Derived from T11336. Note that the pattern synonym makes it so that the
+ pattern match checker can't see any complete sets: It only sees @s a@
+ instead of @Proxy a@ for the match in 'PProxy'.
+module Bug where
+
+import Data.Proxy
+
+class Prj s where
+ prj :: Proxy a > s a
+
+instance Prj Proxy where
+ prj = id
+
+pattern PProxy :: Prj s => s a > Proxy a
+pattern PProxy s < (prj > s)
+
+  Although this is technically a complete match, the pattern match checker
+ can't in general look through the pattern synonym. So, it should warn that
+ some pattern wasn't matched. It should still flag the redundant second
+ clause, though!
+fun :: Proxy a > ()
+fun (PProxy Proxy) = ()
+fun (PProxy Proxy) = ()
diff git a/testsuite/tests/pmcheck/should_compile/T11336b.stderr b/testsuite/tests/pmcheck/should_compile/T11336b.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..5d479c37563a3ef7b7d8c058a3f3cb46e021c9d7
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T11336b.stderr
@@ 0,0 +1,4 @@
+
+T11336b.hs:25:1: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In an equation for ‘fun’: Patterns not matched: _
diff git a/testsuite/tests/pmcheck/should_compile/T15753c.hs b/testsuite/tests/pmcheck/should_compile/T15753c.hs
new file mode 100644
index 0000000000000000000000000000000000000000..d04a889e004d94abe6d552fbd683afa69014509c
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T15753c.hs
@@ 0,0 +1,51 @@
+{# LANGUAGE DataKinds #}
+{# LANGUAGE EmptyCase #}
+{# LANGUAGE GADTs #}
+{# LANGUAGE TypeFamilies #}
+{# LANGUAGE TypeOperators #}
+{# LANGUAGE UndecidableInstances #}
+{# OPTIONS_GHC Wincompletepatterns #}
+module Bug where
+
+import Data.Kind (Type)
+import Data.Type.Equality ((:~:)(..))
+import Data.Void (Void)
+
+data SBool :: Bool > Type where
+ SFalse :: SBool False
+ STrue :: SBool True
+data SUnit :: () > Type where
+ SUnit :: SUnit '()
+
+type family IsUnit (u :: ()) :: Bool where
+ IsUnit '() = True
+
+sIsUnit :: SUnit u > SBool (IsUnit u)
+sIsUnit SUnit = STrue
+
+type family If (c :: Bool) (t :: Type) (f :: Type) :: Type where
+ If True t _ = t
+ If False _ f = f
+
+type family F (u1 :: ()) (u2 :: ()) :: Type where
+ F u1 u2 =
+ If (IsUnit u1) (Case u2) Int
+
+type family Case (u :: ()) :: Type where
+ Case '() = Int
+
+g1 :: F u1 u2 :~: Char
+ > SUnit u1 > SUnit u2
+ > Void
+g1 Refl su1 su2
+  STrue < sIsUnit su1
+ = case su2 of {}
+
+g2 :: F u1 u2 :~: Char
+ > SUnit u1 > SUnit u2
+ > Void
+g2 Refl su1 su2
+ = case sIsUnit su1 of
+ STrue >
+ case su2 of {}
+
diff git a/testsuite/tests/pmcheck/should_compile/T15753d.hs b/testsuite/tests/pmcheck/should_compile/T15753d.hs
new file mode 100644
index 0000000000000000000000000000000000000000..5935cf7d9d746a830a12d2c0b2e3cfcddd7dfa74
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T15753d.hs
@@ 0,0 +1,95 @@
+{# LANGUAGE DataKinds #}
+{# LANGUAGE EmptyCase #}
+{# LANGUAGE GADTs #}
+{# LANGUAGE PolyKinds #}
+{# LANGUAGE RankNTypes #}
+{# LANGUAGE ScopedTypeVariables #}
+{# LANGUAGE TypeFamilies #}
+{# LANGUAGE TypeOperators #}
+{# LANGUAGE UndecidableInstances #}
+{# OPTIONS_GHC Wall Wnountickedpromotedconstructors #}
+module Bug where
+
+import Data.Kind
+import Data.Type.Bool
+import Data.Type.Equality ((:~:)(..))
+import Data.Void
+
+data family Sing :: forall k. k > Type
+data instance Sing :: Bool > Type where
+ SFalse :: Sing False
+ STrue :: Sing True
+data instance Sing :: forall a. [a] > Type where
+ SNil :: Sing '[]
+ SCons :: Sing x > Sing xs > Sing (x:xs)
+data instance Sing :: forall a b. (a, b) > Type where
+ STuple2 :: Sing x > Sing y > Sing '(x, y)
+newtype instance Sing (f :: k1 ~> k2) =
+ SLambda { (@@) :: forall t. Sing t > Sing (f @@ t) }
+
+data TyFun :: Type > Type > Type
+type a ~> b = TyFun a b > Type
+infixr 0 ~>
+type family (f :: k1 ~> k2) @@ (x :: k1) :: k2
+infixl 9 @@
+
+newtype Map k v = MkMap [(k, v)]
+data instance Sing :: forall k v. Map k v > Type where
+ SMkMap :: Sing x > Sing (MkMap x)
+
+type family MapEmpty :: Map k v where
+ MapEmpty = MkMap '[]
+
+type family MapInsertWith (f :: v ~> v ~> v) (new_k :: k) (new_v :: v) (m :: Map k v) :: Map k v where
+ MapInsertWith _ new_k new_v (MkMap '[]) = MkMap '[ '(new_k, new_v)]
+ MapInsertWith f new_k new_v (MkMap ('(old_k,old_v):old_kvs)) =
+ If (old_k == new_k)
+ (MkMap ('(new_k,f @@ new_v @@ old_v):old_kvs))
+ (Case (MapInsertWith f new_k new_v (MkMap old_kvs)) old_k old_v)
+
+type family Case (m :: Map k v) (old_k :: k) (old_v :: v) :: Map k v where
+ Case (MkMap kvs) old_k old_v = MkMap ('(old_k,old_v) : kvs)
+
+sMapInsertWith :: forall k v (f :: v ~> v ~> v) (new_k :: k) (new_v :: v) (m :: Map k v).
+ SEq k
+ => Sing f > Sing new_k > Sing new_v > Sing m
+ > Sing (MapInsertWith f new_k new_v m)
+sMapInsertWith _ snew_k snew_v (SMkMap SNil) = SMkMap (STuple2 snew_k snew_v `SCons` SNil)
+sMapInsertWith sf snew_k snew_v (SMkMap (STuple2 sold_k sold_v `SCons` sold_kvs)) =
+ case sold_k %== snew_k of
+ STrue > SMkMap (STuple2 snew_k (sf @@ snew_v @@ sold_v) `SCons` sold_kvs)
+ SFalse >
+ case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of
+ SMkMap skvs > SMkMap (STuple2 sold_k sold_v `SCons` skvs)
+
+class PEq a where
+ type (x :: a) == (y :: a) :: Bool
+class SEq a where
+ (%==) :: forall (x :: a) (y :: a).
+ Sing x > Sing y > Sing (x == y)
+
+mapInsertWithNonEmpty1 :: forall k v (f :: v ~> v ~> v) (old_k :: k) (old_v :: v) (old_kvs :: [(k,v)])
+ (new_k :: k) (new_v :: v) (m :: Map k v).
+ SEq k
+ => Sing f > Sing new_k > Sing new_v > Sing m
+ > m :~: MkMap ('(old_k,old_v) : old_kvs)
+ > MapInsertWith f new_k new_v m :~: MapEmpty
+ > Void
+mapInsertWithNonEmpty1 sf snew_k snew_v (SMkMap sm) Refl Refl
+  STuple2 sold_k _ `SCons` sold_kvs < sm
+ , SFalse < sold_k %== snew_k
+ = case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of {}
+
+mapInsertWithNonEmpty2 :: forall k v (f :: v ~> v ~> v) (old_k :: k) (old_v :: v) (old_kvs :: [(k,v)])
+ (new_k :: k) (new_v :: v) (m :: Map k v).
+ SEq k
+ => Sing f > Sing new_k > Sing new_v > Sing m
+ > m :~: MkMap ('(old_k,old_v) : old_kvs)
+ > MapInsertWith f new_k new_v m :~: MapEmpty
+ > Void
+mapInsertWithNonEmpty2 sf snew_k snew_v (SMkMap sm) Refl Refl
+  STuple2 sold_k _ `SCons` sold_kvs < sm
+ = case sold_k %== snew_k of
+ SFalse >
+ case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of {}
+
diff git a/testsuite/tests/pmcheck/should_compile/T17096.hs b/testsuite/tests/pmcheck/should_compile/T17096.hs
new file mode 100755
index 0000000000000000000000000000000000000000..5fc4e6e879497f5e2f67f640d9dce6fb507cda09
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T17096.hs
@@ 0,0 +1,319 @@
+{# language PatternSynonyms #}
+ Taken from the Dhall library
+module T17096 where
+
+data Expr s a
+ = Const String
+  Var Int
+  Lam String (Expr s a) (Expr s a)
+  Pi String (Expr s a) (Expr s a)
+  App (Expr s a) (Expr s a)
+  Let String (Maybe (Expr s a)) (Expr s a) (Expr s a)
+  Annot (Expr s a) (Expr s a)
+  Bool
+  BoolLit Bool
+  BoolAnd (Expr s a) (Expr s a)
+  BoolOr (Expr s a) (Expr s a)
+  BoolEQ (Expr s a) (Expr s a)
+  BoolNE (Expr s a) (Expr s a)
+  BoolIf (Expr s a) (Expr s a) (Expr s a)
+  Natural
+  NaturalLit Integer
+  NaturalFold
+  NaturalBuild
+  NaturalIsZero
+  NaturalEven
+  NaturalOdd
+  NaturalToInteger
+  NaturalShow
+  NaturalSubtract
+  NaturalPlus (Expr s a) (Expr s a)
+  NaturalTimes (Expr s a) (Expr s a)
+  Integer
+  IntegerLit Integer
+  IntegerShow
+  IntegerToDouble
+  Double
+  DoubleLit Double
+  DoubleShow
+  String
+  StringLit String
+  StringAppend (Expr s a) (Expr s a)
+  StringShow
+  List
+  ListLit (Maybe (Expr s a)) [Expr s a]
+  ListAppend (Expr s a) (Expr s a)
+  ListBuild
+  ListFold
+  ListLength
+  ListHead
+  ListLast
+  ListIndexed
+  ListReverse
+  Optional
+  Some (Expr s a)
+  None
+  OptionalFold
+  OptionalBuild
+  Record [(String, Expr s a)]
+  RecordLit [(String, Expr s a)]
+  Union [(String, Maybe (Expr s a))]
+  Combine (Expr s a) (Expr s a)
+  CombineTypes (Expr s a) (Expr s a)
+  Prefer (Expr s a) (Expr s a)
+  Merge (Expr s a) (Expr s a) (Maybe (Expr s a))
+  ToMap (Expr s a) (Maybe (Expr s a))
+  Field (Expr s a) String
+  Project (Expr s a) (Either [String] (Expr s a))
+  Assert (Expr s a)
+  Equivalent (Expr s a) (Expr s a)
+  Note s (Expr s a)
+  ImportAlt (Expr s a) (Expr s a)
+  Embed a
+
+isNormalized :: Eq a => Expr s a > Bool
+isNormalized = loop
+ where
+ loop e = case e of
+ Const _ > True
+ Var _ > True
+ Lam _ a b > loop a && loop b
+ Pi _ a b > loop a && loop b
+ App f a > loop f && loop a && case App f a of
+ App (Lam _ _ _) _ > False
+ App (App ListBuild _) (App (App ListFold _) _) > False
+ App NaturalBuild (App NaturalFold _) > False
+ App (App OptionalBuild _) (App (App OptionalFold _) _) > False
+ App (App (App (App NaturalFold (NaturalLit _)) _) _) _ > False
+ App NaturalFold (NaturalLit _) > False
+ App NaturalBuild _ > False
+ App NaturalIsZero (NaturalLit _) > False
+ App NaturalEven (NaturalLit _) > False
+ App NaturalOdd (NaturalLit _) > False
+ App NaturalShow (NaturalLit _) > False
+ App (App NaturalSubtract (NaturalLit _)) (NaturalLit _) > False
+ App (App NaturalSubtract (NaturalLit 0)) _ > False
+ App (App NaturalSubtract _) (NaturalLit 0) > False
+ App (App NaturalSubtract x) y > not (undefined x y)
+ App NaturalToInteger (NaturalLit _) > False
+ App IntegerShow (IntegerLit _) > False
+ App IntegerToDouble (IntegerLit _) > False
+ App DoubleShow (DoubleLit _) > False
+ App (App OptionalBuild _) _ > False
+ App (App ListBuild _) _ > False
+ App (App (App (App (App ListFold _) (ListLit _ _)) _) _) _ >
+ False
+ App (App ListLength _) (ListLit _ _) > False
+ App (App ListHead _) (ListLit _ _) > False
+ App (App ListLast _) (ListLit _ _) > False
+ App (App ListIndexed _) (ListLit _ _) > False
+ App (App ListReverse _) (ListLit _ _) > False
+ App (App (App (App (App OptionalFold _) (Some _)) _) _) _ >
+ False
+ App (App (App (App (App OptionalFold _) (App None _)) _) _) _ >
+ False
+ App StringShow (StringLit _) >
+ False
+ _ > True
+ Let _ _ _ _ > False
+ Annot _ _ > False
+ Bool > True
+ BoolLit _ > True
+ BoolAnd x y > loop x && loop y && decide x y
+ where
+ decide (BoolLit _) _ = False
+ decide _ (BoolLit _) = False
+ decide l r = not (undefined l r)
+ BoolOr x y > loop x && loop y && decide x y
+ where
+ decide (BoolLit _) _ = False
+ decide _ (BoolLit _) = False
+ decide l r = not (undefined l r)
+ BoolEQ x y > loop x && loop y && decide x y
+ where
+ decide (BoolLit True) _ = False
+ decide _ (BoolLit True) = False
+ decide l r = not (undefined l r)
+ BoolNE x y > loop x && loop y && decide x y
+ where
+ decide (BoolLit False) _ = False
+ decide _ (BoolLit False ) = False
+ decide l r = not (undefined l r)
+ BoolIf x y z >
+ loop x && loop y && loop z && decide x y z
+ where
+ decide (BoolLit _) _ _ = False
+ decide _ (BoolLit True) (BoolLit False) = False
+ decide _ l r = not (undefined l r)
+ Natural > True
+ NaturalLit _ > True
+ NaturalFold > True
+ NaturalBuild > True
+ NaturalIsZero > True
+ NaturalEven > True
+ NaturalOdd > True
+ NaturalShow > True
+ NaturalSubtract > True
+ NaturalToInteger > True
+ NaturalPlus x y > loop x && loop y && decide x y
+ where
+ decide (NaturalLit 0) _ = False
+ decide _ (NaturalLit 0) = False
+ decide (NaturalLit _) (NaturalLit _) = False
+ decide _ _ = True
+ NaturalTimes x y > loop x && loop y && decide x y
+ where
+ decide (NaturalLit 0) _ = False
+ decide _ (NaturalLit 0) = False
+ decide (NaturalLit 1) _ = False
+ decide _ (NaturalLit 1) = False
+ decide (NaturalLit _) (NaturalLit _) = False
+ decide _ _ = True
+ Integer > True
+ IntegerLit _ > True
+ IntegerShow > True
+ IntegerToDouble > True
+ Double > True
+ DoubleLit _ > True
+ DoubleShow > True
+ String > True
+ StringLit _ > False
+ StringAppend _ _ > False
+ StringShow > True
+ List > True
+ ListLit t es > all loop t && all loop es
+ ListAppend x y > loop x && loop y && decide x y
+ where
+ decide (ListLit _ m) _  null m = False
+ decide _ (ListLit _ n)  null n = False
+ decide (ListLit _ _) (ListLit _ _) = False
+ decide _ _ = True
+ ListBuild > True
+ ListFold > True
+ ListLength > True
+ ListHead > True
+ ListLast > True
+ ListIndexed > True
+ ListReverse > True
+ Optional > True
+ Some a > loop a
+ None > True
+ OptionalFold > True
+ OptionalBuild > True
+ Record kts > undefined kts && all loop (map snd kts)
+ RecordLit kvs > undefined kvs && all loop (map snd kvs)
+ Union kts > undefined kts && all (all loop) (map snd kts)
+ Combine x y > loop x && loop y && decide x y
+ where
+ decide (RecordLit m) _  null m = False
+ decide _ (RecordLit n)  null n = False
+ decide (RecordLit _) (RecordLit _) = False
+ decide _ _ = True
+ CombineTypes x y > loop x && loop y && decide x y
+ where
+ decide (Record m) _  null m = False
+ decide _ (Record n)  null n = False
+ decide (Record _) (Record _) = False
+ decide _ _ = True
+ Prefer x y > loop x && loop y && decide x y
+ where
+ decide (RecordLit m) _  null m = False
+ decide _ (RecordLit n)  null n = False
+ decide (RecordLit _) (RecordLit _) = False
+ decide l r = not (undefined l r)
+ Merge x y t > loop x && loop y && all loop t
+ ToMap x t > case x of
+ RecordLit _ > False
+ _ > loop x && all loop t
+ Field r k > case r of
+ RecordLit _ > False
+ Project _ _ > False
+ Prefer (RecordLit m) _ > map fst m == [k] && loop r
+ Prefer _ (RecordLit _) > False
+ Combine (RecordLit m) _ > map fst m == [k] && loop r
+ Combine _ (RecordLit m) > map fst m == [k] && loop r
+ _ > loop r
+ Project r p > loop r &&
+ case p of
+ Left s > case r of
+ RecordLit _ > False
+ _ > not (null s) && undefined s
+ Right e' > case e' of
+ Record _ > False
+ _ > loop e'
+ Assert t > loop t
+ Equivalent l r > loop l && loop r
+ Note _ e' > loop e'
+ ImportAlt l _r > loop l
+ Embed _ > True
+
+{# COMPLETE
+ Let'
+ , Const
+ , Var
+ , Lam
+ , Pi
+ , App
+ , Annot
+ , Bool
+ , BoolLit
+ , BoolAnd
+ , BoolOr
+ , BoolEQ
+ , BoolNE
+ , BoolIf
+ , Natural
+ , NaturalLit
+ , NaturalFold
+ , NaturalBuild
+ , NaturalIsZero
+ , NaturalEven
+ , NaturalOdd
+ , NaturalToInteger
+ , NaturalShow
+ , NaturalSubtract
+ , NaturalPlus
+ , NaturalTimes
+ , Integer
+ , IntegerLit
+ , IntegerShow
+ , IntegerToDouble
+ , Double
+ , DoubleLit
+ , DoubleShow
+ , String
+ , StringLit
+ , StringAppend
+ , StringShow
+ , List
+ , ListLit
+ , ListAppend
+ , ListBuild
+ , ListFold
+ , ListLength
+ , ListHead
+ , ListLast
+ , ListIndexed
+ , ListReverse
+ , Optional
+ , Some
+ , None
+ , OptionalFold
+ , OptionalBuild
+ , Record
+ , RecordLit
+ , Union
+ , Combine
+ , CombineTypes
+ , Prefer
+ , Merge
+ , ToMap
+ , Field
+ , Project
+ , Assert
+ , Equivalent
+ , Note
+ , ImportAlt
+ , Embed
+ #}
+pattern Let' x mA a b = Let x mA a b
diff git a/testsuite/tests/pmcheck/should_compile/T2204.stderr b/testsuite/tests/pmcheck/should_compile/T2204.stderr
index c2e2251fc91323ef18f23ca5846a67f3d10c7107..26f70352e2e057ff8a4d3c3d8e0c39ee7a3d3397 100644
 a/testsuite/tests/pmcheck/should_compile/T2204.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T2204.stderr
@@ 3,10 +3,10 @@ T2204.hs:6:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘f’:
Patterns not matched:
 []
 (p:_) where p is not one of {'0'}
 ['0']
+ ('0':'1':_:_)
('0':p:_) where p is not one of {'1'}
+ ['0']
+ (p:_) where p is not one of {'0'}
...
T2204.hs:9:1: warning: [Wincompletepatterns (in Wextra)]
diff git a/testsuite/tests/pmcheck/should_compile/T9951b.stderr b/testsuite/tests/pmcheck/should_compile/T9951b.stderr
index ff6696b466c9ce2520c17b19f57d1d51aef04bee..e4e337b1536342d1e0fef89426d3c70babe6703e 100644
 a/testsuite/tests/pmcheck/should_compile/T9951b.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T9951b.stderr
@@ 3,8 +3,8 @@ T9951b.hs:7:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘f’:
Patterns not matched:
 []
 (p:_) where p is not one of {'a'}
 ['a']
+ ('a':'b':_:_)
('a':p:_) where p is not one of {'b'}
+ ['a']
+ (p:_) where p is not one of {'a'}
...
diff git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index 87874f81c822b3cf27f81270afdba85d8bf35628..5a24832945ce7a8d0950c82fa9cb41f9d9f70b65 100644
 a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ 68,14 +68,20 @@ test('T15584', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T15713', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T15753a', expect_broken(15753), compile,
+test('T15753a', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T15753b', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('T15753c', normal, compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('T15753d', normal, compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T15884', expect_broken(15884), compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T16289', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('T17096', collect_compiler_stats('bytes allocated',10), compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns +RTS M2G RTS'])
test('T17112', normal, compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
@@ 94,8 +100,14 @@ test('pmc006', [], compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('pmc007', [], compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('pmc008', [], compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('pmc009', [], compile,
+ ['package ghc fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T11245', [], compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('T11336b', [], compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('T12949', [], compile, ['fwarnoverlappingpatterns'])
test('T12957', [], compile, ['fwarnoverlappingpatterns'])
test('T12957a', [], compile, ['fwarnoverlappingpatterns'])
@@ 103,6 +115,8 @@ test('PmExprVars', [], compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
test('CyclicSubst', [], compile,
['fwarnincompletepatterns fwarnoverlappingpatterns'])
+test('CaseOfKnownCon', [], compile,
+ ['fwarnincompletepatterns fwarnoverlappingpatterns'])
# EmptyCase
test('T10746', [], compile,
diff git a/testsuite/tests/pmcheck/should_compile/pmc007.stderr b/testsuite/tests/pmcheck/should_compile/pmc007.stderr
index f6e4ece88c641486db905385e55d321dc8f479ce..d4bbe8fb73893c02ef71f99d612225c515242e83 100644
 a/testsuite/tests/pmcheck/should_compile/pmc007.stderr
+++ b/testsuite/tests/pmcheck/should_compile/pmc007.stderr
@@ 2,24 +2,24 @@
pmc007.hs:7:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘f’:
 Patterns not matched: p where p is not one of {"ac", "ab"}
+ Patterns not matched: p where p is not one of {"ab", "ac"}
pmc007.hs:12:1: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In an equation for ‘g’:
Patterns not matched:
 []
 (p:_) where p is not one of {'a'}
+ ('a':'b':_:_)
+ ('a':'c':_:_)
+ ('a':p:_) where p is not one of {'b', 'c'}
['a']
 ('a':p:_) where p is not one of {'c', 'b'}
...
pmc007.hs:18:11: warning: [Wincompletepatterns (in Wextra)]
Pattern match(es) are nonexhaustive
In a case alternative:
Patterns not matched:
 []
 (p:_) where p is not one of {'a'}
+ ('a':'b':_:_)
+ ('a':'c':_:_)
+ ('a':p:_) where p is not one of {'b', 'c'}
['a']
 ('a':p:_) where p is not one of {'c', 'b'}
...
diff git a/testsuite/tests/pmcheck/should_compile/pmc008.hs b/testsuite/tests/pmcheck/should_compile/pmc008.hs
new file mode 100644
index 0000000000000000000000000000000000000000..29e39b573b79ca4efd7dc4df2fdf0db6f57b7c50
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/pmc008.hs
@@ 0,0 +1,9 @@
+{# OPTIONS_GHC fwarnincompletepatterns fwarnoverlappingpatterns #}
+
+module PMC008 where
+
+ complete match, but because of the guard, the information that `x` is not
+ `Just` has to flow through the term oracle.
+foo :: Maybe Int > Int
+foo x  Just y < x = y
+foo Nothing = 43
diff git a/testsuite/tests/pmcheck/should_compile/pmc009.hs b/testsuite/tests/pmcheck/should_compile/pmc009.hs
new file mode 100644
index 0000000000000000000000000000000000000000..ac8f5c2dd53e3154f94400a1362cab6ac072a9ed
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/pmc009.hs
@@ 0,0 +1,12 @@
+module HsUtils where
+import HsBinds
+import SrcLoc
+
+
+  We have to be careful to normalise @SrcSpanLess (LHsBind)@ to
+ @LHsBindLR l r@ before passing the representative of @unLoc bind@ on to
+ @mkOneConFull@, otherwise this triggers a panic in @zipTvSubst@.
+addPatSynSelector:: LHsBind p > [a]
+addPatSynSelector bind
+  PatSynBind _ _ < unLoc bind
+ = []
diff git a/testsuite/tests/pmcheck/should_compile/pmc009.stderr b/testsuite/tests/pmcheck/should_compile/pmc009.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..8eaa4ab61a184daa20c3e7d99879fd21943db951
 /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/pmc009.stderr
@@ 0,0 +1,4 @@
+
+pmc009.hs:10:1: warning: [Wincompletepatterns (in Wextra)]
+ Pattern match(es) are nonexhaustive
+ In an equation for ‘addPatSynSelector’: Patterns not matched: _
diff git a/testsuite/tests/th/TH_repUnboxedTuples.stderr b/testsuite/tests/th/TH_repUnboxedTuples.stderr
index 5b1f2b386ef468daeb8235e23bbefc7ddb2075b9..8439b1254752f75929beb8451d068fee90d0cba3 100644
 a/testsuite/tests/th/TH_repUnboxedTuples.stderr
+++ b/testsuite/tests/th/TH_repUnboxedTuples.stderr
@@ 7,3 +7,7 @@ case (# 'b', GHC.Types.False #) of
TH_repUnboxedTuples.hs:18:13: warning: [Woverlappingpatterns (in Wdefault)]
Pattern match is redundant
In a case alternative: (# 'a', True #) > ...
+
+TH_repUnboxedTuples.hs:18:13: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a case alternative: (# _, _ #) > ...
diff git a/testsuite/tests/typecheck/should_compile/Vta2.stderr b/testsuite/tests/typecheck/should_compile/Vta2.stderr
new file mode 100644
index 0000000000000000000000000000000000000000..0598cc022672e4fc78c0cfe380f9ba87380f7195
 /dev/null
+++ b/testsuite/tests/typecheck/should_compile/Vta2.stderr
@@ 0,0 +1,4 @@
+
+Vta2.hs:14:17: warning: [Woverlappingpatterns (in Wdefault)]
+ Pattern match is redundant
+ In a case alternative: False > ...