TcErrors.hs

{-# LANGUAGE CPP, ScopedTypeVariables #-}

module TcErrors(
       reportUnsolved, reportAllUnsolved, warnAllUnsolved,
       warnDefaulting,

       solverDepthErrorTcS
  ) where

#include "HsVersions.h"

import GhcPrelude

import TcRnTypes
import TcRnMonad
import TcMType
import TcUnify( occCheckForErrors, OccCheckResult(..) )
import TcEnv( tcInitTidyEnv )
import TcType
import RnUnbound ( unknownNameSuggestions )
import Type
import TyCoRep
import Kind
import Unify            ( tcMatchTys )
import Module
import FamInst
import FamInstEnv       ( flattenTys )
import Inst
import InstEnv
import TyCon
import Class
import DataCon
import TcEvidence
import HsExpr  ( UnboundVar(..) )
import HsBinds ( PatSynBind(..) )
import Name
import RdrName ( lookupGlobalRdrEnv, lookupGRE_Name, GlobalRdrEnv
               , mkRdrUnqual, isLocalGRE, greSrcSpan, pprNameProvenance
               , GlobalRdrElt (..), globalRdrEnvElts )
import PrelNames ( typeableClassName, hasKey, liftedRepDataConKey )
import Id
import Var
import VarSet
import VarEnv
import NameSet
import Bag
import ErrUtils         ( ErrMsg, errDoc, pprLocErrMsg )
import BasicTypes
import ConLike          ( ConLike(..))
import Util
import TcEnv (tcLookupIdMaybe)
import {-# SOURCE #-} TcSimplify ( tcSubsumes )
import FastString
import Outputable
import SrcLoc
import DynFlags
import ListSetOps       ( equivClasses )
import Maybes
import Pair
import qualified GHC.LanguageExtensions as LangExt
import FV ( fvVarList, unionFV )

import Control.Monad    ( when )
import Data.Foldable    ( toList )
import Data.List        ( partition, mapAccumL, nub, sortBy, unfoldr, foldl')
import qualified Data.Set as Set

import Data.Semigroup   ( Semigroup )
import qualified Data.Semigroup as Semigroup


{-
************************************************************************
*                                                                      *
\section{Errors and contexts}
*                                                                      *
************************************************************************

ToDo: for these error messages, should we note the location as coming
from the insts, or just whatever seems to be around in the monad just
now?

Note [Deferring coercion errors to runtime]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
While developing, sometimes it is desirable to allow compilation to succeed even
if there are type errors in the code. Consider the following case:

  module Main where

  a :: Int
  a = 'a'

  main = print "b"

Even though `a` is ill-typed, it is not used in the end, so if all that we're
interested in is `main` it is handy to be able to ignore the problems in `a`.

Since we treat type equalities as evidence, this is relatively simple. Whenever
we run into a type mismatch in TcUnify, we normally just emit an error. But it
is always safe to defer the mismatch to the main constraint solver. If we do
that, `a` will get transformed into

  co :: Int ~ Char
  co = ...

  a :: Int
  a = 'a' `cast` co

The constraint solver would realize that `co` is an insoluble constraint, and
emit an error with `reportUnsolved`. But we can also replace the right-hand side
of `co` with `error "Deferred type error: Int ~ Char"`. This allows the program
to compile, and it will run fine unless we evaluate `a`. This is what
`deferErrorsToRuntime` does.

It does this by keeping track of which errors correspond to which coercion
in TcErrors. TcErrors.reportTidyWanteds does not print the errors
and does not fail if -fdefer-type-errors is on, so that we can continue
compilation. The errors are turned into warnings in `reportUnsolved`.
-}

-- | Report unsolved goals as errors or warnings. We may also turn some into
-- deferred run-time errors if `-fdefer-type-errors` is on.
reportUnsolved :: WantedConstraints -> TcM (Bag EvBind)
reportUnsolved wanted
  = do { binds_var <- newTcEvBinds
       ; defer_errors <- goptM Opt_DeferTypeErrors
       ; warn_errors <- woptM Opt_WarnDeferredTypeErrors -- implement #10283
       ; let type_errors | not defer_errors = TypeError
                         | warn_errors      = TypeWarn
                         | otherwise        = TypeDefer

       ; defer_holes <- goptM Opt_DeferTypedHoles
       ; warn_holes  <- woptM Opt_WarnTypedHoles
       ; let expr_holes | not defer_holes = HoleError
                        | warn_holes      = HoleWarn
                        | otherwise       = HoleDefer

       ; partial_sigs      <- xoptM LangExt.PartialTypeSignatures
       ; warn_partial_sigs <- woptM Opt_WarnPartialTypeSignatures
       ; let type_holes | not partial_sigs  = HoleError
                        | warn_partial_sigs = HoleWarn
                        | otherwise         = HoleDefer

       ; defer_out_of_scope <- goptM Opt_DeferOutOfScopeVariables
       ; warn_out_of_scope <- woptM Opt_WarnDeferredOutOfScopeVariables
       ; let out_of_scope_holes | not defer_out_of_scope = HoleError
                                | warn_out_of_scope      = HoleWarn
                                | otherwise              = HoleDefer

       ; report_unsolved binds_var False type_errors expr_holes
          type_holes out_of_scope_holes wanted

       ; ev_binds <- getTcEvBindsMap binds_var
       ; return (evBindMapBinds ev_binds)}

-- | Report *all* unsolved goals as errors, even if -fdefer-type-errors is on
-- However, do not make any evidence bindings, because we don't
-- have any convenient place to put them.
-- See Note [Deferring coercion errors to runtime]
-- Used by solveEqualities for kind equalities
--      (see Note [Fail fast on kind errors] in TcSimplify]
-- and for simplifyDefault.
reportAllUnsolved :: WantedConstraints -> TcM ()
reportAllUnsolved wanted
  = do { ev_binds <- newTcEvBinds
       ; report_unsolved ev_binds False TypeError
                         HoleError HoleError HoleError wanted }

-- | Report all unsolved goals as warnings (but without deferring any errors to
-- run-time). See Note [Safe Haskell Overlapping Instances Implementation] in
-- TcSimplify
warnAllUnsolved :: WantedConstraints -> TcM ()
warnAllUnsolved wanted
  = do { ev_binds <- newTcEvBinds
       ; report_unsolved ev_binds True TypeWarn
                         HoleWarn HoleWarn HoleWarn wanted }

-- | Report unsolved goals as errors or warnings.
report_unsolved :: EvBindsVar        -- cec_binds
                -> Bool              -- Errors as warnings
                -> TypeErrorChoice   -- Deferred type errors
                -> HoleChoice        -- Expression holes
                -> HoleChoice        -- Type holes
                -> HoleChoice        -- Out of scope holes
                -> WantedConstraints -> TcM ()
report_unsolved mb_binds_var err_as_warn type_errors expr_holes
    type_holes out_of_scope_holes wanted
  | isEmptyWC wanted
  = return ()
  | otherwise
  = do { traceTc "reportUnsolved (before zonking and tidying)" (ppr wanted)

       ; wanted <- zonkWC wanted   -- Zonk to reveal all information
       ; env0 <- tcInitTidyEnv
            -- If we are deferring we are going to need /all/ evidence around,
            -- including the evidence produced by unflattening (zonkWC)
       ; let tidy_env = tidyFreeTyCoVars env0 free_tvs
             free_tvs = tyCoVarsOfWCList wanted

       ; traceTc "reportUnsolved (after zonking):" $
         vcat [ text "Free tyvars:" <+> pprTyVars free_tvs
              , text "Tidy env:" <+> ppr tidy_env
              , text "Wanted:" <+> ppr wanted ]

       ; warn_redundant <- woptM Opt_WarnRedundantConstraints
       ; let err_ctxt = CEC { cec_encl  = []
                            , cec_tidy  = tidy_env
                            , cec_defer_type_errors = type_errors
                            , cec_errors_as_warns = err_as_warn
                            , cec_expr_holes = expr_holes
                            , cec_type_holes = type_holes
                            , cec_out_of_scope_holes = out_of_scope_holes
                            , cec_suppress = False -- See Note [Suppressing error messages]
                            , cec_warn_redundant = warn_redundant
                            , cec_binds    = mb_binds_var }

       ; tc_lvl <- getTcLevel
       ; reportWanteds err_ctxt tc_lvl wanted }

--------------------------------------------
--      Internal functions
--------------------------------------------

-- | An error Report collects messages categorised by their importance.
-- See Note [Error report] for details.
data Report
  = Report { report_important :: [SDoc]
           , report_relevant_bindings :: [SDoc]
           , report_valid_substitutions :: [SDoc]
           }

instance Outputable Report where   -- Debugging only
  ppr (Report { report_important = imp
              , report_relevant_bindings = rel
              , report_valid_substitutions = val })
    = vcat [ text "important:" <+> vcat imp
           , text "relevant:"  <+> vcat rel
           , text "valid:"  <+> vcat val ]

{- Note [Error report]
The idea is that error msgs are divided into three parts: the main msg, the
context block (\"In the second argument of ...\"), and the relevant bindings
block, which are displayed in that order, with a mark to divide them.  The
idea is that the main msg ('report_important') varies depending on the error
in question, but context and relevant bindings are always the same, which
should simplify visual parsing.

The context is added when the Report is passed off to 'mkErrorReport'.
Unfortunately, unlike the context, the relevant bindings are added in
multiple places so they have to be in the Report.
-}

instance Semigroup Report where
    Report a1 b1 c1 <> Report a2 b2 c2 = Report (a1 ++ a2) (b1 ++ b2) (c1 ++ c2)

instance Monoid Report where
    mempty = Report [] [] []
    mappend = (Semigroup.<>)

-- | Put a doc into the important msgs block.
important :: SDoc -> Report
important doc = mempty { report_important = [doc] }

-- | Put a doc into the relevant bindings block.
relevant_bindings :: SDoc -> Report
relevant_bindings doc = mempty { report_relevant_bindings = [doc] }

-- | Put a doc into the valid substitutions block.
valid_substitutions :: SDoc -> Report
valid_substitutions docs = mempty { report_valid_substitutions = [docs] }

data TypeErrorChoice   -- What to do for type errors found by the type checker
  = TypeError     -- A type error aborts compilation with an error message
  | TypeWarn      -- A type error is deferred to runtime, plus a compile-time warning
  | TypeDefer     -- A type error is deferred to runtime; no error or warning at compile time

data HoleChoice
  = HoleError     -- A hole is a compile-time error
  | HoleWarn      -- Defer to runtime, emit a compile-time warning
  | HoleDefer     -- Defer to runtime, no warning

instance Outputable HoleChoice where
  ppr HoleError = text "HoleError"
  ppr HoleWarn  = text "HoleWarn"
  ppr HoleDefer = text "HoleDefer"

instance Outputable TypeErrorChoice  where
  ppr TypeError = text "TypeError"
  ppr TypeWarn  = text "TypeWarn"
  ppr TypeDefer = text "TypeDefer"

data ReportErrCtxt
    = CEC { cec_encl :: [Implication]  -- Enclosing implications
                                       --   (innermost first)
                                       -- ic_skols and givens are tidied, rest are not
          , cec_tidy  :: TidyEnv

          , cec_binds :: EvBindsVar    -- Make some errors (depending on cec_defer)
                                       -- into warnings, and emit evidence bindings
                                       -- into 'cec_binds' for unsolved constraints

          , cec_errors_as_warns :: Bool   -- Turn all errors into warnings
                                          -- (except for Holes, which are
                                          -- controlled by cec_type_holes and
                                          -- cec_expr_holes)
          , cec_defer_type_errors :: TypeErrorChoice -- Defer type errors until runtime

          -- cec_expr_holes is a union of:
          --   cec_type_holes - a set of typed holes: '_', '_a', '_foo'
          --   cec_out_of_scope_holes - a set of variables which are
          --                            out of scope: 'x', 'y', 'bar'
          , cec_expr_holes :: HoleChoice           -- Holes in expressions
          , cec_type_holes :: HoleChoice           -- Holes in types
          , cec_out_of_scope_holes :: HoleChoice   -- Out of scope holes

          , cec_warn_redundant :: Bool    -- True <=> -Wredundant-constraints

          , cec_suppress :: Bool    -- True <=> More important errors have occurred,
                                    --          so create bindings if need be, but
                                    --          don't issue any more errors/warnings
                                    -- See Note [Suppressing error messages]
      }

instance Outputable ReportErrCtxt where
  ppr (CEC { cec_binds              = bvar
           , cec_errors_as_warns    = ew
           , cec_defer_type_errors  = dte
           , cec_expr_holes         = eh
           , cec_type_holes         = th
           , cec_out_of_scope_holes = osh
           , cec_warn_redundant     = wr
           , cec_suppress           = sup })
    = text "CEC" <+> braces (vcat
         [ text "cec_binds"              <+> equals <+> ppr bvar
         , text "cec_errors_as_warns"    <+> equals <+> ppr ew
         , text "cec_defer_type_errors"  <+> equals <+> ppr dte
         , text "cec_expr_holes"         <+> equals <+> ppr eh
         , text "cec_type_holes"         <+> equals <+> ppr th
         , text "cec_out_of_scope_holes" <+> equals <+> ppr osh
         , text "cec_warn_redundant"     <+> equals <+> ppr wr
         , text "cec_suppress"           <+> equals <+> ppr sup ])

{-
Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The cec_suppress flag says "don't report any errors".  Instead, just create
evidence bindings (as usual).  It's used when more important errors have occurred.

Specifically (see reportWanteds)
  * If there are insoluble Givens, then we are in unreachable code and all bets
    are off.  So don't report any further errors.
  * If there are any insolubles (eg Int~Bool), here or in a nested implication,
    then suppress errors from the simple constraints here.  Sometimes the
    simple-constraint errors are a knock-on effect of the insolubles.
-}

reportImplic :: ReportErrCtxt -> Implication -> TcM ()
reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
                                 , ic_wanted = wanted, ic_binds = evb
                                 , ic_status = status, ic_info = info
                                 , ic_env = tcl_env, ic_tclvl = tc_lvl })
  | BracketSkol <- info
  , not insoluble
  = return ()        -- For Template Haskell brackets report only
                     -- definite errors. The whole thing will be re-checked
                     -- later when we plug it in, and meanwhile there may
                     -- certainly be un-satisfied constraints

  | otherwise
  = do { traceTc "reportImplic" (ppr implic')
       ; reportWanteds ctxt' tc_lvl wanted
       ; when (cec_warn_redundant ctxt) $
         warnRedundantConstraints ctxt' tcl_env info' dead_givens }
  where
    insoluble    = isInsolubleStatus status
    (env1, tvs') = mapAccumL tidyTyCoVarBndr (cec_tidy ctxt) tvs
    info'        = tidySkolemInfo env1 info
    implic' = implic { ic_skols = tvs'
                     , ic_given = map (tidyEvVar env1) given
                     , ic_info  = info' }
    ctxt' = ctxt { cec_tidy     = env1
                 , cec_encl     = implic' : cec_encl ctxt

                 , cec_suppress = insoluble || cec_suppress ctxt
                      -- Suppress inessential errors if there
                      -- are insolubles anywhere in the
                      -- tree rooted here, or we've come across
                      -- a suppress-worthy constraint higher up (Trac #11541)

                 , cec_binds    = evb }

    dead_givens = case status of
                    IC_Solved { ics_dead = dead } -> dead
                    _                             -> []

warnRedundantConstraints :: ReportErrCtxt -> TcLclEnv -> SkolemInfo -> [EvVar] -> TcM ()
-- See Note [Tracking redundant constraints] in TcSimplify
warnRedundantConstraints ctxt env info ev_vars
 | null redundant_evs
 = return ()

 | SigSkol {} <- info
 = setLclEnv env $  -- We want to add "In the type signature for f"
                    -- to the error context, which is a bit tiresome
   addErrCtxt (text "In" <+> ppr info) $
   do { env <- getLclEnv
      ; msg <- mkErrorReport ctxt env (important doc)
      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }

 | otherwise  -- But for InstSkol there already *is* a surrounding
              -- "In the instance declaration for Eq [a]" context
              -- and we don't want to say it twice. Seems a bit ad-hoc
 = do { msg <- mkErrorReport ctxt env (important doc)
      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
 where
   doc = text "Redundant constraint" <> plural redundant_evs <> colon
         <+> pprEvVarTheta redundant_evs

   redundant_evs = case info of -- See Note [Redundant constraints in instance decls]
                     InstSkol -> filterOut improving ev_vars
                     _        -> ev_vars

   improving ev_var = any isImprovementPred $
                      transSuperClasses (idType ev_var)

{- Note [Redundant constraints in instance decls]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For instance declarations, we don't report unused givens if
they can give rise to improvement.  Example (Trac #10100):
    class Add a b ab | a b -> ab, a ab -> b
    instance Add Zero b b
    instance Add a b ab => Add (Succ a) b (Succ ab)
The context (Add a b ab) for the instance is clearly unused in terms
of evidence, since the dictionary has no fields.  But it is still
needed!  With the context, a wanted constraint
   Add (Succ Zero) beta (Succ Zero)
we will reduce to (Add Zero beta Zero), and thence we get beta := Zero.
But without the context we won't find beta := Zero.

This only matters in instance declarations..
-}

reportWanteds :: ReportErrCtxt -> TcLevel -> WantedConstraints -> TcM ()
reportWanteds ctxt tc_lvl (WC { wc_simple = simples, wc_impl = implics })
  = do { traceTc "reportWanteds" (vcat [ text "Simples =" <+> ppr simples
                                       , text "Suppress =" <+> ppr (cec_suppress ctxt)])
       ; let tidy_cts = bagToList (mapBag (tidyCt env) simples)
       ; traceTc "rw2" (ppr tidy_cts)

         -- First deal with things that are utterly wrong
         -- Like Int ~ Bool (incl nullary TyCons)
         -- or  Int ~ t a   (AppTy on one side)
         -- These /ones/ are not suppressed by the incoming context
       ; let ctxt_for_insols = ctxt { cec_suppress = False }
       ; (ctxt1, cts1) <- tryReporters ctxt_for_insols report1 tidy_cts

         -- Now all the other constraints.  We suppress errors here if
         -- any of the first batch failed, or if the enclosing context
         -- says to suppress
       ; let ctxt2 = ctxt { cec_suppress = cec_suppress ctxt || cec_suppress ctxt1 }
       ; (_, leftovers) <- tryReporters ctxt2 report2 cts1
       ; MASSERT2( null leftovers, ppr leftovers )

            -- All the Derived ones have been filtered out of simples
            -- by the constraint solver. This is ok; we don't want
            -- to report unsolved Derived goals as errors
            -- See Note [Do not report derived but soluble errors]

     ; mapBagM_ (reportImplic ctxt2) implics }
            -- NB ctxt1: don't suppress inner insolubles if there's only a
            -- wanted insoluble here; but do suppress inner insolubles
            -- if there's a *given* insoluble here (= inaccessible code)
 where
    env = cec_tidy ctxt

    -- report1: ones that should *not* be suppresed by
    --          an insoluble somewhere else in the tree
    -- It's crucial that anything that is considered insoluble
    -- (see TcRnTypes.insolubleWantedCt) is caught here, otherwise
    -- we might suppress its error message, and proceed on past
    -- type checking to get a Lint error later
    report1 = [ ("custom_error", is_user_type_error,True, mkUserTypeErrorReporter)
              , given_eq_spec
              , ("insoluble2",    utterly_wrong,    True, mkGroupReporter mkEqErr)
              , ("skolem eq1",    very_wrong,       True, mkSkolReporter)
              , ("skolem eq2",    skolem_eq,        True, mkSkolReporter)
              , ("non-tv eq",     non_tv_eq,        True, mkSkolReporter)
              , ("Out of scope",  is_out_of_scope,  True, mkHoleReporter)
              , ("Holes",         is_hole,          False, mkHoleReporter)

                  -- The only remaining equalities are alpha ~ ty,
                  -- where alpha is untouchable; and representational equalities
                  -- Prefer homogeneous equalities over hetero, because the
                  -- former might be holding up the latter.
                  -- See Note [Equalities with incompatible kinds] in TcCanonical
              , ("Homo eqs",      is_homo_equality, True,  mkGroupReporter mkEqErr)
              , ("Other eqs",     is_equality,      False, mkGroupReporter mkEqErr) ]

    -- report2: we suppress these if there are insolubles elsewhere in the tree
    report2 = [ ("Implicit params", is_ip,           False, mkGroupReporter mkIPErr)
              , ("Irreds",          is_irred,        False, mkGroupReporter mkIrredErr)
              , ("Dicts",           is_dict,         False, mkGroupReporter mkDictErr) ]

    -- rigid_nom_eq, rigid_nom_tv_eq,
    is_hole, is_dict,
      is_equality, is_ip, is_irred :: Ct -> PredTree -> Bool

    is_given_eq ct pred
       | EqPred {} <- pred = arisesFromGivens ct
       | otherwise         = False
       -- I think all given residuals are equalities

    -- Things like (Int ~N Bool)
    utterly_wrong _ (EqPred NomEq ty1 ty2) = isRigidTy ty1 && isRigidTy ty2
    utterly_wrong _ _                      = False

    -- Things like (a ~N Int)
    very_wrong _ (EqPred NomEq ty1 ty2) = isSkolemTy tc_lvl ty1 && isRigidTy ty2
    very_wrong _ _                      = False

    -- Things like (a ~N b) or (a  ~N  F Bool)
    skolem_eq _ (EqPred NomEq ty1 _) = isSkolemTy tc_lvl ty1
    skolem_eq _ _                    = False

    -- Things like (F a  ~N  Int)
    non_tv_eq _ (EqPred NomEq ty1 _) = not (isTyVarTy ty1)
    non_tv_eq _ _                    = False

    is_out_of_scope ct _ = isOutOfScopeCt ct
    is_hole         ct _ = isHoleCt ct

    is_user_type_error ct _ = isUserTypeErrorCt ct

    is_homo_equality _ (EqPred _ ty1 ty2) = typeKind ty1 `tcEqType` typeKind ty2
    is_homo_equality _ _                  = False

    is_equality _ (EqPred {}) = True
    is_equality _ _           = False

    is_dict _ (ClassPred {}) = True
    is_dict _ _              = False

    is_ip _ (ClassPred cls _) = isIPClass cls
    is_ip _ _                 = False

    is_irred _ (IrredPred {}) = True
    is_irred _ _              = False

    given_eq_spec = case find_gadt_match (cec_encl ctxt) of
       Just imp -> ("insoluble1a", is_given_eq, True,  mkGivenErrorReporter imp)
       Nothing  -> ("insoluble1b", is_given_eq, False, ignoreErrorReporter)
                  -- False means don't suppress subsequent errors
                  -- Reason: we don't report all given errors
                  --         (see mkGivenErrorReporter), and we should only suppress
                  --         subsequent errors if we actually report this one!
                  --         Trac #13446 is an example

    find_gadt_match [] = Nothing
    find_gadt_match (implic : implics)
      | PatSkol {} <- ic_info implic
      , not (ic_no_eqs implic)
      = Just implic
      | otherwise
      = find_gadt_match implics

---------------
isSkolemTy :: TcLevel -> Type -> Bool
-- The type is a skolem tyvar
isSkolemTy tc_lvl ty
  | Just tv <- getTyVar_maybe ty
  =  isSkolemTyVar tv
  || (isSigTyVar tv && isTouchableMetaTyVar tc_lvl tv)
     -- The last case is for touchable SigTvs
     -- we postpone untouchables to a latter test (too obscure)

  | otherwise
  = False

isTyFun_maybe :: Type -> Maybe TyCon
isTyFun_maybe ty = case tcSplitTyConApp_maybe ty of
                      Just (tc,_) | isTypeFamilyTyCon tc -> Just tc
                      _ -> Nothing

--------------------------------------------
--      Reporters
--------------------------------------------

type Reporter
  = ReportErrCtxt -> [Ct] -> TcM ()
type ReporterSpec
  = ( String                     -- Name
    , Ct -> PredTree -> Bool     -- Pick these ones
    , Bool                       -- True <=> suppress subsequent reporters
    , Reporter)                  -- The reporter itself

mkSkolReporter :: Reporter
-- Suppress duplicates with either the same LHS, or same location
mkSkolReporter ctxt cts
  = mapM_ (reportGroup mkEqErr ctxt) (group cts)
  where
     group [] = []
     group (ct:cts) = (ct : yeses) : group noes
        where
          (yeses, noes) = partition (group_with ct) cts

     group_with ct1 ct2
       | EQ <- cmp_loc ct1 ct2 = True
       | eq_lhs_type   ct1 ct2 = True
       | otherwise             = False

mkHoleReporter :: Reporter
-- Reports errors one at a time
mkHoleReporter ctxt
  = mapM_ $ \ct -> do { err <- mkHoleError ctxt ct
                      ; maybeReportHoleError ctxt ct err
                      ; maybeAddDeferredHoleBinding ctxt err ct }

mkUserTypeErrorReporter :: Reporter
mkUserTypeErrorReporter ctxt
  = mapM_ $ \ct -> do { err <- mkUserTypeError ctxt ct
                      ; maybeReportError ctxt err
                      ; addDeferredBinding ctxt err ct }

mkUserTypeError :: ReportErrCtxt -> Ct -> TcM ErrMsg
mkUserTypeError ctxt ct = mkErrorMsgFromCt ctxt ct
                        $ important
                        $ pprUserTypeErrorTy
                        $ case getUserTypeErrorMsg ct of
                            Just msg -> msg
                            Nothing  -> pprPanic "mkUserTypeError" (ppr ct)


mkGivenErrorReporter :: Implication -> Reporter
-- See Note [Given errors]
mkGivenErrorReporter implic ctxt cts
  = do { (ctxt, binds_msg, ct) <- relevantBindings True ctxt ct
       ; dflags <- getDynFlags
       ; let ct' = setCtLoc ct (setCtLocEnv (ctLoc ct) (ic_env implic))
                   -- For given constraints we overwrite the env (and hence src-loc)
                  -- with one from the implication.  See Note [Inaccessible code]

             inaccessible_msg = hang (text "Inaccessible code in")
                                   2 (ppr (ic_info implic))
             report = important inaccessible_msg `mappend`
                      relevant_bindings binds_msg

       ; err <- mkEqErr_help dflags ctxt report ct'
                             Nothing ty1 ty2

       ; traceTc "mkGivenErrorReporter" (ppr ct)
       ; maybeReportError ctxt err }
  where
    (ct : _ )  = cts    -- Never empty
    (ty1, ty2) = getEqPredTys (ctPred ct)

ignoreErrorReporter :: Reporter
-- Discard Given errors that don't come from
-- a pattern match; maybe we should warn instead?
ignoreErrorReporter ctxt cts
  = do { traceTc "mkGivenErrorReporter no" (ppr cts $$ ppr (cec_encl ctxt))
       ; return () }


{- Note [Given errors]
~~~~~~~~~~~~~~~~~~~~~~
Given constraints represent things for which we have (or will have)
evidence, so they aren't errors.  But if a Given constraint is
insoluble, this code is inaccessible, and we might want to at least
warn about that.  A classic case is

   data T a where
     T1 :: T Int
     T2 :: T a
     T3 :: T Bool

   f :: T Int -> Bool
   f T1 = ...
   f T2 = ...
   f T3 = ...  -- We want to report this case as inaccessible

We'd like to point out that the T3 match is inaccessible. It
will have a Given constraint [G] Int ~ Bool.

But we don't want to report ALL insoluble Given constraints.  See Trac
#12466 for a long discussion.  For example, if we aren't careful
we'll complain about
   f :: ((Int ~ Bool) => a -> a) -> Int
which arguably is OK.  It's more debatable for
   g :: (Int ~ Bool) => Int -> Int
but it's tricky to distinguish these cases to we don't report
either.

The bottom line is this: find_gadt_match looks for an enclosing
pattern match which binds some equality constraints.  If we
find one, we report the insoluble Given.
-}

mkGroupReporter :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg)
                             -- Make error message for a group
                -> Reporter  -- Deal with lots of constraints
-- Group together errors from same location,
-- and report only the first (to avoid a cascade)
mkGroupReporter mk_err ctxt cts
  = mapM_ (reportGroup mk_err ctxt . toList) (equivClasses cmp_loc cts)

eq_lhs_type :: Ct -> Ct -> Bool
eq_lhs_type ct1 ct2
  = case (classifyPredType (ctPred ct1), classifyPredType (ctPred ct2)) of
       (EqPred eq_rel1 ty1 _, EqPred eq_rel2 ty2 _) ->
         (eq_rel1 == eq_rel2) && (ty1 `eqType` ty2)
       _ -> pprPanic "mkSkolReporter" (ppr ct1 $$ ppr ct2)

cmp_loc :: Ct -> Ct -> Ordering
cmp_loc ct1 ct2 = ctLocSpan (ctLoc ct1) `compare` ctLocSpan (ctLoc ct2)

reportGroup :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg) -> ReportErrCtxt
            -> [Ct] -> TcM ()
reportGroup mk_err ctxt cts =
  case partition isMonadFailInstanceMissing cts of
        -- Only warn about missing MonadFail constraint when
        -- there are no other missing constraints!
        (monadFailCts, []) ->
            do { err <- mk_err ctxt monadFailCts
               ; reportWarning (Reason Opt_WarnMissingMonadFailInstances) err }

        (_, cts') -> do { err <- mk_err ctxt cts'
                        ; maybeReportError ctxt err
                            -- But see Note [Always warn with -fdefer-type-errors]
                        ; traceTc "reportGroup" (ppr cts')
                        ; mapM_ (addDeferredBinding ctxt err) cts' }
                            -- Add deferred bindings for all
                            -- Redundant if we are going to abort compilation,
                            -- but that's hard to know for sure, and if we don't
                            -- abort, we need bindings for all (e.g. Trac #12156)
  where
    isMonadFailInstanceMissing ct =
        case ctLocOrigin (ctLoc ct) of
            FailablePattern _pat -> True
            _otherwise           -> False

maybeReportHoleError :: ReportErrCtxt -> Ct -> ErrMsg -> TcM ()
maybeReportHoleError ctxt ct err
  -- When -XPartialTypeSignatures is on, warnings (instead of errors) are
  -- generated for holes in partial type signatures.
  -- Unless -fwarn_partial_type_signatures is not on,
  -- in which case the messages are discarded.
  | isTypeHoleCt ct
  = -- For partial type signatures, generate warnings only, and do that
    -- only if -fwarn_partial_type_signatures is on
    case cec_type_holes ctxt of
       HoleError -> reportError err
       HoleWarn  -> reportWarning (Reason Opt_WarnPartialTypeSignatures) err
       HoleDefer -> return ()

  -- Always report an error for out-of-scope variables
  -- Unless -fdefer-out-of-scope-variables is on,
  -- in which case the messages are discarded.
  -- See Trac #12170, #12406
  | isOutOfScopeCt ct
  = -- If deferring, report a warning only if -Wout-of-scope-variables is on
    case cec_out_of_scope_holes ctxt of
      HoleError -> reportError err
      HoleWarn  ->
        reportWarning (Reason Opt_WarnDeferredOutOfScopeVariables) err
      HoleDefer -> return ()

  -- Otherwise this is a typed hole in an expression,
  -- but not for an out-of-scope variable
  | otherwise
  = -- If deferring, report a warning only if -Wtyped-holes is on
    case cec_expr_holes ctxt of
       HoleError -> reportError err
       HoleWarn  -> reportWarning (Reason Opt_WarnTypedHoles) err
       HoleDefer -> return ()

maybeReportError :: ReportErrCtxt -> ErrMsg -> TcM ()
-- Report the error and/or make a deferred binding for it
maybeReportError ctxt err
  | cec_suppress ctxt    -- Some worse error has occurred;
  = return ()            -- so suppress this error/warning

  | cec_errors_as_warns ctxt
  = reportWarning NoReason err

  | otherwise
  = case cec_defer_type_errors ctxt of
      TypeDefer -> return ()
      TypeWarn  -> reportWarning (Reason Opt_WarnDeferredTypeErrors) err
      TypeError -> reportError err

addDeferredBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
-- See Note [Deferring coercion errors to runtime]
addDeferredBinding ctxt err ct
  | CtWanted { ctev_pred = pred, ctev_dest = dest } <- ctEvidence ct
    -- Only add deferred bindings for Wanted constraints
  = do { dflags <- getDynFlags
       ; let err_msg = pprLocErrMsg err
             err_fs  = mkFastString $ showSDoc dflags $
                       err_msg $$ text "(deferred type error)"
             err_tm  = EvDelayedError pred err_fs
             ev_binds_var = cec_binds ctxt

       ; case dest of
           EvVarDest evar
             -> addTcEvBind ev_binds_var $ mkWantedEvBind evar err_tm
           HoleDest hole
             -> do { -- See Note [Deferred errors for coercion holes]
                     let co_var = coHoleCoVar hole
                   ; addTcEvBind ev_binds_var $ mkWantedEvBind co_var err_tm
                   ; fillCoercionHole hole (mkTcCoVarCo co_var) }}

  | otherwise   -- Do not set any evidence for Given/Derived
  = return ()

maybeAddDeferredHoleBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
maybeAddDeferredHoleBinding ctxt err ct
  | isExprHoleCt ct
  = addDeferredBinding ctxt err ct  -- Only add bindings for holes in expressions
  | otherwise                       -- not for holes in partial type signatures
  = return ()

tryReporters :: ReportErrCtxt -> [ReporterSpec] -> [Ct] -> TcM (ReportErrCtxt, [Ct])
-- Use the first reporter in the list whose predicate says True
tryReporters ctxt reporters cts
  = do { let (vis_cts, invis_cts) = partition (isVisibleOrigin . ctOrigin) cts
       ; traceTc "tryReporters {" (ppr vis_cts $$ ppr invis_cts)
       ; (ctxt', cts') <- go ctxt reporters vis_cts invis_cts
       ; traceTc "tryReporters }" (ppr cts')
       ; return (ctxt', cts') }
  where
    go ctxt [] vis_cts invis_cts
      = return (ctxt, vis_cts ++ invis_cts)

    go ctxt (r : rs) vis_cts invis_cts
       -- always look at *visible* Origins before invisible ones
       -- this is the whole point of isVisibleOrigin
      = do { (ctxt', vis_cts') <- tryReporter ctxt r vis_cts
           ; (ctxt'', invis_cts') <- tryReporter ctxt' r invis_cts
           ; go ctxt'' rs vis_cts' invis_cts' }
                -- Carry on with the rest, because we must make
                -- deferred bindings for them if we have -fdefer-type-errors
                -- But suppress their error messages

tryReporter :: ReportErrCtxt -> ReporterSpec -> [Ct] -> TcM (ReportErrCtxt, [Ct])
tryReporter ctxt (str, keep_me,  suppress_after, reporter) cts
  | null yeses = return (ctxt, cts)
  | otherwise  = do { traceTc "tryReporter{ " (text str <+> ppr yeses)
                    ; reporter ctxt yeses
                    ; let ctxt' = ctxt { cec_suppress = suppress_after || cec_suppress ctxt }
                    ; traceTc "tryReporter end }" (text str <+> ppr (cec_suppress ctxt) <+> ppr suppress_after)
                    ; return (ctxt', nos) }
  where
    (yeses, nos) = partition (\ct -> keep_me ct (classifyPredType (ctPred ct))) cts


pprArising :: CtOrigin -> SDoc
-- Used for the main, top-level error message
-- We've done special processing for TypeEq, KindEq, Given
pprArising (TypeEqOrigin {}) = empty
pprArising (KindEqOrigin {}) = empty
pprArising (GivenOrigin {})  = empty
pprArising orig              = pprCtOrigin orig

-- Add the "arising from..." part to a message about bunch of dicts
addArising :: CtOrigin -> SDoc -> SDoc
addArising orig msg = hang msg 2 (pprArising orig)

pprWithArising :: [Ct] -> (CtLoc, SDoc)
-- Print something like
--    (Eq a) arising from a use of x at y
--    (Show a) arising from a use of p at q
-- Also return a location for the error message
-- Works for Wanted/Derived only
pprWithArising []
  = panic "pprWithArising"
pprWithArising (ct:cts)
  | null cts
  = (loc, addArising (ctLocOrigin loc)