TcErrors.lhs 57.2 KB
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\begin{code}
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{-# LANGUAGE ScopedTypeVariables #-}
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{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
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--     http://ghc.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
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-- for details

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module TcErrors( 
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       reportUnsolved, reportAllUnsolved,
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       warnDefaulting,
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       solverDepthErrorTcS
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  ) where

#include "HsVersions.h"

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import TcRnTypes
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import TcRnMonad
import TcMType
import TcType
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import TypeRep
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import Type
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import Kind ( isKind )
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import Unify            ( tcMatchTys )
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import Inst
import InstEnv
import TyCon
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import DataCon
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import TcEvidence
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import TysWiredIn       ( coercibleClass )
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import Name
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import RdrName          ( lookupGRE_Name )
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import Id 
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import Var
import VarSet
import VarEnv
import Bag
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import Maybes
import ErrUtils         ( ErrMsg, makeIntoWarning, pprLocErrMsg )
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import BasicTypes 
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import Util
import FastString
import Outputable
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import SrcLoc
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import DynFlags
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import Data.List        ( partition, mapAccumL, zip4 )
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\end{code}

%************************************************************************
%*									*
\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?

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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
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  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
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and does not fail if -fdefer-type-errors is on, so that we can continue
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compilation. The errors are turned into warnings in `reportUnsolved`.

\begin{code}
reportUnsolved :: WantedConstraints -> TcM (Bag EvBind)
reportUnsolved wanted
  = do { binds_var <- newTcEvBinds
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       ; defer <- goptM Opt_DeferTypeErrors
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       ; report_unsolved (Just binds_var) defer wanted
       ; getTcEvBinds binds_var }

reportAllUnsolved :: WantedConstraints -> TcM ()
-- Report all unsolved goals, even if -fdefer-type-errors is on
-- See Note [Deferring coercion errors to runtime]
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reportAllUnsolved wanted = report_unsolved Nothing False wanted
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report_unsolved :: Maybe EvBindsVar  -- cec_binds
                -> Bool              -- cec_defer
                -> WantedConstraints -> TcM ()
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-- Important precondition:
-- WantedConstraints are fully zonked and unflattened, that is,
-- zonkWC has already been applied to these constraints.
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report_unsolved mb_binds_var defer wanted
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  | isEmptyWC wanted
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  = return ()
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  | otherwise
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  = do { traceTc "reportUnsolved (before unflattening)" (ppr wanted)
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       ; env0 <- tcInitTidyEnv
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            -- If we are deferring we are going to need /all/ evidence around,
            -- including the evidence produced by unflattening (zonkWC)
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       ; let tidy_env = tidyFreeTyVars env0 free_tvs
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             free_tvs = tyVarsOfWC wanted
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             err_ctxt = CEC { cec_encl  = []
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                            , cec_tidy  = tidy_env
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                            , cec_defer    = defer
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                            , cec_suppress = False -- See Note [Suppressing error messages]
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                            , cec_binds    = mb_binds_var }
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       ; traceTc "reportUnsolved (after unflattening):" $ 
         vcat [ pprTvBndrs (varSetElems free_tvs)
              , ppr wanted ]
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       ; reportWanteds err_ctxt wanted }
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--------------------------------------------
--      Internal functions
--------------------------------------------
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data ReportErrCtxt 
    = CEC { cec_encl :: [Implication]  -- Enclosing implications
                	       	       --   (innermost first)
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                                       -- ic_skols and givens are tidied, rest are not
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          , cec_tidy  :: TidyEnv
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          , cec_binds :: Maybe EvBindsVar 
                         -- Nothinng <=> Report all errors, including holes; no bindings
                         -- Just ev  <=> make some errors (depending on cec_defer)
                         --              into warnings, and emit evidence bindings
                         --              into 'ev' for unsolved constraints
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          , cec_defer :: Bool       -- True <=> -fdefer-type-errors
                                    -- Irrelevant if cec_binds = Nothing
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          , cec_suppress :: Bool    -- True <=> More important errors have occurred,
                                    --          so create bindings if need be, but
                                    --          don't issue any more errors/warnings
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                                    -- See Note [Suppressing error messages]
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      }
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\end{code}
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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 flat constraints here.  Sometimes the
    flat-constraint errors are a knock-on effect of the insolubles.


\begin{code}
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reportImplic :: ReportErrCtxt -> Implication -> TcM ()
reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
                                 , ic_wanted = wanted, ic_binds = evb
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                                 , ic_insol = ic_insoluble, ic_info = info })
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  | BracketSkol <- info
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  , not ic_insoluble -- For Template Haskell brackets report only
  = return ()        -- definite errors. The whole thing will be re-checked
                     -- later when we plug it in, and meanwhile there may
                     -- certainly be un-satisfied constraints
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  | otherwise
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  = reportWanteds ctxt' wanted
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  where
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    (env1, tvs') = mapAccumL tidyTyVarBndr (cec_tidy ctxt) tvs
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    (env2, info') = tidySkolemInfo env1 info
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    implic' = implic { ic_skols = tvs'
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                     , ic_given = map (tidyEvVar env2) given
                     , ic_info  = info' }
    ctxt' = ctxt { cec_tidy  = env2
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                 , cec_encl  = implic' : cec_encl ctxt
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                 , cec_binds = case cec_binds ctxt of
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                                 Nothing -> Nothing
                                 Just {} -> Just evb }

reportWanteds :: ReportErrCtxt -> WantedConstraints -> TcM ()
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reportWanteds ctxt wanted@(WC { wc_flat = flats, wc_insol = insols, wc_impl = implics })
  = do { reportFlats ctxt  (mapBag (tidyCt env) insol_given)
       ; reportFlats ctxt1 (mapBag (tidyCt env) insol_wanted)
       ; reportFlats ctxt2 (mapBag (tidyCt env) flats)
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            -- All the Derived ones have been filtered out of flats 
            -- 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]
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       ; mapBagM_ (reportImplic ctxt1) 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
    (insol_given, insol_wanted) = partitionBag isGivenCt insols
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    env = cec_tidy ctxt

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      -- See Note [Suppressing error messages]
    suppress0 = cec_suppress ctxt
    suppress1 = suppress0 || not (isEmptyBag insol_given)
    suppress2 = suppress0 || insolubleWC wanted
    ctxt1     = ctxt { cec_suppress = suppress1 }
    ctxt2     = ctxt { cec_suppress = suppress2 }

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reportFlats :: ReportErrCtxt -> Cts -> TcM ()
reportFlats ctxt flats    -- Here 'flats' includes insolble goals
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  = traceTc "reportFlats" (vcat [ ptext (sLit "Flats =") <+> ppr flats
                                , ptext (sLit "Suppress =") <+> ppr (cec_suppress ctxt)]) >>
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    tryReporters 
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      [ -- First deal with things that are utterly wrong
        -- Like Int ~ Bool (incl nullary TyCons)
        -- or  Int ~ t a   (AppTy on one side)
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        ("Utterly wrong",  utterly_wrong,   mkGroupReporter mkEqErr)
      , ("Holes",          is_hole,         mkUniReporter mkHoleError)
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        -- Report equalities of form (a~ty).  They are usually
        -- skolem-equalities, and they cause confusing knock-on 
        -- effects in other errors; see test T4093b.
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      , ("Skolem equalities",    skolem_eq,   mkUniReporter mkEqErr1) ]
      reportFlatErrs
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      ctxt (bagToList flats)
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  where
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    utterly_wrong, skolem_eq :: Ct -> PredTree -> Bool
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    utterly_wrong _ (EqPred ty1 ty2) = isRigid ty1 && isRigid ty2 
    utterly_wrong _ _ = False

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    is_hole ct _ = isHoleCt ct

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    skolem_eq _ (EqPred ty1 ty2) = isRigidOrSkol ty1 && isRigidOrSkol ty2 
    skolem_eq _ _ = False

---------------
isRigid, isRigidOrSkol :: Type -> Bool
isRigid ty 
  | Just (tc,_) <- tcSplitTyConApp_maybe ty = isDecomposableTyCon tc
  | Just {} <- tcSplitAppTy_maybe ty        = True
  | isForAllTy ty                           = True
  | otherwise                               = False

isRigidOrSkol ty 
  | Just tv <- getTyVar_maybe ty = isSkolemTyVar tv
  | otherwise                    = isRigid ty

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

-----------------
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reportFlatErrs :: Reporter
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-- Called once for non-ambigs, once for ambigs
-- Report equality errors, and others only if we've done all 
-- the equalities.  The equality errors are more basic, and
-- can lead to knock on type-class errors
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reportFlatErrs
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  = tryReporters
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      [ ("Equalities", is_equality, mkGroupReporter mkEqErr) ]
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      (\ctxt cts -> do { let (dicts, ips, irreds) = go cts [] [] []
                       ; mkGroupReporter mkIPErr    ctxt ips   
                       ; mkGroupReporter mkIrredErr ctxt irreds
                       ; mkGroupReporter mkDictErr  ctxt dicts })
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  where
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    is_equality _ (EqPred {}) = True
    is_equality _ _           = False

    go [] dicts ips irreds
      = (dicts, ips, irreds)
    go (ct:cts) dicts ips irreds
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      | isIPPred (ctPred ct) 
      = go cts dicts (ct:ips) irreds
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      | otherwise
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      = case classifyPredType (ctPred ct) of
          ClassPred {}  -> go cts (ct:dicts) ips irreds
          IrredPred {}  -> go cts dicts ips (ct:irreds)
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          _             -> panic "reportFlatErrs"
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    -- TuplePreds should have been expanded away by the constraint
    -- simplifier, so they shouldn't show up at this point
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    -- And EqPreds are dealt with by the is_equality test

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

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type Reporter = ReportErrCtxt -> [Ct] -> TcM ()

mkUniReporter :: (ReportErrCtxt -> Ct -> TcM ErrMsg) -> Reporter
-- Reports errors one at a time
mkUniReporter mk_err ctxt 
  = mapM_ $ \ct -> 
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    do { err <- mk_err ctxt ct
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       ; maybeReportError ctxt err
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       ; maybeAddDeferredBinding ctxt err ct }
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mkGroupReporter :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg)
                             -- Make error message for a group
                -> Reporter  -- Deal with lots of constraints
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-- Group together insts from same location
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-- We want to report them together in error messages

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mkGroupReporter _ _ [] 
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  = return ()
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mkGroupReporter mk_err ctxt (ct1 : rest)
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  = do { err <- mk_err ctxt first_group
       ; maybeReportError ctxt err
       ; mapM_ (maybeAddDeferredBinding ctxt err) first_group
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               -- Add deferred bindings for all
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               -- But see Note [Always warn with -fdefer-type-errors]
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       ; mkGroupReporter mk_err ctxt others }
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  where
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   loc               = cc_loc ct1
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   first_group       = ct1 : friends
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   (friends, others) = partition is_friend rest
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   is_friend friend  = cc_loc friend `same_loc` loc
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   same_loc :: CtLoc -> CtLoc -> Bool
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   same_loc l1 l2 = ctLocSpan l1 == ctLocSpan l2

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maybeReportError :: ReportErrCtxt -> ErrMsg -> TcM ()
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-- Report the error and/or make a deferred binding for it
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maybeReportError ctxt err
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  | cec_defer ctxt  -- See Note [Always warn with -fdefer-type-errors]
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  = reportWarning (makeIntoWarning err)
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  | cec_suppress ctxt
  = return ()
  | otherwise
  = reportError err

maybeAddDeferredBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
-- See Note [Deferring coercion errors to runtime]
maybeAddDeferredBinding ctxt err ct
  | CtWanted { ctev_pred = pred, ctev_evar = ev_id } <- cc_ev ct
    -- Only add deferred bindings for Wanted constraints
  , isHoleCt ct || cec_defer ctxt  -- And it's a hole or we have -fdefer-type-errors
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  , Just ev_binds_var <- cec_binds ctxt  -- We have somewhere to put the bindings
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  = do { dflags <- getDynFlags
       ; let err_msg = pprLocErrMsg err
             err_fs  = mkFastString $ showSDoc dflags $
                       err_msg $$ text "(deferred type error)"

         -- Create the binding
       ; addTcEvBind ev_binds_var ev_id (EvDelayedError pred err_fs) }

  | otherwise   -- Do not set any evidence for Given/Derived
  = return ()   
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tryReporters :: [(String, Ct -> PredTree -> Bool, Reporter)] 
             -> Reporter -> Reporter
-- Use the first reporter in the list whose predicate says True
tryReporters reporters deflt ctxt cts
  = do { traceTc "tryReporters {" (ppr cts) 
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       ; go ctxt reporters cts
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       ; traceTc "tryReporters }" empty }
  where
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    go ctxt [] cts = deflt ctxt cts 
    go ctxt ((str, pred, reporter) : rs) cts
      | null yeses  = do { traceTc "tryReporters: no" (text str)
                         ; go ctxt rs cts }
      | otherwise   = do { traceTc "tryReporters: yes" (text str <+> ppr yeses)
                         ; reporter ctxt yeses :: TcM ()
                         ; go (ctxt { cec_suppress = True }) rs nos }
                         -- Carry on with the rest, because we must make
                         -- deferred bindings for them if we have 
                         -- -fdefer-type-errors
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                         -- But suppress their error messages
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      where
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       (yeses, nos) = partition keep_me cts
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       keep_me ct = pred ct (classifyPredType (ctPred ct))
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-- Add the "arising from..." part to a message about bunch of dicts
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addArising :: CtOrigin -> SDoc -> SDoc
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addArising orig msg = hang msg 2 (pprArising orig)
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pprWithArising :: [Ct] -> (CtLoc, SDoc)
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-- Print something like
--    (Eq a) arising from a use of x at y
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--    (Show a) arising from a use of p at q
-- Also return a location for the error message
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-- Works for Wanted/Derived only
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pprWithArising [] 
  = panic "pprWithArising"
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pprWithArising (ct:cts)
  | null cts
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  = (loc, addArising (ctLocOrigin loc) 
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                     (pprTheta [ctPred ct]))
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  | otherwise
  = (loc, vcat (map ppr_one (ct:cts)))
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  where
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    loc = cc_loc ct
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    ppr_one ct = hang (parens (pprType (ctPred ct))) 
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                    2 (pprArisingAt (cc_loc ct))
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mkErrorMsg :: ReportErrCtxt -> Ct -> SDoc -> TcM ErrMsg
mkErrorMsg ctxt ct msg 
  = do { let tcl_env = ctLocEnv (cc_loc ct)
       ; err_info <- mkErrInfo (cec_tidy ctxt) (tcl_ctxt tcl_env)
       ; mkLongErrAt (tcl_loc tcl_env) msg err_info }
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type UserGiven = ([EvVar], SkolemInfo, SrcSpan)
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getUserGivens :: ReportErrCtxt -> [UserGiven]
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-- One item for each enclosing implication
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getUserGivens (CEC {cec_encl = ctxt})
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  = reverse $
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    [ (givens, info, tcl_loc env) 
    | Implic {ic_given = givens, ic_env = env, ic_info = info } <- ctxt
    , not (null givens) ]
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\end{code}

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Note [Always warn with -fdefer-type-errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When -fdefer-type-errors is on we warn about *all* type errors, even
if cec_suppress is on.  This can lead to a lot more warnings than you
would get errors without -fdefer-type-errors, but if we suppress any of
them you might get a runtime error that wasn't warned about at compile
time. 

This is an easy design choice to change; just flip the order of the
first two equations for maybeReportError

To be consistent, we should also report multiple warnings from a single
location in mkGroupReporter, when -fdefer-type-errors is on.  But that 
is perhaps a bit *over*-consistent! Again, an easy choice to change.


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Note [Do not report derived but soluble errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The wc_flats include Derived constraints that have not been solved, but are
not insoluble (in that case they'd be in wc_insols).  We do not want to report
these as errors:

* Superclass constraints. If we have an unsolved [W] Ord a, we'll also have
  an unsolved [D] Eq a, and we do not want to report that; it's just noise.

* Functional dependencies.  For givens, consider
      class C a b | a -> b
      data T a where
         MkT :: C a d => [d] -> T a
      f :: C a b => T a -> F Int
      f (MkT xs) = length xs
  Then we get a [D] b~d.  But there *is* a legitimate call to
  f, namely   f (MkT [True]) :: T Bool, in which b=d.  So we should
  not reject the program.

  For wanteds, something similar
      data T a where
        MkT :: C Int b => a -> b -> T a 
      g :: C Int c => c -> ()
      f :: T a -> ()
      f (MkT x y) = g x
  Here we get [G] C Int b, [W] C Int a, hence [D] a~b.
  But again f (MkT True True) is a legitimate call.

(We leave the Deriveds in wc_flat until reportErrors, so that we don't lose
derived superclasses between iterations of the solver.)

For functional dependencies, here is a real example, 
stripped off from libraries/utf8-string/Codec/Binary/UTF8/Generic.hs

  class C a b | a -> b
  g :: C a b => a -> b -> () 
  f :: C a b => a -> b -> () 
  f xa xb = 
      let loop = g xa 
      in loop xb

We will first try to infer a type for loop, and we will succeed:
    C a b' => b' -> ()
Subsequently, we will type check (loop xb) and all is good. But, 
recall that we have to solve a final implication constraint: 
    C a b => (C a b' => .... cts from body of loop .... )) 
And now we have a problem as we will generate an equality b ~ b' and fail to 
solve it. 


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%************************************************************************
%*                  *
                Irreducible predicate errors
%*                  *
%************************************************************************

\begin{code}
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mkIrredErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
mkIrredErr ctxt cts 
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  = do { (ctxt, binds_msg) <- relevantBindings True ctxt ct1
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       ; mkErrorMsg ctxt ct1 (msg $$ binds_msg) }
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  where
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    (ct1:_) = cts
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    orig    = ctLocOrigin (cc_loc ct1)
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    givens  = getUserGivens ctxt
    msg = couldNotDeduce givens (map ctPred cts, orig)
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----------------
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mkHoleError :: ReportErrCtxt -> Ct -> TcM ErrMsg
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mkHoleError ctxt ct@(CHoleCan { cc_occ = occ })
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  = do { let tyvars = varSetElems (tyVarsOfCt ct)
             tyvars_msg = map loc_msg tyvars
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             msg = vcat [ hang (ptext (sLit "Found hole") <+> quotes (ppr occ))
                             2 (ptext (sLit "with type:") <+> pprType (ctEvPred (cc_ev ct)))
                        , ppUnless (null tyvars_msg) (ptext (sLit "Where:") <+> vcat tyvars_msg) ]
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       ; (ctxt, binds_doc) <- relevantBindings False ctxt ct
               -- The 'False' means "don't filter the bindings; see Trac #8191
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       ; mkErrorMsg ctxt ct (msg $$ binds_doc) }
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  where
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    loc_msg tv 
       = case tcTyVarDetails tv of
          SkolemTv {} -> quotes (ppr tv) <+> skol_msg
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          MetaTv {}   -> quotes (ppr tv) <+> ptext (sLit "is an ambiguous type variable")
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          det -> pprTcTyVarDetails det
       where 
          skol_msg = pprSkol (getSkolemInfo (cec_encl ctxt) tv) (getSrcLoc tv)
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mkHoleError _ ct = pprPanic "mkHoleError" (ppr ct)
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----------------
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mkIPErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
mkIPErr ctxt cts
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  = do { (ctxt, bind_msg) <- relevantBindings True ctxt ct1
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       ; mkErrorMsg ctxt ct1 (msg $$ bind_msg) }
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  where
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    (ct1:_) = cts
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    orig    = ctLocOrigin (cc_loc ct1)
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    preds   = map ctPred cts
    givens  = getUserGivens ctxt
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    msg | null givens
        = addArising orig $
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          sep [ ptext (sLit "Unbound implicit parameter") <> plural cts
              , nest 2 (pprTheta preds) ] 
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        | otherwise
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        = couldNotDeduce givens (preds, orig)
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\end{code}


%************************************************************************
%*									*
                Equality errors
%*									*
%************************************************************************

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Note [Inaccessible code]
~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   data T a where
     T1 :: T a
     T2 :: T Bool

   f :: (a ~ Int) => T a -> Int
   f T1 = 3
   f T2 = 4   -- Unreachable code

Here the second equation is unreachable. The original constraint
(a~Int) from the signature gets rewritten by the pattern-match to
(Bool~Int), so the danger is that we report the error as coming from
the *signature* (Trac #7293).  So, for Given errors we replace the
env (and hence src-loc) on its CtLoc with that from the immediately
enclosing implication.

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\begin{code}
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mkEqErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
-- Don't have multiple equality errors from the same location
-- E.g.   (Int,Bool) ~ (Bool,Int)   one error will do!
mkEqErr ctxt (ct:_) = mkEqErr1 ctxt ct
mkEqErr _ [] = panic "mkEqErr"

mkEqErr1 :: ReportErrCtxt -> Ct -> TcM ErrMsg
-- Wanted constraints only!
mkEqErr1 ctxt ct
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  | isGiven ev
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  = do { (ctxt, binds_msg) <- relevantBindings True ctxt ct
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       ; let (given_loc, given_msg) = mk_given (cec_encl ctxt)
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       ; dflags <- getDynFlags
       ; mkEqErr_help dflags ctxt (given_msg $$ binds_msg) 
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                      (ct { cc_loc = given_loc}) -- Note [Inaccessible code]
                      Nothing ty1 ty2 }

  | otherwise   -- Wanted or derived
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  = do { (ctxt, binds_msg) <- relevantBindings True ctxt ct
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       ; (ctxt, tidy_orig) <- zonkTidyOrigin ctxt (ctLocOrigin (cc_loc ct))
       ; let (is_oriented, wanted_msg) = mk_wanted_extra tidy_orig
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       ; dflags <- getDynFlags
       ; mkEqErr_help dflags ctxt (wanted_msg $$ binds_msg) 
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                      ct is_oriented ty1 ty2 }
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  where
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    ev         = cc_ev ct
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    (ty1, ty2) = getEqPredTys (ctEvPred ev)

    mk_given :: [Implication] -> (CtLoc, SDoc)
    -- For given constraints we overwrite the env (and hence src-loc)
    -- with one from the implication.  See Note [Inaccessible code]
    mk_given []           = (cc_loc ct, empty)
    mk_given (implic : _) = (setCtLocEnv (cc_loc ct) (ic_env implic)
                            , hang (ptext (sLit "Inaccessible code in"))
                                 2 (ppr (ic_info implic)))
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       -- If the types in the error message are the same as the types
       -- we are unifying, don't add the extra expected/actual message
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    mk_wanted_extra orig@(TypeEqOrigin {})
      = mkExpectedActualMsg ty1 ty2 orig

    mk_wanted_extra (KindEqOrigin cty1 cty2 sub_o)
      = (Nothing, msg1 $$ msg2)
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      where
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        msg1 = hang (ptext (sLit "When matching types"))
                  2 (vcat [ ppr cty1 <+> dcolon <+> ppr (typeKind cty1)
                          , ppr cty2 <+> dcolon <+> ppr (typeKind cty2) ])
        msg2 = case sub_o of
                 TypeEqOrigin {} -> snd (mkExpectedActualMsg cty1 cty2 sub_o)
                 _ -> empty

    mk_wanted_extra _ = (Nothing, empty)
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mkEqErr_help :: DynFlags -> ReportErrCtxt -> SDoc
             -> Ct          
             -> Maybe SwapFlag   -- Nothing <=> not sure
             -> TcType -> TcType -> TcM ErrMsg
mkEqErr_help dflags ctxt extra ct oriented ty1 ty2
  | Just tv1 <- tcGetTyVar_maybe ty1 = mkTyVarEqErr dflags ctxt extra ct oriented tv1 ty2
  | Just tv2 <- tcGetTyVar_maybe ty2 = mkTyVarEqErr dflags ctxt extra ct swapped  tv2 ty1
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  | otherwise                        = reportEqErr  ctxt extra ct oriented ty1 ty2
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  where
    swapped = fmap flipSwap oriented
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reportEqErr :: ReportErrCtxt -> SDoc
            -> Ct    
            -> Maybe SwapFlag   -- Nothing <=> not sure
            -> TcType -> TcType -> TcM ErrMsg
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reportEqErr ctxt extra1 ct oriented ty1 ty2
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  = do { let extra2 = mkEqInfoMsg ct ty1 ty2
       ; mkErrorMsg ctxt ct (vcat [ misMatchOrCND ctxt ct oriented ty1 ty2
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                                   , extra2, extra1]) }
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mkTyVarEqErr :: DynFlags -> ReportErrCtxt -> SDoc -> Ct 
             -> Maybe SwapFlag -> TcTyVar -> TcType -> TcM ErrMsg
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-- tv1 and ty2 are already tidied
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mkTyVarEqErr dflags ctxt extra ct oriented tv1 ty2
  | isUserSkolem ctxt tv1   -- ty2 won't be a meta-tyvar, or else the thing would
                            -- be oriented the other way round; see TcCanonical.reOrient
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  || isSigTyVar tv1 && not (isTyVarTy ty2)
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  = mkErrorMsg ctxt ct (vcat [ misMatchOrCND ctxt ct oriented ty1 ty2
                             , extraTyVarInfo ctxt ty1 ty2
                             , extra ])
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  -- So tv is a meta tyvar (or started that way before we 
  -- generalised it).  So presumably it is an *untouchable* 
  -- meta tyvar or a SigTv, else it'd have been unified
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  | not (k2 `tcIsSubKind` k1)   	 -- Kind error
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  = mkErrorMsg ctxt ct $ (kindErrorMsg (mkTyVarTy tv1) ty2 $$ extra)
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  | OC_Occurs <- occ_check_expand
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  = do { let occCheckMsg = hang (text "Occurs check: cannot construct the infinite type:")
                              2 (sep [ppr ty1, char '~', ppr ty2])
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             extra2 = mkEqInfoMsg ct ty1 ty2
       ; mkErrorMsg ctxt ct (occCheckMsg $$ extra2 $$ extra) }
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  | OC_Forall <- occ_check_expand
  = do { let msg = vcat [ ptext (sLit "Cannot instantiate unification variable")
                          <+> quotes (ppr tv1)
                        , hang (ptext (sLit "with a type involving foralls:")) 2 (ppr ty2)
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                        , nest 2 (ptext (sLit "Perhaps you want ImpredicativeTypes")) ]
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       ; mkErrorMsg ctxt ct msg }

  -- If the immediately-enclosing implication has 'tv' a skolem, and
  -- we know by now its an InferSkol kind of skolem, then presumably
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  -- it started life as a SigTv, else it'd have been unified, given
  -- that there's no occurs-check or forall problem
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  | (implic:_) <- cec_encl ctxt
  , Implic { ic_skols = skols } <- implic
  , tv1 `elem` skols
  = mkErrorMsg ctxt ct (vcat [ misMatchMsg oriented ty1 ty2
                             , extraTyVarInfo ctxt ty1 ty2
                             , extra ])

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  -- Check for skolem escape
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
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  , Implic { ic_env = env, ic_skols = skols, ic_info = skol_info } <- implic
  , let esc_skols = filter (`elemVarSet` (tyVarsOfType ty2)) skols
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  , not (null esc_skols)
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  = do { let msg = misMatchMsg oriented ty1 ty2
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             esc_doc = sep [ ptext (sLit "because type variable") <> plural esc_skols
                             <+> pprQuotedList esc_skols
                           , ptext (sLit "would escape") <+>
                             if isSingleton esc_skols then ptext (sLit "its scope")
                                                      else ptext (sLit "their scope") ]
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             tv_extra = vcat [ nest 2 $ esc_doc
                             , sep [ (if isSingleton esc_skols 
                                      then ptext (sLit "This (rigid, skolem) type variable is")
                                      else ptext (sLit "These (rigid, skolem) type variables are"))
                               <+> ptext (sLit "bound by")
                             , nest 2 $ ppr skol_info
                             , nest 2 $ ptext (sLit "at") <+> ppr (tcl_loc env) ] ]
       ; mkErrorMsg ctxt ct (msg $$ tv_extra $$ extra) }
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  -- Nastiest case: attempt to unify an untouchable variable
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
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  , Implic { ic_env = env, ic_given = given, ic_info = skol_info } <- implic
  = do { let msg = misMatchMsg oriented ty1 ty2
             untch_extra 
                = nest 2 $
                  sep [ quotes (ppr tv1) <+> ptext (sLit "is untouchable")
                      , nest 2 $ ptext (sLit "inside the constraints") <+> pprEvVarTheta given
                      , nest 2 $ ptext (sLit "bound by") <+> ppr skol_info
                      , nest 2 $ ptext (sLit "at") <+> ppr (tcl_loc env) ]
             tv_extra = extraTyVarInfo ctxt ty1 ty2
       ; mkErrorMsg ctxt ct (vcat [msg, untch_extra, tv_extra, extra]) }
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  | otherwise
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  = reportEqErr ctxt extra ct oriented (mkTyVarTy tv1) ty2
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        -- This *can* happen (Trac #6123, and test T2627b)
        -- Consider an ambiguous top-level constraint (a ~ F a)
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        -- Not an occurs check, because F is a type function.
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  where         
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    occ_check_expand = occurCheckExpand dflags tv1 ty2
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    k1 	= tyVarKind tv1
    k2 	= typeKind ty2
    ty1 = mkTyVarTy tv1
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mkEqInfoMsg :: Ct -> TcType -> TcType -> SDoc
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-- Report (a) ambiguity if either side is a type function application
--            e.g. F a0 ~ Int    
--        (b) warning about injectivity if both sides are the same
--            type function application   F a ~ F b
--            See Note [Non-injective type functions]
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mkEqInfoMsg ct ty1 ty2
  = tyfun_msg $$ ambig_msg
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  where
    mb_fun1 = isTyFun_maybe ty1
    mb_fun2 = isTyFun_maybe ty2
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    ambig_msg | isJust mb_fun1 || isJust mb_fun2 
              = snd (mkAmbigMsg ct)
              | otherwise = empty

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    tyfun_msg | Just tc1 <- mb_fun1
              , Just tc2 <- mb_fun2
              , tc1 == tc2 
              = ptext (sLit "NB:") <+> quotes (ppr tc1) 
                <+> ptext (sLit "is a type function, and may not be injective")
              | otherwise = empty

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isUserSkolem :: ReportErrCtxt -> TcTyVar -> Bool
-- See Note [Reporting occurs-check errors]
isUserSkolem ctxt tv
  = isSkolemTyVar tv && any is_user_skol_tv (cec_encl ctxt)
  where
    is_user_skol_tv (Implic { ic_skols = sks, ic_info = skol_info })
      = tv `elem` sks && is_user_skol_info skol_info

    is_user_skol_info (InferSkol {}) = False
    is_user_skol_info _ = True

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misMatchOrCND :: ReportErrCtxt -> Ct -> Maybe SwapFlag -> TcType -> TcType -> SDoc
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-- If oriented then ty1 is actual, ty2 is expected
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misMatchOrCND ctxt ct oriented ty1 ty2
  | null givens || 
    (isRigid ty1 && isRigid ty2) || 
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    isGivenCt ct
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       -- If the equality is unconditionally insoluble
       -- or there is no context, don't report the context
  = misMatchMsg oriented ty1 ty2
  | otherwise      
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  = couldNotDeduce givens ([mkEqPred ty1 ty2], orig)
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  where
    givens = getUserGivens ctxt
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    orig   = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2 }
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couldNotDeduce :: [UserGiven] -> (ThetaType, CtOrigin) -> SDoc
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couldNotDeduce givens (wanteds, orig)
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  = vcat [ addArising orig (ptext (sLit "Could not deduce") <+> pprTheta wanteds)
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         , vcat (pp_givens givens)]

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pp_givens :: [UserGiven] -> [SDoc]
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pp_givens givens 
   = case givens of
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         []     -> []
         (g:gs) ->      ppr_given (ptext (sLit "from the context")) g
                 : map (ppr_given (ptext (sLit "or from"))) gs
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    where 
       ppr_given herald (gs, skol_info, loc)
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           = hang (herald <+> pprEvVarTheta gs)
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                2 (sep [ ptext (sLit "bound by") <+> ppr skol_info
                       , ptext (sLit "at") <+> ppr loc])
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extraTyVarInfo :: ReportErrCtxt -> TcType -> TcType -> SDoc
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-- Add on extra info about the types themselves
-- NB: The types themselves are already tidied
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extraTyVarInfo ctxt ty1 ty2
  = nest 2 (extra1 $$ extra2)
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  where
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    extra1 = tyVarExtraInfoMsg (cec_encl ctxt) ty1
    extra2 = tyVarExtraInfoMsg (cec_encl ctxt) ty2
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tyVarExtraInfoMsg :: [Implication] -> Type -> SDoc
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-- Shows a bit of extra info about skolem constants
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tyVarExtraInfoMsg implics ty
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  | Just tv <- tcGetTyVar_maybe ty
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  , isTcTyVar tv, isSkolemTyVar tv
  , let pp_tv = quotes (ppr tv)
 = case tcTyVarDetails tv of
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    SkolemTv {}   -> pp_tv <+> pprSkol (getSkolemInfo implics tv) (getSrcLoc tv)
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    FlatSkol {}   -> pp_tv <+> ptext (sLit "is a flattening type variable")
    RuntimeUnk {} -> pp_tv <+> ptext (sLit "is an interactive-debugger skolem")
    MetaTv {}     -> empty

 | otherwise             -- Normal case
 = empty
 
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kindErrorMsg :: TcType -> TcType -> SDoc   -- Types are already tidy
kindErrorMsg ty1 ty2
  = vcat [ ptext (sLit "Kind incompatibility when matching types:")
         , nest 2 (vcat [ ppr ty1 <+> dcolon <+> ppr k1
                        , ppr ty2 <+> dcolon <+> ppr k2 ]) ]
  where
    k1 = typeKind ty1
    k2 = typeKind ty2

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--------------------
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misMatchMsg :: Maybe SwapFlag -> TcType -> TcType -> SDoc	   -- Types are already tidy
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-- If oriented then ty1 is actual, ty2 is expected
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misMatchMsg oriented ty1 ty2  
  | Just IsSwapped <- oriented
  = misMatchMsg (Just NotSwapped) ty2 ty1
  | Just NotSwapped <- oriented
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  = sep [ ptext (sLit "Couldn't match expected") <+> what <+> quotes (ppr ty2)
        , nest 12 $   ptext (sLit "with actual") <+> what <+> quotes (ppr ty1) ]
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  | otherwise
  = sep [ ptext (sLit "Couldn't match") <+> what <+> quotes (ppr ty1)
        , nest 14 $ ptext (sLit "with") <+> quotes (ppr ty2) ]
  where 
    what | isKind ty1 = ptext (sLit "kind")
         | otherwise  = ptext (sLit "type")
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mkExpectedActualMsg :: Type -> Type -> CtOrigin -> (Maybe SwapFlag, SDoc)
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-- NotSwapped means (actual, expected), IsSwapped is the reverse
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mkExpectedActualMsg ty1 ty2 (TypeEqOrigin { uo_actual = act, uo_expected = exp })
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  | act `pickyEqType` ty1, exp `pickyEqType` ty2 = (Just NotSwapped,  empty)
  | exp `pickyEqType` ty1, act `pickyEqType` ty2 = (Just IsSwapped, empty)
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  | otherwise                                    = (Nothing, msg)
  where
    msg = vcat [ text "Expected type:" <+> ppr exp
               , text "  Actual type:" <+> ppr act ]

mkExpectedActualMsg _ _ _ = panic "mkExprectedAcutalMsg"
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\end{code}

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Note [Reporting occurs-check errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (a ~ [a]), if 'a' is a rigid type variable bound by a user-supplied
type signature, then the best thing is to report that we can't unify
a with [a], because a is a skolem variable.  That avoids the confusing
"occur-check" error message.

But nowadays when inferring the type of a function with no type signature,
even if there are errors inside, we still generalise its signature and
carry on. For example
   f x = x:x
Here we will infer somethiing like
   f :: forall a. a -> [a]
with a suspended error of (a ~ [a]).  So 'a' is now a skolem, but not
one bound by the programmer!  Here we really should report an occurs check.

So isUserSkolem distinguishes the two.

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Note [Non-injective type functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's very confusing to get a message like
     Couldn't match expected type `Depend s'
            against inferred type `Depend s1'
so mkTyFunInfoMsg adds:
       NB: `Depend' is type function, and hence may not be injective

Warn of loopy local equalities that were dropped.


%************************************************************************
%*									*
                 Type-class errors
%*									*
%************************************************************************

\begin{code}
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mkDictErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
mkDictErr ctxt cts 
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  = ASSERT( not (null cts) )
    do { inst_envs <- tcGetInstEnvs
       ; lookups   <- mapM (lookup_cls_inst inst_envs) cts
       ; let (no_inst_cts, overlap_cts) = partition is_no_inst lookups

       -- Report definite no-instance errors, 
       -- or (iff there are none) overlap errors
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       -- But we report only one of them (hence 'head') because they all
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       -- have the same source-location origin, to try avoid a cascade
       -- of error from one location
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       ; (ctxt, err) <- mk_dict_err ctxt (head (no_inst_cts ++ overlap_cts))
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       ; mkErrorMsg ctxt ct1 err }
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  where
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    ct1:_ = cts
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    no_givens = null (getUserGivens ctxt)
    is_no_inst (ct, (matches, unifiers, _))
      =  no_givens 
      && null matches 
      && (null unifiers || all (not . isAmbiguousTyVar) (varSetElems (tyVarsOfCt ct)))
           
    lookup_cls_inst inst_envs ct
      = do { tys_flat <- mapM quickFlattenTy tys
                -- Note [Flattening in error message generation]
           ; return (ct, lookupInstEnv inst_envs clas tys_flat) }
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      where
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        (clas, tys) = getClassPredTys (ctPred ct)
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mk_dict_err :: ReportErrCtxt -> (Ct, ClsInstLookupResult)
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            -> TcM (ReportErrCtxt, SDoc)
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-- Report an overlap error if this class constraint results
-- from an overlap (returning Left clas), otherwise return (Right pred)
mk_dict_err ctxt (ct, (matches, unifiers, safe_haskell)) 
  | null matches  -- No matches but perhaps several unifiers
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  = do { let (is_ambig, ambig_msg) = mkAmbigMsg ct
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       ; (ctxt, binds_msg) <- relevantBindings True ctxt ct
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       ; traceTc "mk_dict_err" (ppr ct $$ ppr is_ambig $$ ambig_msg)
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       ; safe_mod <- safeLanguageOn `fmap` getDynFlags
       ; rdr_env <- getGlobalRdrEnv
       ; return (ctxt, cannot_resolve_msg safe_mod rdr_env is_ambig binds_msg ambig_msg) }
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  | not safe_haskell   -- Some matches => overlap errors
  = return (ctxt, overlap_msg)
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  | otherwise
  = return (ctxt, safe_haskell_msg)
  where
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    orig        = ctLocOrigin (cc_loc ct)
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    pred        = ctPred ct
    (clas, tys) = getClassPredTys pred
    ispecs      = [ispec | (ispec, _) <- matches]
    givens      = getUserGivens ctxt
    all_tyvars  = all isTyVarTy tys

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    cannot_resolve_msg safe_mod rdr_env has_ambig_tvs binds_msg ambig_msg
      = vcat [ addArising orig (no_inst_herald <+> pprParendType pred $$
                                coercible_msg safe_mod rdr_env)
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             , vcat (pp_givens givens)
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             , ppWhen (has_ambig_tvs && not (null unifiers && null givens))
               (vcat [ ambig_msg, binds_msg, potential_msg ])
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             , show_fixes (add_to_ctxt_fixes has_ambig_tvs ++ drv_fixes) ]
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    potential_msg
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      = ppWhen (not (null unifiers) && want_potential orig) $
        hang (if isSingleton unifiers 
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              then ptext (sLit "Note: there is a potential instance available:")
              else ptext (sLit "Note: there are several potential instances:"))
    	   2 (ppr_insts unifiers)

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    -- Report "potential instances" only when the constraint arises
    -- directly from the user's use of an overloaded function
    want_potential (AmbigOrigin {})   = False
    want_potential _                  = True

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    add_to_ctxt_fixes has_ambig_tvs
      | not has_ambig_tvs && all_tyvars
      , (orig:origs) <- mapCatMaybes get_good_orig (cec_encl ctxt)
      = [sep [ ptext (sLit "add") <+> pprParendType pred
               <+> ptext (sLit "to the context of")
	     , nest 2 $ ppr_skol orig $$ 
                        vcat [ ptext (sLit "or") <+> ppr_skol orig 
                             | orig <- origs ] ] ]
      | otherwise = []

    ppr_skol (PatSkol dc _) = ptext (sLit "the data constructor") <+> quotes (ppr dc)
    ppr_skol skol_info      = ppr skol_info
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	-- Do not suggest adding constraints to an *inferred* type signature!
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    get_good_orig ic = case ic_info ic of 
                         SigSkol (InfSigCtxt {}) _ -> Nothing
                         origin                    -> Just origin
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    no_inst_herald
      | null givens && null matches = ptext (sLit "No instance for")
      | otherwise                   = ptext (sLit "Could not deduce")
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    drv_fixes = case orig of
                   DerivOrigin -> [drv_fix]
                   _           -> []
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    drv_fix = hang (ptext (sLit "use a standalone 'deriving instance' declaration,"))
                 2 (ptext (sLit "so you can specify the instance context yourself"))