TcErrors.hs 108 KB
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{-# LANGUAGE CPP, ScopedTypeVariables #-}
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module TcErrors(
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       reportUnsolved, reportAllUnsolved, warnAllUnsolved,
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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 RnEnv( unknownNameSuggestions )
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import Type
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import TyCoRep
import Kind
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import Unify            ( tcMatchTys )
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import Module
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import FamInst
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import FamInstEnv       ( flattenTys )
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import Inst
import InstEnv
import TyCon
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import Class
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import DataCon
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import TcEvidence
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import HsExpr  ( UnboundVar(..) )
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import HsBinds ( PatSynBind(..) )
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import Name
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import RdrName ( lookupGlobalRdrEnv, lookupGRE_Name, GlobalRdrEnv
               , mkRdrUnqual, isLocalGRE, greSrcSpan )
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import PrelNames ( typeableClassName, hasKey, ptrRepLiftedDataConKey
                 , ptrRepUnliftedDataConKey )
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import Id
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import Var
import VarSet
import VarEnv
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import NameSet
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import Bag
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import ErrUtils         ( ErrMsg, errDoc, pprLocErrMsg )
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import BasicTypes
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import ConLike          ( ConLike(..) )
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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 StaticFlags      ( opt_PprStyle_Debug )
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import ListSetOps       ( equivClasses )
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import Maybes
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import qualified GHC.LanguageExtensions as LangExt
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import Control.Monad    ( when )
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import Data.List        ( partition, mapAccumL, nub, sortBy )
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import qualified Data.Set as Set
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#if __GLASGOW_HASKELL__ > 710
import Data.Semigroup   ( Semigroup )
import qualified Data.Semigroup as Semigroup
#endif
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{-
************************************************************************
*                                                                      *
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\section{Errors and contexts}
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*                                                                      *
************************************************************************
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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`.
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-}
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-- | Report unsolved goals as errors or warnings. We may also turn some into
-- deferred run-time errors if `-fdefer-type-errors` is on.
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reportUnsolved :: WantedConstraints -> TcM (Bag EvBind)
reportUnsolved wanted
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  = 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
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       ; defer_holes <- goptM Opt_DeferTypedHoles
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       ; warn_holes  <- woptM Opt_WarnTypedHoles
       ; let expr_holes | not defer_holes = HoleError
                        | warn_holes      = HoleWarn
                        | otherwise       = HoleDefer

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

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       ; report_unsolved (Just binds_var) False type_errors expr_holes type_holes wanted
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       ; getTcEvBinds binds_var }

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-- | Report *all* unsolved goals as errors, even if -fdefer-type-errors is on
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-- However, do not make any evidence bindings, because we don't
-- have any convenient place to put them.
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-- See Note [Deferring coercion errors to runtime]
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-- Used by solveEqualities for kind equalities
--      (see Note [Fail fast on kind errors] in TcSimplify]
-- and for simplifyDefault.
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reportAllUnsolved :: WantedConstraints -> TcM ()
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reportAllUnsolved wanted
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  = report_unsolved Nothing False TypeError HoleError HoleError wanted
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-- | 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
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  = report_unsolved Nothing True TypeWarn HoleWarn HoleWarn wanted
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-- | Report unsolved goals as errors or warnings.
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report_unsolved :: Maybe EvBindsVar  -- cec_binds
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                -> Bool              -- Errors as warnings
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                -> TypeErrorChoice   -- Deferred type errors
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                -> HoleChoice        -- Expression holes
                -> HoleChoice        -- Type holes
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                -> WantedConstraints -> TcM ()
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report_unsolved mb_binds_var err_as_warn type_errors expr_holes type_holes 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 zonking and tidying)" (ppr wanted)
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       ; wanted <- zonkWC wanted   -- Zonk to reveal all information
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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 = tidyFreeTyCoVars env0 free_tvs
             free_tvs = tyCoVarsOfWC wanted
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       ; traceTc "reportUnsolved (after zonking and tidying):" $
         vcat [ pprTvBndrs (varSetElems free_tvs)
              , ppr wanted ]

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

{- 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 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.
-}

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#if __GLASGOW_HASKELL__ > 710
instance Semigroup Report where
    Report a1 b1 <> Report a2 b2 = Report (a1 ++ a2) (b1 ++ b2)
#endif

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instance Monoid Report where
    mempty = Report [] []
    mappend (Report a1 b1) (Report a2 b2) = Report (a1 ++ a2) (b1 ++ b2)

-- | 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] }

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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

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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

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data ReportErrCtxt
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    = CEC { cec_encl :: [Implication]  -- Enclosing implications
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                                       --   (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
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                         -- Nothing <=> Report all errors, including holes
                         --             Do not add any evidence bindings, because
                         --             we have no convenient place to put them
                         --             See TcErrors.reportAllUnsolved
                         -- 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_errors_as_warns :: Bool   -- Turn all errors into warnings
                                          -- (except for Holes, which are
                                          -- controlled by cec_type_holes and
                                          -- cec_expr_holes)
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          , cec_defer_type_errors :: TypeErrorChoice -- Defer type errors until runtime
                                                     -- Irrelevant if cec_binds = Nothing
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          , cec_expr_holes :: HoleChoice  -- Holes in expressions
          , cec_type_holes :: HoleChoice  -- Holes in types
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          , cec_warn_redundant :: Bool    -- True <=> -Wredundant-constraints
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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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{-
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Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The cec_suppress flag says "don't report any errors".  Instead, just create
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evidence bindings (as usual).  It's used when more important errors have occurred.
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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.
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  * If there are any insolubles (eg Int~Bool), here or in a nested implication,
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    then suppress errors from the simple constraints here.  Sometimes the
    simple-constraint errors are a knock-on effect of the insolubles.
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-}
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reportImplic :: ReportErrCtxt -> Implication -> TcM ()
reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
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                                 , ic_wanted = wanted, ic_binds = m_evb
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                                 , ic_status = status, ic_info = info
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                                 , ic_env = tcl_env, ic_tclvl = tc_lvl })
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  | BracketSkol <- info
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  , not insoluble
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  = return ()        -- For Template Haskell brackets report only
                     -- definite errors. The whole thing will be re-checked
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                     -- later when we plug it in, and meanwhile there may
                     -- certainly be un-satisfied constraints
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  | otherwise
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  = do { reportWanteds ctxt' tc_lvl wanted
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       ; traceTc "reportImplic" (ppr implic)
       ; when (cec_warn_redundant ctxt) $
         warnRedundantConstraints ctxt' tcl_env info' dead_givens }
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  where
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    insoluble    = isInsolubleStatus status
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    (env1, tvs') = mapAccumL tidyTyCoVarBndr (cec_tidy ctxt) tvs
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    info'        = tidySkolemInfo env1 info
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    implic' = implic { ic_skols = tvs'
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                     , ic_given = map (tidyEvVar env1) given
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                     , ic_info  = info' }
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    ctxt' = ctxt { cec_tidy     = env1
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                 , cec_encl     = implic' : cec_encl ctxt
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                 , cec_suppress = insoluble || cec_suppress ctxt
                      -- Suppress inessential errors if there
                      -- are are insolubles anywhere in the
                      -- tree rooted here, or we've come across
                      -- a suppress-worthy constraint higher up (Trac #11541)

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                 , cec_binds    = cec_binds ctxt *> m_evb }
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                      -- If cec_binds ctxt is Nothing, that means
                      -- we're reporting *all* errors. Don't change
                      -- that behavior just because we're going into
                      -- an implication.

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

warnRedundantConstraints :: ReportErrCtxt -> TcLclEnv -> SkolemInfo -> [EvVar] -> TcM ()
warnRedundantConstraints ctxt env info ev_vars
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 | null redundant_evs
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 = return ()

 | SigSkol {} <- info
 = setLclEnv env $  -- We want to add "In the type signature for f"
                    -- to the error context, which is a bit tiresome
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   addErrCtxt (text "In" <+> ppr info) $
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   do { env <- getLclEnv
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      ; msg <- mkErrorReport ctxt env (important doc)
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      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
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 | 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
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 = do { msg <- mkErrorReport ctxt env (important doc)
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      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
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 where
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   doc = text "Redundant constraint" <> plural redundant_evs <> colon
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         <+> 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 $
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                      transSuperClasses (idType ev_var)
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{- 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 feilds.  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..
-}
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reportWanteds :: ReportErrCtxt -> TcLevel -> WantedConstraints -> TcM ()
reportWanteds ctxt tc_lvl (WC { wc_simple = simples, wc_insol = insols, wc_impl = implics })
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  = do { traceTc "reportWanteds" (vcat [ text "Simples =" <+> ppr simples
                                       , text "Suppress =" <+> ppr (cec_suppress ctxt)])
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       ; let tidy_cts = bagToList (mapBag (tidyCt env) (insols `unionBags` simples))
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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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         -- 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
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       ; MASSERT2( null leftovers, ppr leftovers )

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            -- All the Derived ones have been filtered out of simples
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            -- 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 ctxt2) implics }
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            -- NB ctxt1: don't suppress inner insolubles if there's only a
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            -- wanted insoluble here; but do suppress inner insolubles
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            -- if there's a *given* insoluble here (= inaccessible code)
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 where
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    env = cec_tidy ctxt
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    -- 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.trulyInsoluble) is caught here, otherwise
    -- we might suppress its error message, and proceed on past
    -- type checking to get a Lint error later
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    report1 = [ ("custom_error", is_user_type_error,
                                                  True, mkUserTypeErrorReporter)
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              , ("insoluble1",   is_given_eq,     True, mkGroupReporter mkEqErr)
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              , ("insoluble2",   utterly_wrong,   True, mkGroupReporter mkEqErr)
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              , ("skolem eq1",   very_wrong,      True, mkSkolReporter)
              , ("skolem eq2",   skolem_eq,       True, mkSkolReporter)
              , ("non-tv eq",    non_tv_eq,       True, mkSkolReporter)
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              , ("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
              , ("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) ]

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    -- rigid_nom_eq, rigid_nom_tv_eq,
    is_hole, is_dict,
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      is_equality, is_ip, is_irred :: Ct -> PredTree -> Bool
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    is_given_eq ct pred
       | EqPred {} <- pred = arisesFromGivens ct
       | otherwise         = False
       -- I think all given residuals are equalities

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    -- 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
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    -- Things like (a ~N b) or (a  ~N  F Bool)
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    skolem_eq _ (EqPred NomEq ty1 _) = isSkolemTy tc_lvl ty1
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    skolem_eq _ _                    = False
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    -- Things like (F a  ~N  Int)
    non_tv_eq _ (EqPred NomEq ty1 _) = not (isTyVarTy ty1)
    non_tv_eq _ _                    = False

--    rigid_nom_eq _ pred = isRigidEqPred tc_lvl pred
--
--    rigid_nom_tv_eq _ pred
--      | EqPred _ ty1 _ <- pred = isRigidEqPred tc_lvl pred && isTyVarTy ty1
--      | otherwise              = False

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

    is_user_type_error ct _ = isUserTypeErrorCt ct
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    is_equality _ (EqPred {}) = True
    is_equality _ _           = False

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

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

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


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---------------
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isSkolemTy :: TcLevel -> Type -> Bool
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-- The type is a skolem tyvar
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isSkolemTy tc_lvl ty
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  | 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
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isTyFun_maybe :: Type -> Maybe TyCon
isTyFun_maybe ty = case tcSplitTyConApp_maybe ty of
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                      Just (tc,_) | isTypeFamilyTyCon tc -> Just tc
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                      _ -> Nothing

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

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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
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-- Suppress duplicates with either the same LHS, or same location
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mkSkolReporter ctxt cts
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  = mapM_ (reportGroup mkEqErr ctxt) (group cts)
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  where
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     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 <- cmp_lhs_type ct1 ct2 = True
       | otherwise                  = False
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mkHoleReporter :: Reporter
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-- Reports errors one at a time
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mkHoleReporter ctxt
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  = mapM_ $ \ct ->
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    do { err <- mkHoleError ctxt ct
       ; maybeReportHoleError ctxt ct err
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       ; maybeAddDeferredHoleBinding ctxt err ct }
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mkUserTypeErrorReporter :: Reporter
mkUserTypeErrorReporter ctxt
  = mapM_ $ \ct -> maybeReportError ctxt =<< mkUserTypeError ctxt ct

mkUserTypeError :: ReportErrCtxt -> Ct -> TcM ErrMsg
mkUserTypeError ctxt ct = mkErrorMsgFromCt ctxt ct
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                        $ important
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                        $ pprUserTypeErrorTy
                        $ case getUserTypeErrorMsg ct of
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                            Just msg -> msg
                            Nothing  -> pprPanic "mkUserTypeError" (ppr 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 errors from same location,
-- and report only the first (to avoid a cascade)
mkGroupReporter mk_err ctxt cts
  = mapM_ (reportGroup mk_err ctxt) (equivClasses cmp_loc cts)
  where
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cmp_lhs_type :: Ct -> Ct -> Ordering
cmp_lhs_type ct1 ct2
  = case (classifyPredType (ctPred ct1), classifyPredType (ctPred ct2)) of
       (EqPred eq_rel1 ty1 _, EqPred eq_rel2 ty2 _) ->
         (eq_rel1 `compare` eq_rel2) `thenCmp` (ty1 `cmpType` ty2)
       _ -> pprPanic "mkSkolReporter" (ppr ct1 $$ ppr ct2)

cmp_loc :: Ct -> Ct -> Ordering
cmp_loc ct1 ct2 = ctLocSpan (ctLoc ct1) `compare` ctLocSpan (ctLoc ct2)
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reportGroup :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg) -> ReportErrCtxt
            -> [Ct] -> TcM ()
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reportGroup mk_err ctxt cts =
  case partition isMonadFailInstanceMissing cts of
        -- Only warn about missing MonadFail constraint when
        -- there are no other missing contstraints!
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        (monadFailCts, []) ->
            do { err <- mk_err ctxt monadFailCts
               ; reportWarning (Reason Opt_WarnMissingMonadFailInstances) err }
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        (_, cts') -> do { err <- mk_err ctxt cts'
                        ; maybeReportError ctxt err
                        ; mapM_ (maybeAddDeferredBinding ctxt err) cts' }
                                -- Add deferred bindings for all
                                -- But see Note [Always warn with -fdefer-type-errors]
  where
    isMonadFailInstanceMissing ct =
        case ctLocOrigin (ctLoc ct) of
            FailablePattern _pat -> True
            _otherwise           -> False
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maybeReportHoleError :: ReportErrCtxt -> Ct -> ErrMsg -> TcM ()
maybeReportHoleError ctxt ct err
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  -- When -XPartialTypeSignatures is on, warnings (instead of errors) are
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  -- generated for holes in partial type signatures.
  -- Unless -fwarn_partial_type_signatures is not on,
  -- in which case the messages are discarded.
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  | 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
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       HoleWarn  -> reportWarning (Reason Opt_WarnPartialTypeSignatures) err
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       HoleDefer -> return ()

  -- Otherwise this is a typed hole in an expression
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  | otherwise
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  = -- If deferring, report a warning only if -Wtyped-holds is on
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    case cec_expr_holes ctxt of
       HoleError -> reportError err
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       HoleWarn  -> reportWarning (Reason Opt_WarnTypedHoles) err
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       HoleDefer -> return ()
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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_suppress ctxt    -- Some worse error has occurred;
  = return ()            -- so suppress this error/warning

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  | cec_errors_as_warns ctxt
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  = reportWarning NoReason err
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  | otherwise
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  = case cec_defer_type_errors ctxt of
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      TypeDefer -> return ()
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      TypeWarn  -> reportWarning (Reason Opt_WarnDeferredTypeErrors) err
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      TypeError -> reportError err
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addDeferredBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
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-- See Note [Deferring coercion errors to runtime]
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addDeferredBinding ctxt err ct
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  | CtWanted { ctev_pred = pred, ctev_dest = dest } <- ctEvidence ct
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    -- Only add deferred bindings for Wanted constraints
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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)"
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             err_tm  = EvDelayedError pred err_fs
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       ; case dest of
           EvVarDest evar
             -> addTcEvBind ev_binds_var $ mkWantedEvBind evar err_tm
           HoleDest hole
             -> do { -- See Note [Deferred errors for coercion holes]
                     evar <- newEvVar pred
                   ; addTcEvBind ev_binds_var $ mkWantedEvBind evar err_tm
                   ; fillCoercionHole hole (mkTcCoVarCo evar) }}
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  | otherwise   -- Do not set any evidence for Given/Derived
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  = return ()
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maybeAddDeferredHoleBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
maybeAddDeferredHoleBinding ctxt err ct
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    | isExprHoleCt ct
    , case cec_expr_holes ctxt of
        HoleDefer -> True
        HoleWarn  -> True
        HoleError -> False
    = addDeferredBinding ctxt err ct  -- Only add bindings for holes in expressions
    | otherwise                       -- not for holes in partial type signatures
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    = return ()

maybeAddDeferredBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
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maybeAddDeferredBinding ctxt err ct =
  case cec_defer_type_errors ctxt of
        TypeDefer -> deferred
        TypeWarn -> deferred
        TypeError -> return ()
  where
    deferred = addDeferredBinding ctxt err ct
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tryReporters :: ReportErrCtxt -> [ReporterSpec] -> [Ct] -> TcM (ReportErrCtxt, [Ct])
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-- Use the first reporter in the list whose predicate says True
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tryReporters ctxt reporters cts
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  = do { traceTc "tryReporters {" (ppr cts)
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       ; (ctxt', cts') <- go ctxt reporters cts
       ; traceTc "tryReporters }" (ppr cts')
       ; return (ctxt', cts') }
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  where
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    go ctxt [] cts
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      = return (ctxt, cts)

    go ctxt (r : rs) cts
      = do { (ctxt', cts') <- tryReporter ctxt r cts
           ; go ctxt' rs 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 }
                    ; return (ctxt', nos) }
  where
    (yeses, nos) = partition (\ct -> keep_me ct (classifyPredType (ctPred ct))) cts
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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

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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 []
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  = 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 = ctLoc ct
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    ppr_one ct' = hang (parens (pprType (ctPred ct')))
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                     2 (pprCtLoc (ctLoc ct'))
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mkErrorMsgFromCt :: ReportErrCtxt -> Ct -> Report -> TcM ErrMsg
mkErrorMsgFromCt ctxt ct report
  = mkErrorReport ctxt (ctLocEnv (ctLoc ct)) report
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mkErrorReport :: ReportErrCtxt -> TcLclEnv -> Report -> TcM ErrMsg
mkErrorReport ctxt tcl_env (Report important relevant_bindings)
  = do { context <- mkErrInfo (cec_tidy ctxt) (tcl_ctxt tcl_env)
       ; mkErrDocAt (RealSrcSpan (tcl_loc tcl_env))
            (errDoc important [context] relevant_bindings)
       }
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type UserGiven = Implication
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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 = implics}) = getUserGivensFromImplics implics

getUserGivensFromImplics :: [Implication] -> [UserGiven]
getUserGivensFromImplics implics
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  = reverse (filterOut (null . ic_given) implics)
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{-
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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
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time.
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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
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location in mkGroupReporter, when -fdefer-type-errors is on.  But that
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is perhaps a bit *over*-consistent! Again, an easy choice to change.

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With #10283, you can now opt out of deferred type error warnings.

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Note [Deferred errors for coercion holes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we need to defer a type error where the destination for the evidence
is a coercion hole. We can't just put the error in the hole, because we can't
make an erroneous coercion. (Remember that coercions are erased for runtime.)
Instead, we invent a new EvVar, bind it to an error and then make a coercion
from that EvVar, filling the hole with that coercion. Because coercions'
types are unlifted, the error is guaranteed to be hit before we get to the
coercion.
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Note [Do not report derived but soluble errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The wc_simples include Derived constraints that have not been solved, but are
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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
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        MkT :: C Int b => a -> b -> T a
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      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.

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

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

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

We will first try to infer a type for loop, and we will succeed:
    C a b' => b' -> ()
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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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************************************************************************
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*                                                                      *
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                Irreducible predicate errors
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*                                                                      *
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************************************************************************
-}
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mkIrredErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
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mkIrredErr ctxt cts
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  = do { (ctxt, binds_msg, ct1) <- relevantBindings True ctxt ct1
       ; let orig = ctOrigin ct1
             msg  = couldNotDeduce (getUserGivens ctxt) (map ctPred cts, orig)
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       ; mkErrorMsgFromCt ctxt ct1 $
            important msg `mappend` relevant_bindings binds_msg }
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  where
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    (ct1:_) = cts
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----------------
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mkHoleError :: ReportErrCtxt -> Ct -> TcM ErrMsg
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mkHoleError _ctxt ct@(CHoleCan { cc_hole = ExprHole (OutOfScope occ rdr_env0) })
  -- Out-of-scope variables, like 'a', where 'a' isn't bound; suggest possible
  -- in-scope variables in the message, and note inaccessible exact matches
  = do { dflags   <- getDynFlags
       ; imp_info <- getImports
       ; let suggs_msg = unknownNameSuggestions dflags rdr_env0
                                                (tcl_rdr lcl_env) imp_info rdr
       ; rdr_env     <- getGlobalRdrEnv
       ; splice_locs <- getTopLevelSpliceLocs
       ; let match_msgs = mk_match_msgs rdr_env splice_locs
       ; mkErrDocAt (RealSrcSpan err_loc) $
                    errDoc [out_of_scope_msg] [] (match_msgs ++ [suggs_msg]) }
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  where
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    rdr         = mkRdrUnqual occ
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    ct_loc      = ctLoc ct
    lcl_env     = ctLocEnv ct_loc