TcErrors.hs 145 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 GhcPrelude

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import TcRnTypes
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import TcRnMonad
import TcMType
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import TcUnify( occCheckForErrors, OccCheckResult(..) )
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import TcEnv( tcInitTidyEnv )
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import TcType
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import RnUnbound ( 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 TcEvTerm
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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
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               , mkRdrUnqual, isLocalGRE, greSrcSpan, pprNameProvenance
               , GlobalRdrElt (..), globalRdrEnvElts )
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import PrelNames ( typeableClassName, hasKey, liftedRepDataConKey, tYPETyConKey )
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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
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import TcEnv (tcLookup)
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import {-# SOURCE #-} TcSimplify ( tcCheckHoleFit, tcSubsumes )
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import FastString
import Outputable
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import SrcLoc
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import DynFlags
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import ListSetOps       ( equivClasses )
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import Maybes
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import Pair
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import qualified GHC.LanguageExtensions as LangExt
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import FV ( fvVarList, fvVarSet, unionFV )
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import Control.Monad    ( when, filterM, replicateM )
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import Data.Foldable    ( toList )
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import Data.List        ( partition, mapAccumL, nub
                        , sortBy, sort, unfoldr, foldl' )
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import qualified Data.Set as Set
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import Data.Graph       ( graphFromEdges, topSort )
import Data.Function    ( on )
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import Data.Semigroup   ( Semigroup )
import qualified Data.Semigroup as Semigroup
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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
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                         | warn_errors      = TypeWarn (Reason Opt_WarnDeferredTypeErrors)
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                         | 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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       ; defer_out_of_scope <- goptM Opt_DeferOutOfScopeVariables
       ; warn_out_of_scope <- woptM Opt_WarnDeferredOutOfScopeVariables
       ; let out_of_scope_holes | not defer_out_of_scope = HoleError
                                | warn_out_of_scope      = HoleWarn
                                | otherwise              = HoleDefer

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       ; report_unsolved binds_var type_errors expr_holes
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          type_holes out_of_scope_holes wanted
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       ; ev_binds <- getTcEvBindsMap binds_var
       ; return (evBindMapBinds ev_binds)}
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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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  = do { ev_binds <- newNoTcEvBinds
       ; report_unsolved ev_binds TypeError
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                         HoleError 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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  = do { ev_binds <- newTcEvBinds
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       ; report_unsolved ev_binds (TypeWarn NoReason)
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                         HoleWarn HoleWarn HoleWarn wanted }
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-- | Report unsolved goals as errors or warnings.
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report_unsolved :: EvBindsVar        -- cec_binds
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                -> TypeErrorChoice   -- Deferred type errors
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                -> HoleChoice        -- Expression holes
                -> HoleChoice        -- Type holes
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                -> HoleChoice        -- Out of scope holes
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                -> WantedConstraints -> TcM ()
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report_unsolved mb_binds_var type_errors expr_holes
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    type_holes out_of_scope_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 warning/error settings:" $
           vcat [ text "type errors:" <+> ppr type_errors
                , text "expr holes:" <+> ppr expr_holes
                , text "type holes:" <+> ppr type_holes
                , text "scope holes:" <+> ppr out_of_scope_holes ]
       ; 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
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             free_tvs = tyCoVarsOfWCList wanted
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       ; traceTc "reportUnsolved (after zonking):" $
         vcat [ text "Free tyvars:" <+> pprTyVars free_tvs
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              , text "Tidy env:" <+> ppr tidy_env
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              , text "Wanted:" <+> ppr wanted ]
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       ; 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_expr_holes = expr_holes
                            , cec_type_holes = type_holes
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                            , cec_out_of_scope_holes = out_of_scope_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]
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           , report_valid_substitutions :: [SDoc]
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           }

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

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

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instance Semigroup Report where
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    Report a1 b1 c1 <> Report a2 b2 c2 = Report (a1 ++ a2) (b1 ++ b2) (c1 ++ c2)
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instance Monoid Report where
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    mempty = Report [] [] []
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    mappend = (Semigroup.<>)
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-- | 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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-- | Put a doc into the valid substitutions block.
valid_substitutions :: SDoc -> Report
valid_substitutions docs = mempty { report_valid_substitutions = [docs] }

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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
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  | TypeWarn WarnReason
                  -- A type error is deferred to runtime, plus a compile-time warning
                  -- The WarnReason should usually be (Reason Opt_WarnDeferredTypeErrors)
                  -- but it isn't for the Safe Haskell Overlapping Instances warnings
                  -- see warnAllUnsolved
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  | 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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instance Outputable HoleChoice where
  ppr HoleError = text "HoleError"
  ppr HoleWarn  = text "HoleWarn"
  ppr HoleDefer = text "HoleDefer"

instance Outputable TypeErrorChoice  where
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  ppr TypeError         = text "TypeError"
  ppr (TypeWarn reason) = text "TypeWarn" <+> ppr reason
  ppr TypeDefer         = text "TypeDefer"
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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 :: EvBindsVar    -- Make some errors (depending on cec_defer)
                                       -- into warnings, and emit evidence bindings
                                       -- into 'cec_binds' for unsolved constraints
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          , cec_defer_type_errors :: TypeErrorChoice -- Defer type errors until runtime
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          -- cec_expr_holes is a union of:
          --   cec_type_holes - a set of typed holes: '_', '_a', '_foo'
          --   cec_out_of_scope_holes - a set of variables which are
          --                            out of scope: 'x', 'y', 'bar'
          , cec_expr_holes :: HoleChoice           -- Holes in expressions
          , cec_type_holes :: HoleChoice           -- Holes in types
          , cec_out_of_scope_holes :: HoleChoice   -- Out of scope holes
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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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instance Outputable ReportErrCtxt where
  ppr (CEC { cec_binds              = bvar
           , cec_defer_type_errors  = dte
           , cec_expr_holes         = eh
           , cec_type_holes         = th
           , cec_out_of_scope_holes = osh
           , cec_warn_redundant     = wr
           , cec_suppress           = sup })
    = text "CEC" <+> braces (vcat
         [ text "cec_binds"              <+> equals <+> ppr bvar
         , text "cec_defer_type_errors"  <+> equals <+> ppr dte
         , text "cec_expr_holes"         <+> equals <+> ppr eh
         , text "cec_type_holes"         <+> equals <+> ppr th
         , text "cec_out_of_scope_holes" <+> equals <+> ppr osh
         , text "cec_warn_redundant"     <+> equals <+> ppr wr
         , text "cec_suppress"           <+> equals <+> ppr sup ])

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-- | Returns True <=> the ReportErrCtxt indicates that something is deferred
deferringAnyBindings :: ReportErrCtxt -> Bool
  -- Don't check cec_type_holes, as these don't cause bindings to be deferred
deferringAnyBindings (CEC { cec_defer_type_errors  = TypeError
                          , cec_expr_holes         = HoleError
                          , cec_out_of_scope_holes = HoleError }) = False
deferringAnyBindings _                                            = True

-- | Transforms a 'ReportErrCtxt' into one that does not defer any bindings
-- at all.
noDeferredBindings :: ReportErrCtxt -> ReportErrCtxt
noDeferredBindings ctxt = ctxt { cec_defer_type_errors  = TypeError
                               , cec_expr_holes         = HoleError
                               , cec_out_of_scope_holes = HoleError }

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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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This suppression behaviour is controlled by the Bool flag in
ReportErrorSpec, as used in reportWanteds.

But we need to take care: flags can turn errors into warnings, and we
don't want those warnings to suppress subsequent errors (including
suppressing the essential addTcEvBind for them: Trac #15152). So in
tryReporter we use askNoErrs to see if any error messages were
/actually/ produced; if not, we don't switch on suppression.

A consequence is that warnings never suppress warnings, so turning an
error into a warning may allow subsequent warnings to appear that were
previously suppressed.   (e.g. partial-sigs/should_fail/T14584)
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-}
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reportImplic :: ReportErrCtxt -> Implication -> TcM ()
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reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_telescope = m_telescope
                                 , ic_given = given
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                                 , ic_wanted = wanted, ic_binds = 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 { traceTc "reportImplic" (ppr implic')
       ; reportWanteds ctxt' tc_lvl wanted
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       ; when (cec_warn_redundant ctxt) $
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         warnRedundantConstraints ctxt' tcl_env info' dead_givens
       ; when bad_telescope $ reportBadTelescope ctxt tcl_env m_telescope tvs }
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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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    ctxt1 | NoEvBindsVar{} <- evb    = noDeferredBindings ctxt
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          | otherwise                = ctxt
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          -- If we go inside an implication that has no term
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          -- evidence (e.g. unifying under a forall), we can't defer
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          -- type errors.  You could imagine using the /enclosing/
          -- bindings (in cec_binds), but that may not have enough stuff
          -- in scope for the bindings to be well typed.  So we just
          -- switch off deferred type errors altogether.  See Trac #14605.

    ctxt' = ctxt1 { cec_tidy     = env1
                  , cec_encl     = implic' : cec_encl ctxt

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

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

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    bad_telescope = case status of
              IC_BadTelescope -> True
              _               -> False

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warnRedundantConstraints :: ReportErrCtxt -> TcLclEnv -> SkolemInfo -> [EvVar] -> TcM ()
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-- See Note [Tracking redundant constraints] in TcSimplify
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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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reportBadTelescope :: ReportErrCtxt -> TcLclEnv -> Maybe SDoc -> [TcTyVar] -> TcM ()
reportBadTelescope ctxt env (Just telescope) skols
  = do { msg <- mkErrorReport ctxt env (important doc)
       ; reportError msg }
  where
    doc = hang (text "These kind and type variables:" <+> telescope $$
                text "are out of dependency order. Perhaps try this ordering:")
             2 (pprTyVars sorted_tvs)

    sorted_tvs = toposortTyVars skols

reportBadTelescope _ _ Nothing skols
  = pprPanic "reportBadTelescope" (ppr skols)

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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
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of evidence, since the dictionary has no fields.  But it is still
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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 ()
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reportWanteds ctxt tc_lvl (WC { wc_simple = simples, 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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       ; traceTc "rw2" (ppr tidy_cts)
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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
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       ; 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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    tidy_cts = bagToList (mapBag (tidyCt env) simples)
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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
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    -- (see TcRnTypes.insolubleWantedCt) is caught here, otherwise
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    -- we might suppress its error message, and proceed on past
    -- type checking to get a Lint error later
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    report1 = [ ("Out of scope", is_out_of_scope,    True,  mkHoleReporter tidy_cts)
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              , ("Holes",        is_hole,            False, mkHoleReporter tidy_cts)
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              , ("custom_error", is_user_type_error, True,  mkUserTypeErrorReporter)
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              , given_eq_spec
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              , ("insoluble2",   utterly_wrong,  True, mkGroupReporter mkEqErr)
              , ("skolem eq1",   very_wrong,     True, mkSkolReporter)
              , ("skolem eq2",   skolem_eq,      True, mkSkolReporter)
              , ("non-tv eq",    non_tv_eq,      True, mkSkolReporter)
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                  -- The only remaining equalities are alpha ~ ty,
                  -- where alpha is untouchable; and representational equalities
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                  -- Prefer homogeneous equalities over hetero, because the
                  -- former might be holding up the latter.
                  -- See Note [Equalities with incompatible kinds] in TcCanonical
              , ("Homo eqs",      is_homo_equality, True,  mkGroupReporter mkEqErr)
              , ("Other eqs",     is_equality,      False, mkGroupReporter mkEqErr) ]
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    -- 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

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

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

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

    find_gadt_match [] = Nothing
    find_gadt_match (implic : implics)
      | PatSkol {} <- ic_info implic
      , not (ic_no_eqs implic)
      = Just implic
      | otherwise
      = find_gadt_match implics
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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
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       | EQ <- cmp_loc ct1 ct2 = True
       | eq_lhs_type   ct1 ct2 = True
       | otherwise             = False
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mkHoleReporter :: [Ct] -> Reporter
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-- Reports errors one at a time
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mkHoleReporter tidy_simples ctxt
  = mapM_ $ \ct -> do { err <- mkHoleError tidy_simples ctxt ct
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                      ; maybeReportHoleError ctxt ct err
                      ; maybeAddDeferredHoleBinding ctxt err ct }
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mkUserTypeErrorReporter :: Reporter
mkUserTypeErrorReporter ctxt
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  = mapM_ $ \ct -> do { err <- mkUserTypeError ctxt ct
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                      ; maybeReportError ctxt err
                      ; addDeferredBinding ctxt err ct }
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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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mkGivenErrorReporter :: Implication -> Reporter
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-- See Note [Given errors]
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mkGivenErrorReporter implic ctxt cts
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  = do { (ctxt, binds_msg, ct) <- relevantBindings True ctxt ct
       ; dflags <- getDynFlags
       ; let ct' = setCtLoc ct (setCtLocEnv (ctLoc ct) (ic_env implic))
                   -- For given constraints we overwrite the env (and hence src-loc)
                  -- with one from the implication.  See Note [Inaccessible code]

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

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

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

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

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

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

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

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

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

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

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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
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  = mapM_ (reportGroup mk_err ctxt . toList) (equivClasses cmp_loc cts)
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eq_lhs_type :: Ct -> Ct -> Bool
eq_lhs_type ct1 ct2
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  = case (classifyPredType (ctPred ct1), classifyPredType (ctPred ct2)) of
       (EqPred eq_rel1 ty1 _, EqPred eq_rel2 ty2 _) ->
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         (eq_rel1 == eq_rel2) && (ty1 `eqType` ty2)
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       _ -> 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
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        -- there are no other missing constraints!
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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'
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                        ; traceTc "About to maybeReportErr" $
                          vcat [ text "Constraint:"             <+> ppr cts'
                               , text "cec_suppress ="          <+> ppr (cec_suppress ctxt)
                               , text "cec_defer_type_errors =" <+> ppr (cec_defer_type_errors ctxt) ]
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                        ; maybeReportError ctxt err
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                            -- But see Note [Always warn with -fdefer-type-errors]
                        ; traceTc "reportGroup" (ppr cts')
                        ; mapM_ (addDeferredBinding ctxt err) cts' }
                            -- Add deferred bindings for all
                            -- Redundant if we are going to abort compilation,
                            -- but that's hard to know for sure, and if we don't
                            -- abort, we need bindings for all (e.g. Trac #12156)
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  where
    isMonadFailInstanceMissing ct =
        case ctLocOrigin (ctLoc ct) of
            FailablePattern _pat -> True
            _otherwise           -> False
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maybeReportHoleError :: ReportErrCtxt -> Ct -> ErrMsg -> TcM ()
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-- Unlike maybeReportError, these "hole" errors are
-- /not/ suppressed by cec_suppress.  We want to see them!
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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 ()

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  -- Always report an error for out-of-scope variables
  -- Unless -fdefer-out-of-scope-variables is on,
  -- in which case the messages are discarded.
  -- See Trac #12170, #12406
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  | isOutOfScopeCt ct
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  = -- If deferring, report a warning only if -Wout-of-scope-variables is on
    case cec_out_of_scope_holes ctxt of
      HoleError -> reportError err
      HoleWarn  ->
        reportWarning (Reason Opt_WarnDeferredOutOfScopeVariables) err
      HoleDefer -> return ()
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  -- Otherwise this is a typed hole in an expression,
  -- but not for an out-of-scope variable
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  | otherwise
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  = -- If deferring, report a warning only if -Wtyped-holes 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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  | otherwise
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  = case cec_defer_type_errors ctxt of
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      TypeDefer       -> return ()
      TypeWarn reason -> reportWarning reason err
      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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  | deferringAnyBindings ctxt
  , CtWanted { ctev_pred = pred, ctev_dest = dest } <- ctEvidence ct
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    -- Only add deferred bindings for Wanted constraints
  = do { dflags <- getDynFlags
       ; let err_msg = pprLocErrMsg err
             err_fs  = mkFastString $ showSDoc dflags $
                       err_msg $$ text "(deferred type error)"
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             err_tm  = evDelayedError pred err_fs
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             ev_binds_var = cec_binds ctxt
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       ; case dest of
           EvVarDest evar
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             -> addTcEvBind ev_binds_var $ mkWantedEvBind evar (EvExpr err_tm)
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           HoleDest hole
             -> do { -- See Note [Deferred errors for coercion holes]
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                     let co_var = coHoleCoVar hole
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                   ; addTcEvBind ev_binds_var $ mkWantedEvBind co_var (EvExpr err_tm)
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                   ; fillCoercionHole hole (mkTcCoVarCo co_var) }}
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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
  = addDeferredBinding ctxt err ct  -- Only add bindings for holes in expressions
  | otherwise                       -- not for holes in partial type signatures
  = return ()
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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 { let (vis_cts, invis_cts) = partition (isVisibleOrigin . ctOrigin) cts
       ; traceTc "tryReporters {" (ppr vis_cts $$ ppr invis_cts)
       ; (ctxt', cts') <- go ctxt reporters vis_cts invis_cts
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       ; traceTc "tryReporters }" (ppr cts')
       ; return (ctxt', cts') }
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  where
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    go ctxt [] vis_cts invis_cts
      = return (ctxt, vis_cts ++ invis_cts)

    go ctxt (r : rs) vis_cts invis_cts
       -- always look at *visible* Origins before invisible ones
       -- this is the whole point of isVisibleOrigin
      = do { (ctxt', vis_cts') <- tryReporter ctxt r vis_cts
           ; (ctxt'', invis_cts') <- tryReporter ctxt' r invis_cts
           ; go ctxt'' rs vis_cts' invis_cts' }
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                -- 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
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  | null yeses
  = return (ctxt, cts)
  | otherwise
  = do { traceTc "tryReporter{ " (text str <+> ppr yeses)
       ; (_, no_errs) <- askNoErrs (reporter ctxt yeses)
       ; let suppress_now = not no_errs && suppress_after
                            -- See Note [Suppressing error messages]
             ctxt' = ctxt { cec_suppress = suppress_now || cec_suppress ctxt }
       ; traceTc "tryReporter end }" (text str <+> ppr (cec_suppress ctxt) <+> ppr suppress_after)
       ; return (ctxt', nos) }
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  where
    (yeses, nos) = partition (\ct -> keep_me ct (classifyPredType (ctPred ct))) cts
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