path: root/compiler/typecheck/TcErrors.lhs

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

module TcErrors( 
       reportUnsolved, reportAllUnsolved,
       warnDefaulting,

       solverDepthErrorTcS
  ) where

#include "HsVersions.h"

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

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

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

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

  module Main where

  a :: Int
  a = 'a'

  main = print "b"

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

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

  co :: Int ~ Char
  co = ...

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

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

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

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

reportAllUnsolved :: WantedConstraints -> TcM ()
-- Report all unsolved goals, even if -fdefer-type-errors is on
-- See Note [Deferring coercion errors to runtime]
reportAllUnsolved wanted = report_unsolved Nothing False wanted

report_unsolved :: Maybe EvBindsVar  -- cec_binds
                -> Bool              -- cec_defer
                -> WantedConstraints -> TcM ()
-- Important precondition:
-- WantedConstraints are fully zonked and unflattened, that is,
-- zonkWC has already been applied to these constraints.
report_unsolved mb_binds_var defer wanted
  | isEmptyWC wanted
  = return ()
  | otherwise
  = do { traceTc "reportUnsolved (before unflattening)" (ppr wanted)

       ; env0 <- tcInitTidyEnv
                 
            -- If we are deferring we are going to need /all/ evidence around,
            -- including the evidence produced by unflattening (zonkWC)
       ; let tidy_env = tidyFreeTyVars env0 free_tvs
             free_tvs = tyVarsOfWC wanted
             err_ctxt = CEC { cec_encl  = []
                            , cec_tidy  = tidy_env
                            , cec_defer    = defer
                            , cec_suppress = False -- See Note [Suppressing error messages]
                            , cec_binds    = mb_binds_var }

       ; traceTc "reportUnsolved (after unflattening):" $ 
         vcat [ pprTvBndrs (varSetElems free_tvs)
              , ppr wanted ]

       ; reportWanteds err_ctxt wanted }

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

data ReportErrCtxt 
    = CEC { cec_encl :: [Implication]  -- Enclosing implications
                	       	       --   (innermost first)
                                       -- ic_skols and givens are tidied, rest are not
          , cec_tidy  :: TidyEnv
          , cec_binds :: Maybe EvBindsVar 
                         -- Nothinng <=> Report all errors, including holes; no bindings
                         -- Just ev  <=> make some errors (depending on cec_defer)
                         --              into warnings, and emit evidence bindings
                         --              into 'ev' for unsolved constraints

          , cec_defer :: Bool       -- True <=> -fdefer-type-errors
                                    -- Irrelevant if cec_binds = Nothing
          , cec_suppress :: Bool    -- True <=> More important errors have occurred,
                                    --          so create bindings if need be, but
                                    --          don't issue any more errors/warnings
                                    -- See Note [Suppressing error messages]
      }
\end{code}

Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The cec_suppress flag says "don't report any errors.  Instead, just create
evidence bindings (as usual).  It's used when more important errors have occurred.
Specifically (see reportWanteds)
  * If there are insoluble Givens, then we are in unreachable code and all bets
    are off.  So don't report any further errors.
  * If there are any insolubles (eg Int~Bool), here or in a nested implication, 
    then suppress errors from the flat constraints here.  Sometimes the
    flat-constraint errors are a knock-on effect of the insolubles.


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

  | otherwise
  = reportWanteds ctxt' wanted
  where
    (env1, tvs') = mapAccumL tidyTyVarBndr (cec_tidy ctxt) tvs
    (env2, info') = tidySkolemInfo env1 info
    implic' = implic { ic_skols = tvs'
                     , ic_given = map (tidyEvVar env2) given
                     , ic_info  = info' }
    ctxt' = ctxt { cec_tidy  = env2
                 , cec_encl  = implic' : cec_encl ctxt
                 , cec_binds = case cec_binds ctxt of
                                 Nothing -> Nothing
                                 Just {} -> Just evb }

reportWanteds :: ReportErrCtxt -> WantedConstraints -> TcM ()
reportWanteds ctxt wanted@(WC { wc_flat = flats, wc_insol = insols, wc_impl = implics })
  = do { reportFlats ctxt  (mapBag (tidyCt env) insol_given)
       ; reportFlats ctxt1 (mapBag (tidyCt env) insol_wanted)
       ; reportFlats ctxt2 (mapBag (tidyCt env) flats)
            -- All the Derived ones have been filtered out of flats 
            -- by the constraint solver. This is ok; we don't want
            -- to report unsolved Derived goals as errors
            -- See Note [Do not report derived but soluble errors]
       ; mapBagM_ (reportImplic ctxt1) implics }
            -- NB ctxt1: don't suppress inner insolubles if there's only a
            -- wanted insoluble here; but do suppress inner insolubles
            -- if there's a given insoluble here (= inaccessible code)
 where
    (insol_given, insol_wanted) = partitionBag isGivenCt insols
    env = cec_tidy ctxt

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

reportFlats :: ReportErrCtxt -> Cts -> TcM ()
reportFlats ctxt flats    -- Here 'flats' includes insolble goals
  = traceTc "reportFlats" (vcat [ ptext (sLit "Flats =") <+> ppr flats
                                , ptext (sLit "Suppress =") <+> ppr (cec_suppress ctxt)]) >>
    tryReporters 
      [ -- First deal with things that are utterly wrong
        -- Like Int ~ Bool (incl nullary TyCons)
        -- or  Int ~ t a   (AppTy on one side)
        ("Utterly wrong",  utterly_wrong,   mkGroupReporter mkEqErr)
      , ("Holes",          is_hole,         mkUniReporter mkHoleError)

        -- Report equalities of form (a~ty).  They are usually
        -- skolem-equalities, and they cause confusing knock-on 
        -- effects in other errors; see test T4093b.
      , ("Skolem equalities",    skolem_eq,   mkUniReporter mkEqErr1) ]
      reportFlatErrs
      ctxt (bagToList flats)
  where
    utterly_wrong, skolem_eq :: Ct -> PredTree -> Bool
    utterly_wrong _ (EqPred ty1 ty2) = isRigid ty1 && isRigid ty2 
    utterly_wrong _ _ = False

    is_hole ct _ = isHoleCt ct

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

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

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

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

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

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


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

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

mkUniReporter :: (ReportErrCtxt -> Ct -> TcM ErrMsg) -> Reporter
-- Reports errors one at a time
mkUniReporter mk_err ctxt 
  = mapM_ $ \ct -> 
    do { err <- mk_err ctxt ct
       ; maybeReportError ctxt err
       ; maybeAddDeferredBinding ctxt err ct }

mkGroupReporter :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg)
                             -- Make error message for a group
                -> Reporter  -- Deal with lots of constraints
-- Group together insts from same location
-- We want to report them together in error messages

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

   same_loc :: CtLoc -> CtLoc -> Bool
   same_loc l1 l2 = ctLocSpan l1 == ctLocSpan l2

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

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

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

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

tryReporters :: [(String, Ct -> PredTree -> Bool, Reporter)] 
             -> Reporter -> Reporter
-- Use the first reporter in the list whose predicate says True
tryReporters reporters deflt ctxt cts
  = do { traceTc "tryReporters {" (ppr cts) 
       ; go ctxt reporters cts
       ; traceTc "tryReporters }" empty }
  where
    go ctxt [] cts = deflt ctxt cts 
    go ctxt ((str, pred, reporter) : rs) cts
      | null yeses  = do { traceTc "tryReporters: no" (text str)
                         ; go ctxt rs cts }
      | otherwise   = do { traceTc "tryReporters: yes" (text str <+> ppr yeses)
                         ; reporter ctxt yeses :: TcM ()
                         ; go (ctxt { cec_suppress = True }) rs nos }
                         -- Carry on with the rest, because we must make
                         -- deferred bindings for them if we have 
                         -- -fdefer-type-errors
                         -- But suppress their error messages
      where
       (yeses, nos) = partition keep_me cts
       keep_me ct = pred ct (classifyPredType (ctPred ct))

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

pprWithArising :: [Ct] -> (CtLoc, SDoc)
-- Print something like
--    (Eq a) arising from a use of x at y
--    (Show a) arising from a use of p at q
-- Also return a location for the error message
-- Works for Wanted/Derived only
pprWithArising [] 
  = panic "pprWithArising"
pprWithArising (ct:cts)
  | null cts
  = (loc, addArising (ctLocOrigin loc) 
                     (pprTheta [ctPred ct]))
  | otherwise
  = (loc, vcat (map ppr_one (ct:cts)))
  where
    loc = cc_loc ct
    ppr_one ct = hang (parens (pprType (ctPred ct))) 
                    2 (pprArisingAt (cc_loc ct))

mkErrorMsg :: ReportErrCtxt -> Ct -> SDoc -> TcM ErrMsg
mkErrorMsg ctxt ct msg 
  = do { let tcl_env = ctLocEnv (cc_loc ct)
       ; err_info <- mkErrInfo (cec_tidy ctxt) (tcl_ctxt tcl_env)
       ; mkLongErrAt (tcl_loc tcl_env) msg err_info }

type UserGiven = ([EvVar], SkolemInfo, SrcSpan)

getUserGivens :: ReportErrCtxt -> [UserGiven]
-- One item for each enclosing implication
getUserGivens (CEC {cec_encl = ctxt})
  = reverse $
    [ (givens, info, tcl_loc env) 
    | Implic {ic_given = givens, ic_env = env, ic_info = info } <- ctxt
    , not (null givens) ]
\end{code}

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

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

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


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

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

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

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

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

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

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

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


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

\begin{code}
mkIrredErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
mkIrredErr ctxt cts 
  = do { (ctxt, binds_msg) <- relevantBindings True ctxt ct1
       ; mkErrorMsg ctxt ct1 (msg $$ binds_msg) }
  where
    (ct1:_) = cts
    orig    = ctLocOrigin (cc_loc ct1)
    givens  = getUserGivens ctxt
    msg = couldNotDeduce givens (map ctPred cts, orig)

----------------
mkHoleError :: ReportErrCtxt -> Ct -> TcM ErrMsg
mkHoleError ctxt ct@(CHoleCan { cc_occ = occ })
  = do { let tyvars = varSetElems (tyVarsOfCt ct)
             tyvars_msg = map loc_msg tyvars
             msg = vcat [ hang (ptext (sLit "Found hole") <+> quotes (ppr occ))
                             2 (ptext (sLit "with type:") <+> pprType (ctEvPred (cc_ev ct)))
                        , ppUnless (null tyvars_msg) (ptext (sLit "Where:") <+> vcat tyvars_msg) ]
       ; (ctxt, binds_doc) <- relevantBindings False ctxt ct
               -- The 'False' means "don't filter the bindings; see Trac #8191
       ; mkErrorMsg ctxt ct (msg $$ binds_doc) }
  where
    loc_msg tv 
       = case tcTyVarDetails tv of
          SkolemTv {} -> quotes (ppr tv) <+> skol_msg
          MetaTv {}   -> quotes (ppr tv) <+> ptext (sLit "is an ambiguous type variable")
          det -> pprTcTyVarDetails det
       where 
          skol_msg = pprSkol (getSkolemInfo (cec_encl ctxt) tv) (getSrcLoc tv)

mkHoleError _ ct = pprPanic "mkHoleError" (ppr ct)

----------------
mkIPErr :: ReportErrCtxt -> [Ct] -> TcM ErrMsg
mkIPErr ctxt cts
  = do { (ctxt, bind_msg) <- relevantBindings True ctxt ct1
       ; mkErrorMsg ctxt ct1 (msg $$ bind_msg) }
  where
    (ct1:_) = cts
    orig    = ctLocOrigin (cc_loc ct1)
    preds   = map ctPred cts
    givens  = getUserGivens ctxt
    msg | null givens
        = addArising orig $
          sep [ ptext (sLit "Unbound implicit parameter") <> plural cts
              , nest 2 (pprTheta preds) ] 
        | otherwise
        = couldNotDeduce givens (preds, orig)
\end{code}


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

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

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

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

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

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

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

    mk_given :: [Implication] -> (CtLoc, SDoc)
    -- For given constraints we overwrite the env (and hence src-loc)
    -- with one from the implication.  See Note [Inaccessible code]
    mk_given []           = (cc_loc ct, empty)
    mk_given (implic : _) = (setCtLocEnv (cc_loc ct) (ic_env implic)
                            , hang (ptext (sLit "Inaccessible code in"))
                                 2 (ppr (ic_info implic)))

       -- If the types in the error message are the same as the types
       -- we are unifying, don't add the extra expected/actual message
    mk_wanted_extra orig@(TypeEqOrigin {})
      = mkExpectedActualMsg ty1 ty2 orig

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

    mk_wanted_extra _ = (Nothing, empty)

mkEqErr_help :: DynFlags -> ReportErrCtxt -> SDoc
             -> Ct          
             -> Maybe SwapFlag   -- Nothing <=> not sure
             -> TcType -> TcType -> TcM ErrMsg
mkEqErr_help dflags ctxt extra ct oriented ty1 ty2
  | Just tv1 <- tcGetTyVar_maybe ty1 = mkTyVarEqErr dflags ctxt extra ct oriented tv1 ty2
  | Just tv2 <- tcGetTyVar_maybe ty2 = mkTyVarEqErr dflags ctxt extra ct swapped  tv2 ty1
  | otherwise                        = reportEqErr  ctxt extra ct oriented ty1 ty2
  where
    swapped = fmap flipSwap oriented

reportEqErr :: ReportErrCtxt -> SDoc
            -> Ct    
            -> Maybe SwapFlag   -- Nothing <=> not sure
            -> TcType -> TcType -> TcM ErrMsg
reportEqErr ctxt extra1 ct oriented ty1 ty2
  = do { let extra2 = mkEqInfoMsg ct ty1 ty2
       ; mkErrorMsg ctxt ct (vcat [ misMatchOrCND ctxt ct oriented ty1 ty2
                                   , extra2, extra1]) }

mkTyVarEqErr :: DynFlags -> ReportErrCtxt -> SDoc -> Ct 
             -> Maybe SwapFlag -> TcTyVar -> TcType -> TcM ErrMsg
-- tv1 and ty2 are already tidied
mkTyVarEqErr dflags ctxt extra ct oriented tv1 ty2
  | isUserSkolem ctxt tv1   -- ty2 won't be a meta-tyvar, or else the thing would
                            -- be oriented the other way round; see TcCanonical.reOrient
  || isSigTyVar tv1 && not (isTyVarTy ty2)
  = mkErrorMsg ctxt ct (vcat [ misMatchOrCND ctxt ct oriented ty1 ty2
                             , extraTyVarInfo ctxt ty1 ty2
                             , extra ])

  -- So tv is a meta tyvar (or started that way before we 
  -- generalised it).  So presumably it is an *untouchable* 
  -- meta tyvar or a SigTv, else it'd have been unified
  | not (k2 `tcIsSubKind` k1)   	 -- Kind error
  = mkErrorMsg ctxt ct $ (kindErrorMsg (mkTyVarTy tv1) ty2 $$ extra)

  | OC_Occurs <- occ_check_expand
  = do { let occCheckMsg = hang (text "Occurs check: cannot construct the infinite type:")
                              2 (sep [ppr ty1, char '~', ppr ty2])
             extra2 = mkEqInfoMsg ct ty1 ty2
       ; mkErrorMsg ctxt ct (occCheckMsg $$ extra2 $$ extra) }

  | OC_Forall <- occ_check_expand
  = do { let msg = vcat [ ptext (sLit "Cannot instantiate unification variable")
                          <+> quotes (ppr tv1)
                        , hang (ptext (sLit "with a type involving foralls:")) 2 (ppr ty2)
                        , nest 2 (ptext (sLit "Perhaps you want ImpredicativeTypes")) ]
       ; mkErrorMsg ctxt ct msg }

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

  -- Check for skolem escape
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
  , Implic { ic_env = env, ic_skols = skols, ic_info = skol_info } <- implic
  , let esc_skols = filter (`elemVarSet` (tyVarsOfType ty2)) skols
  , not (null esc_skols)
  = do { let msg = misMatchMsg oriented ty1 ty2
             esc_doc = sep [ ptext (sLit "because type variable") <> plural esc_skols
                             <+> pprQuotedList esc_skols
                           , ptext (sLit "would escape") <+>
                             if isSingleton esc_skols then ptext (sLit "its scope")
                                                      else ptext (sLit "their scope") ]
             tv_extra = vcat [ nest 2 $ esc_doc
                             , sep [ (if isSingleton esc_skols 
                                      then ptext (sLit "This (rigid, skolem) type variable is")
                                      else ptext (sLit "These (rigid, skolem) type variables are"))
                               <+> ptext (sLit "bound by")
                             , nest 2 $ ppr skol_info
                             , nest 2 $ ptext (sLit "at") <+> ppr (tcl_loc env) ] ]
       ; mkErrorMsg ctxt ct (msg $$ tv_extra $$ extra) }

  -- Nastiest case: attempt to unify an untouchable variable
  | (implic:_) <- cec_encl ctxt   -- Get the innermost context
  , Implic { ic_env = env, ic_given = given, ic_info = skol_info } <- implic
  = do { let msg = misMatchMsg oriented ty1 ty2
             untch_extra 
                = nest 2 $
                  sep [ quotes (ppr tv1) <+> ptext (sLit "is untouchable")
                      , nest 2 $ ptext (sLit "inside the constraints") <+> pprEvVarTheta given
                      , nest 2 $ ptext (sLit "bound by") <+> ppr skol_info
                      , nest 2 $ ptext (sLit "at") <+> ppr (tcl_loc env) ]
             tv_extra = extraTyVarInfo ctxt ty1 ty2
       ; mkErrorMsg ctxt ct (vcat [msg, untch_extra, tv_extra, extra]) }

  | otherwise
  = reportEqErr ctxt extra ct oriented (mkTyVarTy tv1) ty2
        -- This *can* happen (Trac #6123, and test T2627b)
        -- Consider an ambiguous top-level constraint (a ~ F a)
        -- Not an occurs check, because F is a type function.
  where         
    occ_check_expand = occurCheckExpand dflags tv1 ty2
    k1 	= tyVarKind tv1
    k2 	= typeKind ty2
    ty1 = mkTyVarTy tv1

mkEqInfoMsg :: Ct -> TcType -> TcType -> SDoc
-- Report (a) ambiguity if either side is a type function application
--            e.g. F a0 ~ Int    
--        (b) warning about injectivity if both sides are the same
--            type function application   F a ~ F b
--            See Note [Non-injective type functions]
mkEqInfoMsg ct ty1 ty2
  = tyfun_msg $$ ambig_msg
  where
    mb_fun1 = isTyFun_maybe ty1
    mb_fun2 = isTyFun_maybe ty2

    ambig_msg | isJust mb_fun1 || isJust mb_fun2 
              = snd (mkAmbigMsg ct)
              | otherwise = empty

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

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

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

misMatchOrCND :: ReportErrCtxt -> Ct -> Maybe SwapFlag -> TcType -> TcType -> SDoc
-- If oriented then ty1 is actual, ty2 is expected
misMatchOrCND ctxt ct oriented ty1 ty2
  | null givens || 
    (isRigid ty1 && isRigid ty2) || 
    isGivenCt ct
       -- If the equality is unconditionally insoluble
       -- or there is no context, don't report the context
  = misMatchMsg oriented ty1 ty2
  | otherwise      
  = couldNotDeduce givens ([mkEqPred ty1 ty2], orig)
  where
    givens = getUserGivens ctxt
    orig   = TypeEqOrigin { uo_actual = ty1, uo_expected = ty2 }

couldNotDeduce :: [UserGiven] -> (ThetaType, CtOrigin) -> SDoc
couldNotDeduce givens (wanteds, orig)
  = vcat [ addArising orig (ptext (sLit "Could not deduce") <+> pprTheta wanteds)
         , vcat (pp_givens givens)]

pp_givens :: [UserGiven] -> [SDoc]
pp_givens givens 
   = case givens of
         []     -> []
         (g:gs) ->      ppr_given (ptext (sLit "from the context")) g
                 : map (ppr_given (ptext (sLit "or from"))) gs
    where 
       ppr_given herald (gs, skol_info, loc)
           = hang (herald <+> pprEvVarTheta gs)
                2 (sep [ ptext (sLit "bound by") <+> ppr skol_info
                       , ptext (sLit "at") <+> ppr loc])

extraTyVarInfo :: ReportErrCtxt -> TcType -> TcType -> SDoc
-- Add on extra info about the types themselves
-- NB: The types themselves are already tidied
extraTyVarInfo ctxt ty1 ty2
  = nest 2 (extra1 $$ extra2)
  where
    extra1 = tyVarExtraInfoMsg (cec_encl ctxt) ty1
    extra2 = tyVarExtraInfoMsg (cec_encl ctxt) ty2

tyVarExtraInfoMsg :: [Implication] -> Type -> SDoc
-- Shows a bit of extra info about skolem constants
tyVarExtraInfoMsg implics ty
  | Just tv <- tcGetTyVar_maybe ty
  , isTcTyVar tv, isSkolemTyVar tv
  , let pp_tv = quotes (ppr tv)
 = case tcTyVarDetails tv of
    SkolemTv {}   -> pp_tv <+> pprSkol (getSkolemInfo implics tv) (getSrcLoc tv)
    FlatSkol {}   -> pp_tv <+> ptext (sLit "is a flattening type variable")
    RuntimeUnk {} -> pp_tv <+> ptext (sLit "is an interactive-debugger skolem")
    MetaTv {}     -> empty

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

--------------------
misMatchMsg :: Maybe SwapFlag -> TcType -> TcType -> SDoc	   -- Types are already tidy
-- If oriented then ty1 is actual, ty2 is expected
misMatchMsg oriented ty1 ty2  
  | Just IsSwapped <- oriented
  = misMatchMsg (Just NotSwapped) ty2 ty1
  | Just NotSwapped <- oriented
  = sep [ ptext (sLit "Couldn't match expected") <+> what <+> quotes (ppr ty2)
        , nest 12 $   ptext (sLit "with actual") <+> what <+> quotes (ppr ty1) ]
  | otherwise
  = sep [ ptext (sLit "Couldn't match") <+> what <+> quotes (ppr ty1)
        , nest 14 $ ptext (sLit "with") <+> quotes (ppr ty2) ]
  where 
    what | isKind ty1 = ptext (sLit "kind")
         | otherwise  = ptext (sLit "type")

mkExpectedActualMsg :: Type -> Type -> CtOrigin -> (Maybe SwapFlag, SDoc)
-- NotSwapped means (actual, expected), IsSwapped is the reverse
mkExpectedActualMsg ty1 ty2 (TypeEqOrigin { uo_actual = act, uo_expected = exp })
  | act `pickyEqType` ty1, exp `pickyEqType` ty2 = (Just NotSwapped,  empty)
  | exp `pickyEqType` ty1, act `pickyEqType` ty2 = (Just IsSwapped, empty)
  | otherwise                                    = (Nothing, msg)
  where
    msg = vcat [ text "Expected type:" <+> ppr exp
               , text "  Actual type:" <+> ppr act ]

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

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

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

So isUserSkolem distinguishes the two.

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

Warn of loopy local equalities that were dropped.


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

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

       -- Report definite no-instance errors, 
       -- or (iff there are none) overlap errors
       -- But we report only one of them (hence 'head') because they all
       -- have the same source-location origin, to try avoid a cascade
       -- of error from one location
       ; (ctxt, err) <- mk_dict_err ctxt (head (no_inst_cts ++ overlap_cts))
       ; mkErrorMsg ctxt ct1 err }
  where
    ct1:_ = cts
    no_givens = null (getUserGivens ctxt)
    is_no_inst (ct, (matches, unifiers, _))
      =  no_givens 
      && null matches 
      && (null unifiers || all (not . isAmbiguousTyVar) (varSetElems (tyVarsOfCt ct)))
           
    lookup_cls_inst inst_envs ct
      = do { tys_flat <- mapM quickFlattenTy tys
                -- Note [Flattening in error message generation]
           ; return (ct, lookupInstEnv inst_envs clas tys_flat) }
      where
        (clas, tys) = getClassPredTys (ctPred ct)

mk_dict_err :: ReportErrCtxt -> (Ct, ClsInstLookupResult)
            -> TcM (ReportErrCtxt, SDoc)
-- Report an overlap error if this class constraint results
-- from an overlap (returning Left clas), otherwise return (Right pred)
mk_dict_err ctxt (ct, (matches, unifiers, safe_haskell)) 
  | null matches  -- No matches but perhaps several unifiers
  = do { let (is_ambig, ambig_msg) = mkAmbigMsg ct
       ; (ctxt, binds_msg) <- relevantBindings True ctxt ct
       ; traceTc "mk_dict_err" (ppr ct $$ ppr is_ambig $$ ambig_msg)
       ; safe_mod <- safeLanguageOn `fmap` getDynFlags
       ; rdr_env <- getGlobalRdrEnv
       ; return (ctxt, cannot_resolve_msg safe_mod rdr_env is_ambig binds_msg ambig_msg) }

  | not safe_haskell   -- Some matches => overlap errors
  = return (ctxt, overlap_msg)

  | otherwise
  = return (ctxt, safe_haskell_msg)
  where
    orig        = ctLocOrigin (cc_loc ct)
    pred        = ctPred ct
    (clas, tys) = getClassPredTys pred
    ispecs      = [ispec | (ispec, _) <- matches]
    givens      = getUserGivens ctxt
    all_tyvars  = all isTyVarTy tys

    cannot_resolve_msg safe_mod rdr_env has_ambig_tvs binds_msg ambig_msg
      = vcat [ addArising orig (no_inst_herald <+> pprParendType pred $$
                                coercible_msg safe_mod rdr_env)
             , vcat (pp_givens givens)
             , ppWhen (has_ambig_tvs && not (null unifiers && null givens))
               (vcat [ ambig_msg, binds_msg, potential_msg ])
             , show_fixes (add_to_ctxt_fixes has_ambig_tvs ++ drv_fixes) ]

    potential_msg
      = ppWhen (not (null unifiers) && want_potential orig) $
        hang (if isSingleton unifiers 
              then ptext (sLit "Note: there is a potential instance available:")
              else ptext (sLit "Note: there are several potential instances:"))
    	   2 (ppr_insts unifiers)

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

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

    ppr_skol (PatSkol dc _) = ptext (sLit "the data constructor") <+> quotes (ppr dc)
    ppr_skol skol_info      = ppr skol_info

	-- Do not suggest adding constraints to an *inferred* type signature!
    get_good_orig ic = case ic_info ic of 
                         SigSkol (InfSigCtxt {}) _ -> Nothing
                         origin                    -> Just origin

    no_inst_herald
      | null givens && null matches = ptext (sLit "No instance for")
      | otherwise                   = ptext (sLit "Could not deduce")

    drv_fixes = case orig of
                   DerivOrigin -> [drv_fix]
                   _           -> []

    drv_fix = hang (ptext (sLit "use a standalone 'deriving instance' declaration,"))
                 2 (ptext (sLit "so you can specify the instance context yourself"))

    -- Normal overlap error
    overlap_msg
      = ASSERT( not (null matches) )
        vcat [	addArising orig (ptext (sLit "Overlapping instances for") 
				<+> pprType (mkClassPred clas tys))

             ,  ppUnless (null matching_givens) $
                  sep [ptext (sLit "Matching givens (or their superclasses):") 
                      , nest 2 (vcat matching_givens)]

    	     ,	sep [ptext (sLit "Matching instances:"),
    		     nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])]

             ,  ppWhen (null matching_givens && isSingleton matches && null unifiers) $
                -- Intuitively, some given matched the wanted in their
                -- flattened or rewritten (from given equalities) form
                -- but the matcher can't figure that out because the
                -- constraints are non-flat and non-rewritten so we
                -- simply report back the whole given
                -- context. Accelerate Smart.hs showed this problem.
                  sep [ ptext (sLit "There exists a (perhaps superclass) match:") 
                      , nest 2 (vcat (pp_givens givens))]

	     ,	ppWhen (isSingleton matches) $
		parens (vcat [ ptext (sLit "The choice depends on the instantiation of") <+>
	    		          quotes (pprWithCommas ppr (varSetElems (tyVarsOfTypes tys)))
			     , ppWhen (null (matching_givens)) $
                               vcat [ ptext (sLit "To pick the first instance above, use IncoherentInstances")
			            , ptext (sLit "when compiling the other instance declarations")]
                        ])]
        where
            ispecs = [ispec | (ispec, _) <- matches]

            givens = getUserGivens ctxt
            matching_givens = mapCatMaybes matchable givens

            matchable (evvars,skol_info,loc) 
              = case ev_vars_matching of
                     [] -> Nothing
                     _  -> Just $ hang (pprTheta ev_vars_matching)
                                    2 (sep [ ptext (sLit "bound by") <+> ppr skol_info
                                           , ptext (sLit "at") <+> ppr loc])
                where ev_vars_matching = filter ev_var_matches (map evVarPred evvars)
                      ev_var_matches ty = case getClassPredTys_maybe ty of
                         Just (clas', tys')
                           | clas' == clas
                           , Just _ <- tcMatchTys (tyVarsOfTypes tys) tys tys'
                           -> True 
                           | otherwise
                           -> any ev_var_matches (immSuperClasses clas' tys')
                         Nothing -> False

    -- Overlap error because of Safe Haskell (first 
    -- match should be the most specific match)
    safe_haskell_msg
      = ASSERT( length matches > 1 )
        vcat [ addArising orig (ptext (sLit "Unsafe overlapping instances for") 
                        <+> pprType (mkClassPred clas tys))
             , sep [ptext (sLit "The matching instance is:"),
                    nest 2 (pprInstance $ head ispecs)]
             , vcat [ ptext $ sLit "It is compiled in a Safe module and as such can only"
                    , ptext $ sLit "overlap instances from the same module, however it"
                    , ptext $ sLit "overlaps the following instances from different modules:"
                    , nest 2 (vcat [pprInstances $ tail ispecs])
                    ]
             ]

    -- This function tries to reconstruct why a "Coercible ty1 ty2" constraint
    -- is left over. Therefore its logic has to stay in sync with
    -- getCoericbleInst in TcInteract. See Note [Coercible Instances]
    coercible_msg safe_mod rdr_env
      | clas /= coercibleClass = empty
      | Just (tc1,tyArgs1) <- splitTyConApp_maybe ty1,
        Just (tc2,tyArgs2) <- splitTyConApp_maybe ty2,
        tc1 == tc2
      = nest 2 $ vcat $
          -- Only for safe haskell: First complain if tc is abstract, only if
          -- not check if the type constructors therein are abstract
          (if safe_mod
           then case tyConAbstractMsg rdr_env tc1 empty of
                    Just msg ->
                       [ msg $$ ptext (sLit "as required in SafeHaskell mode") ]
                    Nothing ->
                       [ msg
                       | tc <- tyConsOfTyCon tc1
                       , Just msg <- return $
                           tyConAbstractMsg rdr_env tc $
                             parens $ ptext (sLit "used within") <+> quotes (ppr tc1)
                       ]
           else []
          ) ++
          [ fsep [ hsep [ ptext $ sLit "because the", speakNth n, ptext $ sLit "type argument"]
                 , hsep [ ptext $ sLit "of", quotes (ppr tc1), ptext $ sLit "has role Nominal,"]
                 , ptext $ sLit "but the arguments"
                 , quotes (ppr t1)
                 , ptext $ sLit "and"
                 , quotes (ppr t2)
                 , ptext $ sLit "differ" ]
          | (n,Nominal,t1,t2) <- zip4 [1..] (tyConRoles tc1) tyArgs1 tyArgs2
          , not (t1 `eqType` t2)
          ]
      | Just (tc,_) <- splitTyConApp_maybe ty1,
        Just msg <- coercible_msg_for_tycon rdr_env tc
      = msg
      | Just (tc,_) <- splitTyConApp_maybe ty2,
        Just msg <- coercible_msg_for_tycon rdr_env tc
      = msg
      | otherwise
      = nest 2 $ hsep [ ptext $ sLit "because", quotes (ppr ty1),
                        ptext $ sLit "and", quotes (ppr ty2),
                        ptext $ sLit "are different types." ]
      where
        (clas, ~[ty1,ty2]) = getClassPredTys (ctPred ct)

    dataConMissing rdr_env tc =
        all (null . lookupGRE_Name rdr_env) (map dataConName (tyConDataCons tc))

    coercible_msg_for_tycon rdr_env tc
        | isRecursiveTyCon tc
        = Just $ nest 2 $ hsep [ ptext $ sLit "because", quotes (ppr tc)
                               , ptext $ sLit "is a recursive type constuctor" ]
        | isNewTyCon tc
        = tyConAbstractMsg rdr_env tc empty
        | otherwise
        = Nothing

    tyConAbstractMsg rdr_env tc occExpl
        | isAbstractTyCon tc || dataConMissing rdr_env tc = Just $ vcat $
            [ fsep [ ptext $ sLit "because the type constructor", quotes (ppr tc) <+> occExpl
                   , ptext $ sLit "is abstract" ]
            | isAbstractTyCon tc
            ] ++
            [ fsep [ ptext (sLit "because the constructor") <> plural (tyConDataCons tc)
                   , ptext (sLit "of") <+> quotes (ppr tc) <+> occExpl
                   , isOrAre (tyConDataCons tc) <+> ptext (sLit "not imported") ]
            | dataConMissing rdr_env tc
            ]
        | otherwise = Nothing

show_fixes :: [SDoc] -> SDoc
show_fixes []     = empty
show_fixes (f:fs) = sep [ ptext (sLit "Possible fix:")
                        , nest 2 (vcat (f : map (ptext (sLit "or") <+>) fs))]

ppr_insts :: [ClsInst] -> SDoc
ppr_insts insts
  = pprInstances (take 3 insts) $$ dot_dot_message
  where
    n_extra = length insts - 3
    dot_dot_message 
       | n_extra <= 0 = empty
       | otherwise    = ptext (sLit "...plus") 
                        <+> speakNOf n_extra (ptext (sLit "other"))

----------------------
quickFlattenTy :: TcType -> TcM TcType
-- See Note [Flattening in error message generation]
quickFlattenTy ty | Just ty' <- tcView ty = quickFlattenTy ty'
quickFlattenTy ty@(TyVarTy {})  = return ty
quickFlattenTy ty@(ForAllTy {}) = return ty     -- See
quickFlattenTy ty@(LitTy {})    = return ty
  -- Don't flatten because of the danger or removing a bound variable
quickFlattenTy (AppTy ty1 ty2) = do { fy1 <- quickFlattenTy ty1
                                    ; fy2 <- quickFlattenTy ty2
                                    ; return (AppTy fy1 fy2) }
quickFlattenTy (FunTy ty1 ty2) = do { fy1 <- quickFlattenTy ty1
                                    ; fy2 <- quickFlattenTy ty2
                                    ; return (FunTy fy1 fy2) }
quickFlattenTy (TyConApp tc tys)
    | not (isSynFamilyTyCon tc)
    = do { fys <- mapM quickFlattenTy tys 
         ; return (TyConApp tc fys) }
    | otherwise
    = do { let (funtys,resttys) = splitAt (tyConArity tc) tys
                -- Ignore the arguments of the type family funtys
         ; v <- newMetaTyVar TauTv (typeKind (TyConApp tc funtys))
         ; flat_resttys <- mapM quickFlattenTy resttys
         ; return (foldl AppTy (mkTyVarTy v) flat_resttys) }
\end{code}

Note [Flattening in error message generation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (C (Maybe (F x))), where F is a type function, and we have
instances
                C (Maybe Int) and C (Maybe a)
Since (F x) might turn into Int, this is an overlap situation, and
indeed (because of flattening) the main solver will have refrained
from solving.  But by the time we get to error message generation, we've
un-flattened the constraint.  So we must *re*-flatten it before looking
up in the instance environment, lest we only report one matching
instance when in fact there are two.

Re-flattening is pretty easy, because we don't need to keep track of
evidence.  We don't re-use the code in TcCanonical because that's in
the TcS monad, and we are in TcM here.

Note [Quick-flatten polytypes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see C (Ix a => blah) or C (forall a. blah) we simply refrain from
flattening any further.  After all, there can be no instance declarations
that match such things.  And flattening under a for-all is problematic
anyway; consider C (forall a. F a)

\begin{code}
mkAmbigMsg :: Ct -> (Bool, SDoc)
mkAmbigMsg ct
  | isEmptyVarSet ambig_tv_set = (False, empty)
  | otherwise                  = (True,  msg)
  where
    ambig_tv_set = filterVarSet isAmbiguousTyVar (tyVarsOfCt ct)
    ambig_tvs = varSetElems ambig_tv_set
    
    is_or_are | isSingleton ambig_tvs = text "is"
              | otherwise             = text "are"
                 
    msg | any isRuntimeUnkSkol ambig_tvs  -- See Note [Runtime skolems]
        =  vcat [ ptext (sLit "Cannot resolve unknown runtime type") <> plural ambig_tvs
                     <+> pprQuotedList ambig_tvs
                , ptext (sLit "Use :print or :force to determine these types")]
        | otherwise
        = vcat [ text "The type variable" <> plural ambig_tvs
                    <+> pprQuotedList ambig_tvs
                    <+> is_or_are <+> text "ambiguous" ]

pprSkol :: SkolemInfo -> SrcLoc -> SDoc
pprSkol UnkSkol   _ 
  = ptext (sLit "is an unknown type variable")
pprSkol skol_info tv_loc 
  = sep [ ptext (sLit "is a rigid type variable bound by"),
          sep [ppr skol_info, ptext (sLit "at") <+> ppr tv_loc]]

getSkolemInfo :: [Implication] -> TcTyVar -> SkolemInfo
-- Get the skolem info for a type variable 
-- from the implication constraint that binds it
getSkolemInfo [] tv
  = pprPanic "No skolem info:" (ppr tv)

getSkolemInfo (implic:implics) tv
  | tv `elem` ic_skols implic = ic_info implic
  | otherwise                 = getSkolemInfo implics tv

-----------------------
-- relevantBindings looks at the value environment and finds values whose
-- types mention any of the offending type variables.  It has to be
-- careful to zonk the Id's type first, so it has to be in the monad.
-- We must be careful to pass it a zonked type variable, too.
--
-- We always remove closed top-level bindings, though, 
-- since they are never relevant (cf Trac #8233)

relevantBindings :: Bool  -- True <=> filter by tyvar; False <=> no filtering
                          -- See Trac #8191
                 -> ReportErrCtxt -> Ct
                 -> TcM (ReportErrCtxt, SDoc)
relevantBindings want_filtering ctxt ct
  = do { dflags <- getDynFlags
       ; (tidy_env', docs, discards) 
              <- go (cec_tidy ctxt) (maxRelevantBinds dflags) 
                    emptyVarSet [] False
                    (tcl_bndrs lcl_env)
         -- tcl_bndrs has the innermost bindings first, 
         -- which are probably the most relevant ones

       ; traceTc "relevantBindings" (ppr [id | TcIdBndr id _ <- tcl_bndrs lcl_env])
       ; let doc = hang (ptext (sLit "Relevant bindings include")) 
                      2 (vcat docs $$ max_msg)
             max_msg | discards 
                     = ptext (sLit "(Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds)")
                     | otherwise = empty

       ; if null docs 
         then return (ctxt, empty)
         else do { traceTc "rb" doc
                 ; return (ctxt { cec_tidy = tidy_env' }, doc) } } 
  where
    lcl_env = ctLocEnv (cc_loc ct)
    ct_tvs = tyVarsOfCt ct

    run_out :: Maybe Int -> Bool
    run_out Nothing = False
    run_out (Just n) = n <= 0

    dec_max :: Maybe Int -> Maybe Int
    dec_max = fmap (\n -> n - 1)

    go :: TidyEnv -> Maybe Int -> TcTyVarSet -> [SDoc] 
       -> Bool                          -- True <=> some filtered out due to lack of fuel
       -> [TcIdBinder] 
       -> TcM (TidyEnv, [SDoc], Bool)   -- The bool says if we filtered any out
                                        -- because of lack of fuel
    go tidy_env _ _ docs discards []
       = return (tidy_env, reverse docs, discards)
    go tidy_env n_left tvs_seen docs discards (TcIdBndr id top_lvl : tc_bndrs)
       = do { (tidy_env', tidy_ty) <- zonkTidyTcType tidy_env (idType id)
            ; let id_tvs = tyVarsOfType tidy_ty
                  doc = sep [ pprPrefixOcc id <+> dcolon <+> ppr tidy_ty
		            , nest 2 (parens (ptext (sLit "bound at")
			    	 <+> ppr (getSrcLoc id)))]
                  new_seen = tvs_seen `unionVarSet` id_tvs

            ; if (want_filtering && id_tvs `disjointVarSet` ct_tvs)
                       -- We want to filter out this binding anyway
                       -- so discard it silently
              then go tidy_env n_left tvs_seen docs discards tc_bndrs

              else if isTopLevel top_lvl && not (isNothing n_left)
                       -- It's a top-level binding and we have not specified
                       -- -fno-max-relevant-bindings, so discard it silently
              then go tidy_env n_left tvs_seen docs discards tc_bndrs

              else if run_out n_left && id_tvs `subVarSet` tvs_seen
                       -- We've run out of n_left fuel and this binding only
                       -- mentions aleady-seen type variables, so discard it
              then go tidy_env n_left tvs_seen docs True tc_bndrs

                       -- Keep this binding, decrement fuel
              else go tidy_env' (dec_max n_left) new_seen (doc:docs) discards tc_bndrs }

-----------------------
warnDefaulting :: Cts -> Type -> TcM ()
warnDefaulting wanteds default_ty
  = do { warn_default <- woptM Opt_WarnTypeDefaults
       ; env0 <- tcInitTidyEnv
       ; let tidy_env = tidyFreeTyVars env0 $
                        tyVarsOfCts wanteds
             tidy_wanteds = mapBag (tidyCt tidy_env) wanteds
             (loc, ppr_wanteds) = pprWithArising (bagToList tidy_wanteds)
             warn_msg  = hang (ptext (sLit "Defaulting the following constraint(s) to type")
                                <+> quotes (ppr default_ty))
                            2 ppr_wanteds
       ; setCtLoc loc $ warnTc warn_default warn_msg }
\end{code}

Note [Runtime skolems]
~~~~~~~~~~~~~~~~~~~~~~
We want to give a reasonably helpful error message for ambiguity
arising from *runtime* skolems in the debugger.  These
are created by in RtClosureInspect.zonkRTTIType.  

%************************************************************************
%*									*
                 Error from the canonicaliser
	 These ones are called *during* constraint simplification
%*									*
%************************************************************************

\begin{code}
solverDepthErrorTcS :: Ct -> TcM a
solverDepthErrorTcS ct
  = setCtLoc loc $
    do { pred <- zonkTcType (ctPred ct)
       ; env0 <- tcInitTidyEnv
       ; let tidy_env  = tidyFreeTyVars env0 (tyVarsOfType pred)
             tidy_pred = tidyType tidy_env pred
       ; failWithTcM (tidy_env, hang msg 2 (ppr tidy_pred)) }
  where
    loc   = cc_loc ct
    depth = ctLocDepth loc
    msg = vcat [ ptext (sLit "Context reduction stack overflow; size =") <+> int depth
               , ptext (sLit "Use -fcontext-stack=N to increase stack size to N") ]
\end{code}

%************************************************************************
%*									*
                 Tidying
%*									*
%************************************************************************

\begin{code}
zonkTidyTcType :: TidyEnv -> TcType -> TcM (TidyEnv, TcType)
zonkTidyTcType env ty = do { ty' <- zonkTcType ty
                           ; return (tidyOpenType env ty') }

zonkTidyOrigin :: ReportErrCtxt -> CtOrigin -> TcM (ReportErrCtxt, CtOrigin)
zonkTidyOrigin ctxt (GivenOrigin skol_info)
  = do { skol_info1 <- zonkSkolemInfo skol_info
       ; let (env1, skol_info2) = tidySkolemInfo (cec_tidy ctxt) skol_info1
       ; return (ctxt { cec_tidy = env1 }, GivenOrigin skol_info2) }
zonkTidyOrigin ctxt (TypeEqOrigin { uo_actual = act, uo_expected = exp })
  = do { (env1, act') <- zonkTidyTcType (cec_tidy ctxt) act
       ; (env2, exp') <- zonkTidyTcType env1            exp
       ; return ( ctxt { cec_tidy = env2 }
                , TypeEqOrigin { uo_actual = act', uo_expected = exp' }) }
zonkTidyOrigin ctxt (KindEqOrigin ty1 ty2 orig)
  = do { (env1, ty1') <- zonkTidyTcType (cec_tidy ctxt) ty1
       ; (env2, ty2') <- zonkTidyTcType env1            ty2
       ; (ctxt2, orig') <- zonkTidyOrigin (ctxt { cec_tidy = env2 }) orig
       ; return (ctxt2, KindEqOrigin ty1' ty2' orig') }
zonkTidyOrigin ctxt orig = return (ctxt, orig)
\end{code}