TcErrors.hs 123 KB
Newer Older
1
{-# LANGUAGE CPP, ScopedTypeVariables #-}
2 3

module TcErrors(
4
       reportUnsolved, reportAllUnsolved, warnAllUnsolved,
5
       warnDefaulting,
6 7

       solverDepthErrorTcS
8 9 10 11
  ) where

#include "HsVersions.h"

12
import TcRnTypes
13 14
import TcRnMonad
import TcMType
15
import TcUnify( occCheckForErrors, OccCheckResult(..) )
16
import TcType
17
import RnUnbound ( unknownNameSuggestions )
batterseapower's avatar
batterseapower committed
18
import Type
19 20
import TyCoRep
import Kind
21
import Unify            ( tcMatchTys )
22
import Module
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
23
import FamInst
24
import FamInstEnv       ( flattenTys )
25 26 27
import Inst
import InstEnv
import TyCon
28
import Class
29
import DataCon
30
import TcEvidence
31
import HsExpr  ( UnboundVar(..) )
32
import HsBinds ( PatSynBind(..) )
33
import Name
34
import RdrName ( lookupGlobalRdrEnv, lookupGRE_Name, GlobalRdrEnv
35 36
               , mkRdrUnqual, isLocalGRE, greSrcSpan, pprNameProvenance
               , GlobalRdrElt (..), globalRdrEnvElts )
Richard Eisenberg's avatar
Richard Eisenberg committed
37
import PrelNames ( typeableClassName, hasKey, liftedRepDataConKey )
38
import Id
39 40 41
import Var
import VarSet
import VarEnv
42
import NameSet
43
import Bag
44
import ErrUtils         ( ErrMsg, errDoc, pprLocErrMsg )
45
import BasicTypes
46
import ConLike          ( ConLike(..), conLikeWrapId_maybe )
47
import Util
48 49
import HscTypes (HscEnv, lookupTypeHscEnv, TypeEnv, lookupTypeEnv )
import NameEnv (lookupNameEnv)
50 51
import FastString
import Outputable
52
import SrcLoc
53
import DynFlags
54
import ListSetOps       ( equivClasses )
55
import Maybes
56
import Pair
57
import qualified GHC.LanguageExtensions as LangExt
niteria's avatar
niteria committed
58
import FV ( fvVarList, unionFV )
Icelandjack's avatar
Icelandjack committed
59

60
import Control.Monad    ( when )
61
import Data.Foldable    ( toList )
62
import Data.List        ( partition, mapAccumL, nub, sortBy, unfoldr )
63
import qualified Data.Set as Set
64

65 66
import Data.Semigroup   ( Semigroup )
import qualified Data.Semigroup as Semigroup
67 68


Austin Seipp's avatar
Austin Seipp committed
69 70 71
{-
************************************************************************
*                                                                      *
72
\section{Errors and contexts}
Austin Seipp's avatar
Austin Seipp committed
73 74
*                                                                      *
************************************************************************
75 76 77 78 79

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?

80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98
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
99

100 101 102 103 104 105 106 107 108 109 110 111 112 113
  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
114
and does not fail if -fdefer-type-errors is on, so that we can continue
115
compilation. The errors are turned into warnings in `reportUnsolved`.
Austin Seipp's avatar
Austin Seipp committed
116
-}
117

118 119
-- | Report unsolved goals as errors or warnings. We may also turn some into
-- deferred run-time errors if `-fdefer-type-errors` is on.
120 121
reportUnsolved :: WantedConstraints -> TcM (Bag EvBind)
reportUnsolved wanted
122 123 124 125 126 127
  = do { binds_var <- newTcEvBinds
       ; defer_errors <- goptM Opt_DeferTypeErrors
       ; warn_errors <- woptM Opt_WarnDeferredTypeErrors -- implement #10283
       ; let type_errors | not defer_errors = TypeError
                         | warn_errors      = TypeWarn
                         | otherwise        = TypeDefer
128

129
       ; defer_holes <- goptM Opt_DeferTypedHoles
130 131 132 133 134
       ; warn_holes  <- woptM Opt_WarnTypedHoles
       ; let expr_holes | not defer_holes = HoleError
                        | warn_holes      = HoleWarn
                        | otherwise       = HoleDefer

135
       ; partial_sigs      <- xoptM LangExt.PartialTypeSignatures
thomasw's avatar
thomasw committed
136
       ; warn_partial_sigs <- woptM Opt_WarnPartialTypeSignatures
137 138 139 140
       ; let type_holes | not partial_sigs  = HoleError
                        | warn_partial_sigs = HoleWarn
                        | otherwise         = HoleDefer

141 142 143 144 145 146
       ; 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

147
       ; report_unsolved binds_var False type_errors expr_holes
148
          type_holes out_of_scope_holes wanted
149 150 151

       ; ev_binds <- getTcEvBindsMap binds_var
       ; return (evBindMapBinds ev_binds)}
152

153
-- | Report *all* unsolved goals as errors, even if -fdefer-type-errors is on
Simon Peyton Jones's avatar
Simon Peyton Jones committed
154 155
-- However, do not make any evidence bindings, because we don't
-- have any convenient place to put them.
156
-- See Note [Deferring coercion errors to runtime]
Simon Peyton Jones's avatar
Simon Peyton Jones committed
157 158 159
-- Used by solveEqualities for kind equalities
--      (see Note [Fail fast on kind errors] in TcSimplify]
-- and for simplifyDefault.
160
reportAllUnsolved :: WantedConstraints -> TcM ()
161
reportAllUnsolved wanted
162 163 164
  = do { ev_binds <- newTcEvBinds
       ; report_unsolved ev_binds False TypeError
                         HoleError HoleError HoleError wanted }
165

166 167 168 169 170
-- | 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
171 172 173
  = do { ev_binds <- newTcEvBinds
       ; report_unsolved ev_binds True TypeWarn
                         HoleWarn HoleWarn HoleWarn wanted }
174 175

-- | Report unsolved goals as errors or warnings.
176
report_unsolved :: EvBindsVar        -- cec_binds
177
                -> Bool              -- Errors as warnings
178
                -> TypeErrorChoice   -- Deferred type errors
179 180
                -> HoleChoice        -- Expression holes
                -> HoleChoice        -- Type holes
181
                -> HoleChoice        -- Out of scope holes
182
                -> WantedConstraints -> TcM ()
183 184
report_unsolved mb_binds_var err_as_warn type_errors expr_holes
    type_holes out_of_scope_holes wanted
185
  | isEmptyWC wanted
186
  = return ()
187
  | otherwise
188
  = do { traceTc "reportUnsolved (before zonking and tidying)" (ppr wanted)
189

190
       ; wanted <- zonkWC wanted   -- Zonk to reveal all information
191
       ; env0 <- tcInitTidyEnv
192 193
            -- If we are deferring we are going to need /all/ evidence around,
            -- including the evidence produced by unflattening (zonkWC)
194
       ; let tidy_env = tidyFreeTyCoVars env0 free_tvs
niteria's avatar
niteria committed
195
             free_tvs = tyCoVarsOfWCList wanted
196

197 198 199
       ; traceTc "reportUnsolved (after zonking):" $
         vcat [ text "Free tyvars:" <+> pprTyVars free_tvs
              , text "Wanted:" <+> ppr wanted ]
200 201 202

       ; warn_redundant <- woptM Opt_WarnRedundantConstraints
       ; let err_ctxt = CEC { cec_encl  = []
203
                            , cec_tidy  = tidy_env
204
                            , cec_defer_type_errors = type_errors
205
                            , cec_errors_as_warns = err_as_warn
206 207
                            , cec_expr_holes = expr_holes
                            , cec_type_holes = type_holes
208
                            , cec_out_of_scope_holes = out_of_scope_holes
209
                            , cec_suppress = False -- See Note [Suppressing error messages]
210
                            , cec_warn_redundant = warn_redundant
211
                            , cec_binds    = mb_binds_var }
212

213 214
       ; tc_lvl <- getTcLevel
       ; reportWanteds err_ctxt tc_lvl wanted }
215

216 217 218
--------------------------------------------
--      Internal functions
--------------------------------------------
219

220 221 222 223 224
-- | 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]
225
           , report_valid_substitutions :: [SDoc]
226 227
           }

228
instance Outputable Report where   -- Debugging only
229 230 231
  ppr (Report { report_important = imp
              , report_relevant_bindings = rel
              , report_valid_substitutions = val })
232
    = vcat [ text "important:" <+> vcat imp
233 234
           , text "relevant:"  <+> vcat rel
           , text "valid:"  <+> vcat val ]
235

236 237 238 239 240 241 242 243 244 245 246 247 248
{- Note [Error report]
The idea is that error msgs are divided into three parts: the main msg, the
context block (\"In the second argument of ...\"), and the relevant bindings
block, which are displayed in that order, with a mark to divide them.  The
idea is that the main msg ('report_important') varies depending on the error
in question, but context and relevant bindings are always the same, which
should simplify visual parsing.

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

249
instance Semigroup Report where
250
    Report a1 b1 c1 <> Report a2 b2 c2 = Report (a1 ++ a2) (b1 ++ b2) (c1 ++ c2)
251

252
instance Monoid Report where
253 254 255
    mempty = Report [] [] []
    mappend (Report a1 b1 c1) (Report a2 b2 c2)
      = Report (a1 ++ a2) (b1 ++ b2) (c1 ++ c2)
256 257 258 259 260 261 262 263 264

-- | 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] }

265 266 267 268
-- | Put a doc into the valid substitutions block.
valid_substitutions :: SDoc -> Report
valid_substitutions docs = mempty { report_valid_substitutions = [docs] }

269 270 271 272 273
data TypeErrorChoice   -- What to do for type errors found by the type checker
  = TypeError     -- A type error aborts compilation with an error message
  | TypeWarn      -- A type error is deferred to runtime, plus a compile-time warning
  | TypeDefer     -- A type error is deferred to runtime; no error or warning at compile time

274 275 276 277 278
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

279 280 281 282 283 284 285 286 287 288
instance Outputable HoleChoice where
  ppr HoleError = text "HoleError"
  ppr HoleWarn  = text "HoleWarn"
  ppr HoleDefer = text "HoleDefer"

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

289
data ReportErrCtxt
290
    = CEC { cec_encl :: [Implication]  -- Enclosing implications
291
                                       --   (innermost first)
292
                                       -- ic_skols and givens are tidied, rest are not
293
          , cec_tidy  :: TidyEnv
Simon Peyton Jones's avatar
Simon Peyton Jones committed
294

295 296 297
          , cec_binds :: EvBindsVar    -- Make some errors (depending on cec_defer)
                                       -- into warnings, and emit evidence bindings
                                       -- into 'cec_binds' for unsolved constraints
298

299 300 301 302
          , cec_errors_as_warns :: Bool   -- Turn all errors into warnings
                                          -- (except for Holes, which are
                                          -- controlled by cec_type_holes and
                                          -- cec_expr_holes)
303
          , cec_defer_type_errors :: TypeErrorChoice -- Defer type errors until runtime
304

305 306 307 308 309 310 311
          -- 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
312

313
          , cec_warn_redundant :: Bool    -- True <=> -Wredundant-constraints
314

315 316 317
          , cec_suppress :: Bool    -- True <=> More important errors have occurred,
                                    --          so create bindings if need be, but
                                    --          don't issue any more errors/warnings
318
                                    -- See Note [Suppressing error messages]
319 320
      }

321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339
instance Outputable ReportErrCtxt where
  ppr (CEC { cec_binds              = bvar
           , cec_errors_as_warns    = ew
           , cec_defer_type_errors  = dte
           , cec_expr_holes         = eh
           , cec_type_holes         = th
           , cec_out_of_scope_holes = osh
           , cec_warn_redundant     = wr
           , cec_suppress           = sup })
    = text "CEC" <+> braces (vcat
         [ text "cec_binds"              <+> equals <+> ppr bvar
         , text "cec_errors_as_warns"    <+> equals <+> ppr ew
         , text "cec_defer_type_errors"  <+> equals <+> ppr dte
         , text "cec_expr_holes"         <+> equals <+> ppr eh
         , text "cec_type_holes"         <+> equals <+> ppr th
         , text "cec_out_of_scope_holes" <+> equals <+> ppr osh
         , text "cec_warn_redundant"     <+> equals <+> ppr wr
         , text "cec_suppress"           <+> equals <+> ppr sup ])

Austin Seipp's avatar
Austin Seipp committed
340
{-
341 342
Note [Suppressing error messages]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
343
The cec_suppress flag says "don't report any errors".  Instead, just create
344
evidence bindings (as usual).  It's used when more important errors have occurred.
345

346 347 348
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.
349
  * If there are any insolubles (eg Int~Bool), here or in a nested implication,
350 351
    then suppress errors from the simple constraints here.  Sometimes the
    simple-constraint errors are a knock-on effect of the insolubles.
Austin Seipp's avatar
Austin Seipp committed
352
-}
353

354 355
reportImplic :: ReportErrCtxt -> Implication -> TcM ()
reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
356
                                 , ic_wanted = wanted, ic_binds = evb
357
                                 , ic_status = status, ic_info = info
358
                                 , ic_env = tcl_env, ic_tclvl = tc_lvl })
359
  | BracketSkol <- info
360
  , not insoluble
361 362
  = return ()        -- For Template Haskell brackets report only
                     -- definite errors. The whole thing will be re-checked
Simon Peyton Jones's avatar
Simon Peyton Jones committed
363 364
                     -- later when we plug it in, and meanwhile there may
                     -- certainly be un-satisfied constraints
365 366

  | otherwise
367 368
  = do { traceTc "reportImplic" (ppr implic')
       ; reportWanteds ctxt' tc_lvl wanted
369 370
       ; when (cec_warn_redundant ctxt) $
         warnRedundantConstraints ctxt' tcl_env info' dead_givens }
371
  where
372
    insoluble    = isInsolubleStatus status
373
    (env1, tvs') = mapAccumL tidyTyCoVarBndr (cec_tidy ctxt) tvs
Simon Peyton Jones's avatar
Simon Peyton Jones committed
374
    info'        = tidySkolemInfo env1 info
375
    implic' = implic { ic_skols = tvs'
Simon Peyton Jones's avatar
Simon Peyton Jones committed
376
                     , ic_given = map (tidyEvVar env1) given
377
                     , ic_info  = info' }
Simon Peyton Jones's avatar
Simon Peyton Jones committed
378
    ctxt' = ctxt { cec_tidy     = env1
379
                 , cec_encl     = implic' : cec_encl ctxt
380 381 382 383 384 385 386

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

387
                 , cec_binds    = evb }
388

389 390 391 392 393
    dead_givens = case status of
                    IC_Solved { ics_dead = dead } -> dead
                    _                             -> []

warnRedundantConstraints :: ReportErrCtxt -> TcLclEnv -> SkolemInfo -> [EvVar] -> TcM ()
394
-- See Note [Tracking redundant constraints] in TcSimplify
395
warnRedundantConstraints ctxt env info ev_vars
396
 | null redundant_evs
397 398 399 400 401
 = return ()

 | SigSkol {} <- info
 = setLclEnv env $  -- We want to add "In the type signature for f"
                    -- to the error context, which is a bit tiresome
402
   addErrCtxt (text "In" <+> ppr info) $
403
   do { env <- getLclEnv
404
      ; msg <- mkErrorReport ctxt env (important doc)
405
      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
406 407 408 409

 | 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
410
 = do { msg <- mkErrorReport ctxt env (important doc)
411
      ; reportWarning (Reason Opt_WarnRedundantConstraints) msg }
412
 where
413
   doc = text "Redundant constraint" <> plural redundant_evs <> colon
414 415 416 417 418 419 420
         <+> 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 $
421
                      transSuperClasses (idType ev_var)
422 423 424 425 426 427 428 429 430

{- 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
431
of evidence, since the dictionary has no fields.  But it is still
432 433 434 435 436 437 438
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..
-}
439

440 441
reportWanteds :: ReportErrCtxt -> TcLevel -> WantedConstraints -> TcM ()
reportWanteds ctxt tc_lvl (WC { wc_simple = simples, wc_insol = insols, wc_impl = implics })
442
  = do { traceTc "reportWanteds" (vcat [ text "Simples =" <+> ppr simples
443
                                       , text "Insols =" <+> ppr insols
444
                                       , text "Suppress =" <+> ppr (cec_suppress ctxt)])
445
       ; let tidy_cts = bagToList (mapBag (tidyCt env) (insols `unionBags` simples))
446 447 448 449

         -- First deal with things that are utterly wrong
         -- Like Int ~ Bool (incl nullary TyCons)
         -- or  Int ~ t a   (AppTy on one side)
450
         -- These /ones/ are not suppressed by the incoming context
451 452 453 454 455 456 457 458
       ; 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
459 460
       ; MASSERT2( null leftovers, ppr leftovers )

461
            -- All the Derived ones have been filtered out of simples
462 463 464
            -- 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]
465

466
     ; mapBagM_ (reportImplic ctxt2) implics }
467
            -- NB ctxt1: don't suppress inner insolubles if there's only a
468
            -- wanted insoluble here; but do suppress inner insolubles
469
            -- if there's a *given* insoluble here (= inaccessible code)
470
 where
471
    env = cec_tidy ctxt
472

473 474 475 476 477 478
    -- report1: ones that should *not* be suppresed by
    --          an insoluble somewhere else in the tree
    -- It's crucial that anything that is considered insoluble
    -- (see TcRnTypes.trulyInsoluble) is caught here, otherwise
    -- we might suppress its error message, and proceed on past
    -- type checking to get a Lint error later
479
    report1 = [ ("custom_error", is_user_type_error,True, mkUserTypeErrorReporter)
480
              , given_eq_spec
481
              , ("insoluble2",    utterly_wrong,    True, mkGroupReporter mkEqErr)
482 483 484 485 486
              , ("skolem eq1",    very_wrong,       True, mkSkolReporter)
              , ("skolem eq2",    skolem_eq,        True, mkSkolReporter)
              , ("non-tv eq",     non_tv_eq,        True, mkSkolReporter)
              , ("Out of scope",  is_out_of_scope,  True, mkHoleReporter)
              , ("Holes",         is_hole,          False, mkHoleReporter)
487 488 489

                  -- The only remaining equalities are alpha ~ ty,
                  -- where alpha is untouchable; and representational equalities
490 491 492 493 494
                  -- 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) ]
495 496 497 498 499 500

    -- 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) ]

501 502
    -- rigid_nom_eq, rigid_nom_tv_eq,
    is_hole, is_dict,
503
      is_equality, is_ip, is_irred :: Ct -> PredTree -> Bool
504

505 506 507 508 509
    is_given_eq ct pred
       | EqPred {} <- pred = arisesFromGivens ct
       | otherwise         = False
       -- I think all given residuals are equalities

510 511 512 513 514 515 516
    -- 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
517

518
    -- Things like (a ~N b) or (a  ~N  F Bool)
519
    skolem_eq _ (EqPred NomEq ty1 _) = isSkolemTy tc_lvl ty1
520
    skolem_eq _ _                    = False
521

522 523 524 525 526 527 528 529
    -- 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
530

531 532 533
    is_homo_equality _ (EqPred _ ty1 ty2) = typeKind ty1 `tcEqType` typeKind ty2
    is_homo_equality _ _                  = False

534 535 536
    is_equality _ (EqPred {}) = True
    is_equality _ _           = False

537 538 539 540 541 542 543 544 545
    is_dict _ (ClassPred {}) = True
    is_dict _ _              = False

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

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

546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561
    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
562

563
---------------
564
isSkolemTy :: TcLevel -> Type -> Bool
565
-- The type is a skolem tyvar
566
isSkolemTy tc_lvl ty
567 568 569 570 571 572 573 574
  | 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
575

576 577
isTyFun_maybe :: Type -> Maybe TyCon
isTyFun_maybe ty = case tcSplitTyConApp_maybe ty of
578
                      Just (tc,_) | isTypeFamilyTyCon tc -> Just tc
579 580
                      _ -> Nothing

581
--------------------------------------------
582
--      Reporters
583 584
--------------------------------------------

585 586 587 588 589 590 591 592 593
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
594
-- Suppress duplicates with either the same LHS, or same location
595
mkSkolReporter ctxt cts
596
  = mapM_ (reportGroup mkEqErr ctxt) (group cts)
597
  where
598 599 600 601 602 603
     group [] = []
     group (ct:cts) = (ct : yeses) : group noes
        where
          (yeses, noes) = partition (group_with ct) cts

     group_with ct1 ct2
604 605 606
       | EQ <- cmp_loc ct1 ct2 = True
       | eq_lhs_type   ct1 ct2 = True
       | otherwise             = False
607

608
mkHoleReporter :: Reporter
609
-- Reports errors one at a time
610
mkHoleReporter ctxt
611 612 613
  = mapM_ $ \ct -> do { err <- mkHoleError ctxt ct
                      ; maybeReportHoleError ctxt ct err
                      ; maybeAddDeferredHoleBinding ctxt err ct }
614

615 616
mkUserTypeErrorReporter :: Reporter
mkUserTypeErrorReporter ctxt
617
  = mapM_ $ \ct -> do { err <- mkUserTypeError ctxt ct
618 619
                      ; maybeReportError ctxt err
                      ; addDeferredBinding ctxt err ct }
620 621 622

mkUserTypeError :: ReportErrCtxt -> Ct -> TcM ErrMsg
mkUserTypeError ctxt ct = mkErrorMsgFromCt ctxt ct
623
                        $ important
624 625
                        $ pprUserTypeErrorTy
                        $ case getUserTypeErrorMsg ct of
626 627
                            Just msg -> msg
                            Nothing  -> pprPanic "mkUserTypeError" (ppr ct)
628 629


630
mkGivenErrorReporter :: Implication -> Reporter
631
-- See Note [Given errors]
632
mkGivenErrorReporter implic ctxt cts
633 634 635 636 637 638 639 640 641 642 643 644 645 646
  = 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

Simon Peyton Jones's avatar
Simon Peyton Jones committed
647
       ; traceTc "mkGivenErrorReporter" (ppr ct)
648 649 650 651 652
       ; maybeReportError ctxt err }
  where
    (ct : _ )  = cts    -- Never empty
    (ty1, ty2) = getEqPredTys (ctPred ct)

653 654
ignoreErrorReporter :: Reporter
-- Discard Given errors that don't come from
Simon Peyton Jones's avatar
Simon Peyton Jones committed
655
-- a pattern match; maybe we should warn instead?
656
ignoreErrorReporter ctxt cts
Simon Peyton Jones's avatar
Simon Peyton Jones committed
657
  = do { traceTc "mkGivenErrorReporter no" (ppr cts $$ ppr (cec_encl ctxt))
658 659
       ; return () }

660 661 662

{- Note [Given errors]
~~~~~~~~~~~~~~~~~~~~~~
Gabor Greif's avatar
Gabor Greif committed
663
Given constraints represent things for which we have (or will have)
664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694
evidence, so they aren't errors.  But if a Given constraint is
insoluble, this code is inaccessible, and we might want to at least
warn about that.  A classic case is

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

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

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

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

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

695 696 697
mkGroupReporter :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg)
                             -- Make error message for a group
                -> Reporter  -- Deal with lots of constraints
698 699 700
-- Group together errors from same location,
-- and report only the first (to avoid a cascade)
mkGroupReporter mk_err ctxt cts
701
  = mapM_ (reportGroup mk_err ctxt . toList) (equivClasses cmp_loc cts)
702

703 704
eq_lhs_type :: Ct -> Ct -> Bool
eq_lhs_type ct1 ct2
705 706
  = case (classifyPredType (ctPred ct1), classifyPredType (ctPred ct2)) of
       (EqPred eq_rel1 ty1 _, EqPred eq_rel2 ty2 _) ->
707
         (eq_rel1 == eq_rel2) && (ty1 `eqType` ty2)
708 709 710 711
       _ -> pprPanic "mkSkolReporter" (ppr ct1 $$ ppr ct2)

cmp_loc :: Ct -> Ct -> Ordering
cmp_loc ct1 ct2 = ctLocSpan (ctLoc ct1) `compare` ctLocSpan (ctLoc ct2)
712

713 714
reportGroup :: (ReportErrCtxt -> [Ct] -> TcM ErrMsg) -> ReportErrCtxt
            -> [Ct] -> TcM ()
quchen's avatar
quchen committed
715 716 717
reportGroup mk_err ctxt cts =
  case partition isMonadFailInstanceMissing cts of
        -- Only warn about missing MonadFail constraint when
Gabor Greif's avatar
Gabor Greif committed
718
        -- there are no other missing constraints!
719 720 721
        (monadFailCts, []) ->
            do { err <- mk_err ctxt monadFailCts
               ; reportWarning (Reason Opt_WarnMissingMonadFailInstances) err }
quchen's avatar
quchen committed
722 723 724

        (_, cts') -> do { err <- mk_err ctxt cts'
                        ; maybeReportError ctxt err
725 726 727 728 729 730 731
                            -- 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)
quchen's avatar
quchen committed
732 733 734 735 736
  where
    isMonadFailInstanceMissing ct =
        case ctLocOrigin (ctLoc ct) of
            FailablePattern _pat -> True
            _otherwise           -> False
737

738 739
maybeReportHoleError :: ReportErrCtxt -> Ct -> ErrMsg -> TcM ()
maybeReportHoleError ctxt ct err
thomasw's avatar
thomasw committed
740
  -- When -XPartialTypeSignatures is on, warnings (instead of errors) are
741 742 743
  -- generated for holes in partial type signatures.
  -- Unless -fwarn_partial_type_signatures is not on,
  -- in which case the messages are discarded.
744 745 746 747 748
  | 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
749
       HoleWarn  -> reportWarning (Reason Opt_WarnPartialTypeSignatures) err
750 751
       HoleDefer -> return ()

752 753 754 755
  -- 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
756
  | isOutOfScopeCt ct
757 758 759 760 761 762
  = -- 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 ()
763 764 765

  -- Otherwise this is a typed hole in an expression,
  -- but not for an out-of-scope variable
766
  | otherwise
767
  = -- If deferring, report a warning only if -Wtyped-holes is on
768 769
    case cec_expr_holes ctxt of
       HoleError -> reportError err
770
       HoleWarn  -> reportWarning (Reason Opt_WarnTypedHoles) err
771
       HoleDefer -> return ()
772

773
maybeReportError :: ReportErrCtxt -> ErrMsg -> TcM ()
774
-- Report the error and/or make a deferred binding for it
775
maybeReportError ctxt err
776 777 778
  | cec_suppress ctxt    -- Some worse error has occurred;
  = return ()            -- so suppress this error/warning

779
  | cec_errors_as_warns ctxt
780
  = reportWarning NoReason err
781

782
  | otherwise
783
  = case cec_defer_type_errors ctxt of
784
      TypeDefer -> return ()
785
      TypeWarn  -> reportWarning (Reason Opt_WarnDeferredTypeErrors) err
786
      TypeError -> reportError err
787

788
addDeferredBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
789
-- See Note [Deferring coercion errors to runtime]
790
addDeferredBinding ctxt err ct
791
  | CtWanted { ctev_pred = pred, ctev_dest = dest } <- ctEvidence ct
792 793 794 795 796
    -- 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)"
797
             err_tm  = EvDelayedError pred err_fs
798
             ev_binds_var = cec_binds ctxt
799

800 801 802 803 804 805 806 807
       ; case dest of
           EvVarDest evar
             -> addTcEvBind ev_binds_var $ mkWantedEvBind evar err_tm
           HoleDest hole
             -> do { -- See Note [Deferred errors for coercion holes]
                     evar <- newEvVar pred
                   ; addTcEvBind ev_binds_var $ mkWantedEvBind evar err_tm
                   ; fillCoercionHole hole (mkTcCoVarCo evar) }}
808 809

  | otherwise   -- Do not set any evidence for Given/Derived
810
  = return ()
811

812 813
maybeAddDeferredHoleBinding :: ReportErrCtxt -> ErrMsg -> Ct -> TcM ()
maybeAddDeferredHoleBinding ctxt err ct
814 815 816 817
  | isExprHoleCt ct
  = addDeferredBinding ctxt err ct  -- Only add bindings for holes in expressions
  | otherwise                       -- not for holes in partial type signatures
  = return ()
818

819
tryReporters :: ReportErrCtxt -> [ReporterSpec] -> [Ct] -> TcM (ReportErrCtxt, [Ct])
820
-- Use the first reporter in the list whose predicate says True
821
tryReporters ctxt reporters cts
822 823 824
  = 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
825 826
       ; traceTc "tryReporters }" (ppr cts')
       ; return (ctxt', cts') }
827
  where
828 829 830 831 832 833 834 835 836
    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' }
837 838 839 840 841 842 843
                -- Carry on with the rest, because we must make
                -- deferred bindings for them if we have -fdefer-type-errors
                -- But suppress their error messages

tryReporter :: ReportErrCtxt -> ReporterSpec -> [Ct] -> TcM (ReportErrCtxt, [Ct])
tryReporter ctxt (str, keep_me,  suppress_after, reporter) cts
  | null yeses = return (ctxt, cts)
844
  | otherwise  = do { traceTc "tryReporter{ " (text str <+> ppr yeses)
845 846
                    ; reporter ctxt yeses
                    ; let ctxt' = ctxt { cec_suppress = suppress_after || cec_suppress ctxt }
847
                    ; traceTc "tryReporter end }" (text str <+> ppr (cec_suppress ctxt) <+> ppr suppress_after)
848 849 850
                    ; return (ctxt', nos) }
  where
    (yeses, nos) = partition (\ct -> keep_me ct (classifyPredType (ctPred ct))) cts
851

852 853 854 855 856 857 858 859 860

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

861
-- Add the "arising from..." part to a message about bunch of dicts
862
addArising :: CtOrigin -> SDoc -> SDoc
863
addArising orig msg = hang msg 2 (pprArising orig)
864

865
pprWithArising :: [Ct] -> (CtLoc, SDoc)
866 867
-- Print something like
--    (Eq a) arising from a use of x at y
868 869
--    (Show a) arising from a use of p at q
-- Also return a location for the error message
870
-- Works for Wanted/Derived only
871
pprWithArising []
872
  = panic "pprWithArising"
873 874
pprWithArising (ct:cts)
  | null cts
875
  = (loc, addArising (ctLocOrigin loc)
dimitris's avatar
dimitris committed
876
                     (pprTheta [ctPred ct]))
877 878
  | otherwise
  = (loc, vcat (map ppr_one (ct:cts)))
879
  where
880
    loc = ctLoc ct
881
    ppr_one ct' = hang (parens (pprType (ctPred ct')))
882
                     2 (pprCtLoc (ctLoc ct'))
883

884 885 886
mkErrorMsgFromCt :: ReportErrCtxt -> Ct -> Report -> TcM ErrMsg
mkErrorMsgFromCt ctxt ct report
  = mkErrorReport ctxt (ctLocEnv (ctLoc ct)) report
887

888
mkErrorReport :: ReportErrCtxt -> TcLclEnv -> Report -> TcM ErrMsg
889
mkErrorReport ctxt tcl_env (Report important relevant_bindings valid_subs)
890 891