Simon Peyton Jones pushed to branch wip/T26255 at Glasgow Haskell Compiler / GHC

Commits:

3 changed files:

Changes:

  • compiler/GHC/Tc/Errors.hs
    ... ... @@ -50,6 +50,8 @@ import GHC.Types.Name.Env
    50 50 
     import GHC.Types.SrcLoc
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     import GHC.Types.Basic
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     import GHC.Types.Error
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    +import GHC.Types.Unique( hasKey )
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    +import GHC.Builtin.Names( errorMessageTypeErrorFamKey )
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     import qualified GHC.Types.Unique.Map as UM
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    55 57 
     import GHC.Unit.Module
    ... ... @@ -439,10 +441,10 @@ reportBadTelescope _ _ skol_info skols
    439 441 
     -- See Note [Constraints to ignore].
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     ignoreConstraint :: Ct -> Bool
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     ignoreConstraint ct
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    -  | AssocFamPatOrigin <- ctOrigin ct
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    -  = True
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    -  | otherwise
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    -  = False
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    +  = case ctOrigin ct of
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    +      AssocFamPatOrigin         -> True  -- See (CIG1)
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    +      WantedSuperclassOrigin {} -> True  -- See (CIG2)
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    +      _                         -> False
    446 448
    447 449 
     -- | Makes an error item from a constraint, calculating whether or not
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     -- the item should be suppressed. See Note [Wanteds rewrite Wanteds]
    ... ... @@ -612,14 +614,14 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics
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                   , ("skolem eq2",      skolem_eq,      True, mkSkolReporter)
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    614 616 
                   -- Next, family applications like (F t1 t2 ~ rigid_ty)
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    -              -- These can be solved by doing a type-family reduction for F
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    +              -- These could be solved by doing a type-family reduction for F
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                   -- which probably means fixing a unfication variable in t1/t2
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                   -- See discussion in #26255, where F had an injectivity annotation,
    618 620 
                   -- and we had   [W] F alpha ~ "foo"
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                   -- The real error is that the "foo" should be "bar", because there is
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                   --    type instance F Int = "bar"
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    -              -- We could additionally filter on the injectivty annotation, but
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    -              -- currenlty we don't.
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    +              -- We could additionally filter on the injectivty annotation,
    624 
    +              -- but currently we don't.
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                   , ("fam app",         is_fam_app_eq,  True, mkGroupReporter mkEqErr)
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    625 627 
                   -- Put custom type errors after solid equality errors.  In #26255 we
    ... ... @@ -680,8 +682,11 @@ reportWanteds ctxt tc_lvl wc@(WC { wc_simple = simples, wc_impl = implics
    680 682 
         is_FRR item _ = isJust $ fixedRuntimeRepOrigin_maybe item
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    682 684 
         -- Things like (F t1 t2 ~N Maybe s)
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    -    is_fam_app_eq _ (EqPred NomEq ty1 ty2) = isJust (isSatTyFamApp ty1) && isRigidTy ty2
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    -    is_fam_app_eq _ _                      = False
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    +    -- But only proper type families; not (TypeError t1 t2 ~N blah)
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    +    is_fam_app_eq _ (EqPred NomEq ty1 ty2)
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    +       | Just (tc,_) <- isSatTyFamApp ty1
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    +       = not (tc `hasKey` errorMessageTypeErrorFamKey) && isRigidTy ty2
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    +    is_fam_app_eq _ _  = False
    685 690
    686 691 
         -- Things like (a ~N b) or (a  ~N  F Bool)
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         skolem_eq _ (EqPred NomEq ty1 _) = isSkolemTy tc_lvl ty1
    ... ... @@ -819,7 +824,7 @@ they will remain unfilled, and might have been used to rewrite another constrain
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    820 825 
     Currently, the constraints to ignore are:
    821 826
    822 
    -1) Constraints generated in order to unify associated type instance parameters
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    +(CIG1) Constraints generated in order to unify associated type instance parameters
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        with class parameters. Here are two illustrative examples:
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    825 830 
          class C (a :: k) where
    ... ... @@ -847,6 +852,10 @@ Currently, the constraints to ignore are:
    847 852
    848 853 
        If there is any trouble, checkValidFamInst bleats, aborting compilation.
    849 854
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    +(CIG2) Superclasses of Wanteds.  These are generated on in case they trigger functional
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    +   dependencies.  If such a constraint is unsolved, then its "parent" constraint must
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    +   also be unsolved, and is much more informative to the user (#26255).
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    +
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     Note [Implementation of Unsatisfiable constraints]
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     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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     The Unsatisfiable constraint was introduced in GHC proposal #433 (https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0433-unsatisfiable.rst).

  • testsuite/tests/typecheck/should_fail/T26255c.hs
    1 
    +{-# LANGUAGE DataKinds #-}
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    +{-# LANGUAGE FlexibleContexts #-}
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    +{-# LANGUAGE ScopedTypeVariables #-}
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    +{-# LANGUAGE TypeFamilyDependencies #-}
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    +{-# LANGUAGE TypeOperators #-}
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    +{-# LANGUAGE UndecidableSuperClasses #-}
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    +
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    +module T26255c where
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    +
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    +import Data.Kind
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    +import Data.Proxy
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    +import GHC.TypeLits
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    +
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    +type MinVersion = 1
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    +
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    +class
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    +  ( KnownNat (ProtVerLow era)
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    +  , MinVersion <= ProtVerLow era
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    +  ) =>
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    +  Era era
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    +  where
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    +  type ProtVerLow era :: Nat
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    +
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    +newtype EraFamily era = EraFamily Int
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    +
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    +class Era era => NewEra era where
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    +  eraFamilySize :: EraFamily era -> Int
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    +
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    +printEraFamilySize :: EraFamily era -> IO ()
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    +printEraFamilySize = print . eraFamilySize

  • testsuite/tests/typecheck/should_fail/T26255c.stderr
    1 
    +T26255c.hs:30:30: error: [GHC-39999]
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    +    • No instance for ‘NewEra era’
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    +        arising from a use of ‘eraFamilySize’
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    +      Possible fix:
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    +        add (NewEra era) to the context of
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    +          the type signature for:
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    +            printEraFamilySize :: forall {k} (era :: k). EraFamily era -> IO ()
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    +    • In the second argument of ‘(.)’, namely ‘eraFamilySize’
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    +      In the expression: print . eraFamilySize
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    +      In an equation for ‘printEraFamilySize’:
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    +          printEraFamilySize = print . eraFamilySize

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