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Consider the following program:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug where

import Data.Kind
import Data.Proxy

class C a where
  m :: a -> ()

data Dict c = c => Dict

-----

type family F a :: Type -> Type
class    C (F a b) => CF a b
instance C (F a b) => CF a b

works1        :: (forall z. CF a z) => Proxy (a, b) -> Dict (CF a b)
works1 _ = Dict

works2        :: (          CF a b) => Proxy (a, b) -> Dict (C (F a b))
works2 _ = Dict

works3, fails :: (forall z. CF a z) => Proxy (a, b) -> Dict (C (F a b))
works3 p | Dict <- works1 p = Dict
fails _ = Dict

fails, as its name suggests, fails to typecheck:

$ /opt/ghc/8.6.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:33:11: error:
    • Could not deduce (C (F a b)) arising from a use of ‘Dict’
      from the context: forall z. CF a z
        bound by the type signature for:
                   fails :: forall a b.
                            (forall z. CF a z) =>
                            Proxy (a, b) -> Dict (C (F a b))
        at Bug.hs:31:1-71
    • In the expression: Dict
      In an equation for ‘fails’: fails _ = Dict
   |
33 | fails _ = Dict
   |           ^^^^

But I see no reason why this shouldn't typecheck. After all, the fact that works1 typechecks proves that GHC's constraint solver is perfectly capable of deducing that (forall z. CF a z) implies (CF a b), and the fact that works2 typechecks proves that GHC's constraint solver is perfectly capable of deducing that (CF a b) implies that (C (F a b)). Why then can GHC's constraint solver not connect the dots and deduce that (forall z. CF a z) implies (C (F a b)) (in the type of fails)?

Note that something with the type (forall z. CF a z) => Proxy (a, b) -> Dict (C (F a b)) //can// be made to work if you explicitly guide GHC along with explicit pattern-matching on a Dict, as works3 demonstrates. But I claim that this shouldn't be necessary.

Moreover, in this variation of the program above:

-- <as above>
-----

data G a :: Type -> Type
class    C (G a b) => CG a b
instance C (G a b) => CG a b

works1' :: (forall z. CG a z) => Proxy (a, b) -> Dict (CG a b)
works1' _ = Dict

works2' :: (          CG a b) => Proxy (a, b) -> Dict (C (G a b))
works2' _ = Dict

works3' :: (forall z. CG a z) => Proxy (a, b) -> Dict (C (G a b))
works3' _ = Dict

works3' needs no explicit Dict pattern-matching to typecheck.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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