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Program (compile with !ScopedTypeVariables):

type family H s a b

class D (G v) => C v where
  type G v
  type F v
  foo :: v -> H (F v) (G v) v

class D s where
  bar :: (forall t. Maybe t -> a) -> s -> H a s r -> r

call :: forall v. C v => F v -> v
call x = bar (\_ -> x)
             (undefined :: G v)
             (foo (undefined :: v))

bar' :: C v => (forall t. Maybe t -> F v) -> G v -> H (F v) (G v) v -> v
bar' = bar

The current head (even with today's typechecker patch) complains:

    Could not deduce (H a (G v) v ~ H (F v) (G v) v)
      from the context (C v)

But if I change bar to bar' in the rhs of foo, it all works fine. 6.12.3 accepts both programs.

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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