Misleading error message for higher rank type

Consider

foo :: (forall a b. C a b => b -> b) -> Int
foo = error "urk"

With GHC (for ages, incl 8.6) we get

    • Could not deduce (C a0 b)
      from the context: C a b
        bound by the type signature for:
          foo :: forall a b. C a b => b -> b      <======= Not true!
        at Foo.hs:7:8-43
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘foo’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: foo :: (forall a b. C a b => b -> b) -> Int

But foo's type signature is NOT foo :: forall a b. C a b => b -> b!!

Let's fix this.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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