scoped type variables: bad error message, and wrong semantics?

With this module (a cut-down version of reify):

{-# OPTIONS -fglasgow-exts #-}

module Foo where

type GTypeFun = forall a . a -> ()

gmapType :: Int -> GTypeFun
gmapType _ (_ :: a) = undefined

GHC gives the rather perplexing error:

Foo.hs:8:12:
    The scoped type variables `a' and `a'
      are bound to the same type (variable)
      Distinct scoped type variables must be distinct
    In the pattern: _ :: a
    In the definition of `gmapType': gmapType _ (_ :: a) = undefined

Having locations for the two as would be useful.

This module gives the same error:

{-# OPTIONS -fglasgow-exts #-}

module Foo where

gmapType :: Int -> (forall a . a -> ())
gmapType _ (_ :: a) = undefined

GHC 6.8.2 accepts both modules.

I don't think that the forall in the type signature should bring a into scope for the body, as it doesn't bring it into scope for the type signature.

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case	reify
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Operating system	Unknown
Architecture	Unknown