'Untouchable' error in list comprehension

Hi,

I think this program should compile but it doesn't:

{-# LANGUAGE GADTs #-}

data X a where X :: X Int

foo :: Bool
foo = null [ () | X <- [] ]

It fails with:

    Couldn't match expected type `a0' with actual type `()'
      `a0' is untouchable
           inside the constraints (t_g ~ Int)
           bound at a pattern with constructor
                      X :: X Int,
                    in a pattern binding in
                         list comprehension
    In the pattern: X
    In a stmt of a list comprehension: X <- []
    In the first argument of `null', namely `[() | X <- []]'

If I remove the type declaration GHC compiles without errors and happily infers foo :: Bool.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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