"A lazy (~) pattern cannot bind existential type variables" happens for non-existential GADTs

This program:

{-# LANGUAGE GADTs #-}
module Foo where

data Foo a where
   Foo :: a -> Foo (a, Int)

foo :: Foo (a, Int) -> a
foo ~(Foo a) = a

causes this error:

Foo.hs:8:4:
    A lazy (~) pattern cannot bind existential type variables
      `a' is a rigid type variable bound by
          the constructor `Foo' at Foo.hs:8:6
    In the pattern: ~(Foo a)
    In the definition of `foo': foo ~(Foo a) = a

This doesn't seem like an existential, as there aren't any type variables in the arguments to Foo that aren't in the result type.

If easy it would be nice if the restriction were relaxed to allow for this case, otherwise I think the error message should be improved.

Tested with ghc 6.10.1.20081209 and ghc 6.11.20090107.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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