bad error message with GADTs

[David Roundy]

The following code gives a rather unpleasant error message with ghc 6.6.
Adding the type signature fixes the error.

David

{-# OPTIONS -fglasgow-exts #-}

data PatchSeq p a b where
    Nil   :: PatchSeq p a a
    U  :: p a b -> PatchSeq p a b
    (:-)  :: PatchSeq p a b -> PatchSeq p b c -> PatchSeq p a c

-- is_normal :: PatchSeq p a b -> Bool
is_normal Nil = True
is_normal (U _) = True
is_normal (U _ :- _) = True
is_normal _ = False