GADT regression

[alcremi at free.fr]

Hi,
I am unable to compile the following program with ghc-6.3.20041004, due to a
regression introduced by GADT (I suppose) :

module Definition  where

data UnOp a b =
               OpNegate
             | OpRecip
             | OpNot
             | OpConst
             | OpUnFunc (a -> b)

instance Show (UnOp a b) where
    show OpNegate       = "OpNegate"
    show OpRecip        = "OpRecip"
    show OpNot          = "OpNot"
    show OpConst        = "OpConst"
    show (OpUnFunc _)   = "OpUnFunc <function>"

data (Show a) => Obs a =
                                KonstObs  a
    | forall b.     (Show b) => LiftObs  (UnOp b a)       (Obs b)

instance (Show a) => Show (Obs a) where
    show (KonstObs x)            = "KonstObs " ++ show x


I get the following message :
bug.hs:23:10:
    Ambiguous type variable `a' in the top-level constraint:
      `Show a' arising from use of `LiftObs' at bug.hs:23:10-21


It compiles OK with 'hugs -98'


Alain