GADT: weird error message

In the code below the function "trans" is accepted by GHC 6.4, but 
"trans1" is not. I would expect that "(x,y)" is just syntactic sugar 
for "(,) x y", but apparently it isn't. I guess this is a bug; can 
anyone explain what is going on?

Cheers,

Arthur

The Code:
data Equal a b where
     Eq :: Equal a a

trans :: forall a b c. Equal a b -> Equal b c -> Equal a c
trans = \x -> \y -> case (,) x  y  of
                    (Eq,Eq ) -> Eq

trans1 :: forall a b c. Equal a b -> Equal b c -> Equal a c
trans1 = \x -> \y -> case (x, y)  of
                    (Eq,Eq ) -> Eq

The error message:
Test2.hs:9:0:
    Quantified type variable `c' is unified with another quantified 
type variable a
    When trying to generalise the type inferred for `trans1'
      Signature type:     forall a b c. Equal a b -> Equal b c -> Equal 
a c
      Type to generalise: Equal a b -> Equal b a -> Equal a a
    In the type signature for `trans1'
    When generalising the type(s) for `trans1'