GADTs and infix?

In ghc 6.5, why does this GADT definition get accepted

> data DArrowX :: * -> * -> * where
>   First   :: DArrowX a  a' -> DArrowX (a,b) (a',b)

and also this one

> data DArrowX :: * -> * -> * where
>   First   :: a `DArrowX`  a' -> DArrowX (a,b) (a',b)

but this not one

> data DArrowX :: * -> * -> * where
>   First   :: a `DArrowX`  a' -> (a,b) `DArrowX` (a',b)

which yields

   Malformed constructor signature: (a, b) DArrowX (a', b)
   In the signature of a data constructor: a DArrowX a' -> (a, b)
DArrowX (a', b)
   In the data type declaration for `DArrowX'

Is this a bug?

 - Conal