Deriving Typeable instances

Hi,

we have a problem with deriving Typeable instances in ghc-6.4. The 
following code

data T a b c d e f g h = T a b c d e f g h
 deriving Typeable

fails in ghc-6.4 with the message:
   Can't make a derived instance of `Typeable (T a b c d e f g h)'
   (`T' has too many arguments)
   When deriving instances for type `T'

In ghc 6.2.2, it was no problem to derive Typeable instances for type 
constructors of higher (arbitrary?) arities.

If it is not possible to provide deriving Typeable for arbitrary arity 
of type constructors anymore, would it be possible to extend deriving to 
at least type constructors of arity 20? We are using Haskell as a target 
language of a compiler and here type constructors with many arguments occur.

Regards,
Frank