GADTs and functional dependencies

[Wolfgang Jeltsch]

Hello,

please consider the following code:

> {-# LANGUAGE GADTs, MultiParamTypeClasses, FunctionalDependencies #-}
> 
> data GADT a where
> 
>     GADT :: GADT ()
> 
> class Class a b | a -> b
> 
> instance Class () ()
> 
> fun :: (Class a b) => GADT a -> b
> fun GADT = ()

I’d expect this to work but unfortunately, using GHC 6.8.2, it fails with the 
following message:

> FDGADT.hs:12:11:
>     Couldn't match expected type `b' against inferred type `()'
>       `b' is a rigid type variable bound by
>           the type signature for `fun' at FDGADT.hs:11:16
>     In the expression: ()
>     In the definition of `fun': fun GADT = ()

What’s the reason for this?  Is there a workaround?  Does this work in 6.8.3 
or 6.10.1?

Thank you in advance.

Best wishes,
Wolfgang