GADTs and functional dependencies

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