class not used, error not detected

Hello,

the following code is accepted by GHC but rejected by Hugs. I think, GHC is in 
error here.
    class C a where
        x :: Int

    main :: IO ()
    main = return ()

Is the phenomenon mentioned in
    http://haskell.org/pipermail/haskell/2003-August/012428.html
similar to this problem?

Wolfgang