[Haskell-cafe] GeneralizedNewtypeDeriving and TypeFamilies

So I understand from http://hackage.haskell.org/trac/ghc/ticket/4083 that GeneralizedNewtypeDeriving doesn't play well with TypeFamilies. But, if a typeclass is only using type synonyms, is there any reason why newtype deriving would not work? For a toy example: class Cls a where type MyType a foo :: MyType a -> in this case, if you have something like: instance Cls () where type MyType () = () foo = id would: newtype Bar = Bar () deriving (Cls) be well defined? Thanks, Max