[Haskell-cafe] Class Instance with ExistentialQuantification

Why is the following not allowed? {-# LANGUAGE ExistentialQuantification, ExplicitForAll, RankNTypes, FlexibleInstances #-} class Class a where test :: a -> Bool instance Class (forall m. m -> m) where test _ = True main = do putStrLn $ test id Is there a reason that this is forbidden? Just curious. -- Andrew