problems with impredicativity

Hello, this code is accepted by GHC 7.0.4:

{-# LANGUAGE ImpredicativeTypes #-}

polyId :: (forall a. a) -> a polyId x = x

polyIdMap :: [forall a. a] -> [forall a. a] polyIdMap xs = fmap polyId xs

However, this one isn’t:

{-# LANGUAGE ImpredicativeTypes #-}

polyId :: (forall a. Maybe a) -> Maybe a polyId x = x

polyIdMap :: [forall a. Maybe a] -> [forall a. Maybe a] polyIdMap xs = fmap polyId xs

Is there a way to make it accepted? Best wishes, Wolfgang