[Haskell-cafe] Explicit forall - Strange Error

Hi, I wonder why the following code doesn't typecheck in GHC 7.4.1: {-# LANGUAGE GADTs,RankNTypes #-}data T a where T1 :: (forall b. b -> b) -> (forall a. Int -> T a) {- Error: Data constructor `T1' returns type `forall a. Int -> T a' instead of an instance of its parent type `T a' In the definition of data constructor `T1' In the data type declaration for `T' Failed, modules loaded: none. -} While: {-# LANGUAGE GADTs,RankNTypes #-} f :: (forall b. b -> b) -> (forall a. Int -> Maybe a)f = undefined {- ghci> :t f f :: (forall b. b -> b) -> Int -> Maybe a -} Thanks, Shayan