[Haskell-cafe] Impredicativity confusion

Hello Cafe! Is there some reason why definition of 'boom' in the code below won't compile? Is it documented somewhere? Thanks. {-# OPTIONS_GHC -fglasgow-exts #-} data Foo a foo :: Foo a -> a -> Bool foo = undefined newtype A = A (forall a. a->a) ok = foo f (A id) where f = undefined :: Foo A type B = forall a. a->a boom = foo f (id :: B) where f = undefined :: Foo B