[Haskell] Pattern Matching with rank-2 types

Hello, The following function is a valid function in ghc6.2.2:

func :: (forall a . [a]) -> [b] func [] = []

However, in ghc6.4.1 it does not work anymore: Iterators4.lhs:56:6: Couldn't match `forall a. [a]' against `[a]' When checking the pattern: [] In the definition of `func': func [] = [] Is this supposed to happen? Cheers, Bruno