Given this code:

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ImpredicativeTypes, ConstraintKinds #-}

class P a b
instance P a b

type Test a = forall b. P a b

I would expect to be able to actually use the Test Constraint in a function, like so:

foo :: Test a => a -> a
foo = id

But GHC complains that:

    Illegal polymorphic or qualified type: Test a
    In the type signature for ‘foo’: foo :: Test a => a -> a

I get the exact same error message if I replace ImpredicativeTypes with RankNTypes. If this is intended behavior under ImpredicativeTypes, how difficult would it be to add another extension that allows this type of polymorphism?