Polymorphic implicit parameters

[Roman Cheplyaka]

I'm curious as to why GHC doesn't accept the following (contrived)
module:

  {-# LANGUAGE ImplicitParams, RankNTypes #-}

  f :: (?g :: (forall a . a -> a)) => a -> a
  f = ?g

The error message is:

  Illegal polymorphic or qualified type: forall a. a -> a
  In the type signature for `f':
    f :: ?g :: (forall a. a -> a) => a -> a

It's not a big deal since one can wrap the polymorphism in a newtype,
but it's somewhat unexpected.

Roman