Consider two modules:

{-# LANGUAGE RankNTypes #-}
module X where

foo :: (forall a. a -> a) -> Int
foo = foo

module Y where
import X

g = foo

Note that Y does not use RankNTypes.

Currently, g is accepted. However, the inferred type of g is (forall a. a -> a) -> Int, which cannot be written. We should not allow a value if writing its type requires an additional extension.

Reporting on behalf of Simon PJ.