{-# LANGUAGE RankNTypes, ConstraintKinds #-}

-- This compiles

bar :: (Num a, Num b) => (forall c. Num c => c -> c) -> Either a b -> Either a b

bar f (Left a) = Left (f a)

bar f (Right b) = Right (f b)

bar' = bar (+ 2)

-- This doesn't compile because foo' does not typecheck

foo :: (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b

foo f (Left a) = Left (f a)

foo f (Right b) = Right (f b)

foo' = foo (+ 2)