data List a = Nil | Cons a (List a)

build :: (forall l. l -> (a -> l -> l) -> l) -> List a
build f = f Nil Cons

main = build (\nil cons -> cons (1 :: Integer) (Cons (2 :: Integer) nil))
                                             // ^^^^
-- 
7:48-7:73: Expected <a> got <List Integer>.
7:69-7:72: Expected <List Integer> got <a>.