$ cat incompat.ml
module F(S : sig type 'a t end) =
struct

  type _ ab =
      A : int S.t ab
    | B : float S.t ab

  let f : int S.t ab -> float S.t ab -> string =
    fun (l : int S.t ab) (r : float S.t ab) -> match l, r with
      | A, B -> "f A B"
end

module M = F(struct type 'a t = unit end)

let () = print_endline M.(f A A)

$ ocaml incompat.ml
f A B