$ cat foo.ml
module M = struct
  type t = A | B of u
  and u = C of t
end

module type N = sig
  include module type of M
    with type t = M.t
    and type u = M.u
end

$ ocamlc -c foo.ml
File "foo.ml", line 8, characters 9-21:
Error: This variant or record definition does not match that of type M.t
       The types for field B are not equal.

I guess the "and" in the constraint doesn't make the constraints mutually
recursive, so what does it do?