foo.ml

  type _ foo =
    | A : int -> int foo
    | B : int -> int foo
  module FooSet = Set.Make(struct
                             type t
                             let compare a b =
                               let a' = match a with A x -> x | B x -> x in
                               let b' = match b with A x -> x | B x -> x in
                               Pervasives.compare a' b'
                           end)
                           
(*
Error: Signature mismatch:
       Modules do not match:
         sig type t val compare : int foo -> int foo -> int end
       is not included in
         Set.OrderedType
       Values do not match:
         val compare : int foo -> int foo -> int
       is not included in
         val compare : t -> t -> int
*)