module_constrain.ml

module type Greeting = sig
  type t
  val hello : string
end

(* Hi guys, why this does not type check *)

module type MakeGreeting =
  (functor (M1 : Greeting)
           (M2 : Greeting with type t = M1.t) ->
  sig
     type t = M1.t
     val hello : string
   end with type M1.t = M2.t)


(* while this does? *)

module type MakeGreeting =
  (functor (M1 : Greeting)
           (M2 : Greeting with type t = M1.t) -> 
  sig
     type t = M1.t
     val hello : string
   end with type t = M2.t)


(* assuming OCaml 4.12 *)