test.ml

module type CON = sig
  type 'a t
  val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
end

(* (stdlib) List doesn't include 'a t, add it *)
module ListT = struct
  type 'a t = 'a list
  include List
end

module Test1 (M : CON) : CON = struct
  include M
end

module L1 = Test1 (ListT)

(* Would not type check *)
(* let _ = L1.fold_left (+) 0 [] *)

module Test2 (M : CON) : CON with type 'a t = 'a M.t = struct
  include M
end

module L2 = Test2 (ListT)

let _ = L2.fold_left (+) 0 []