box.ml

type a = [ `A of int ]
type b = [ a | `B of int ]

let pair (a : a) (b : b) = ([a] :> b list) @ [b]  (* works fine *)

module type Config = sig type t end

module type Box = sig
  module C : Config
  type t = { regions : C.t list }
  val empty : t
  val add : C.t -> t -> t
  val get : t -> C.t list
end

module Box (C: Config) : Box with type C.t = C.t = struct
  module C = C
  type t = { regions : C.t list }
  let empty = { regions = [] }
  let add r box = { regions = r :: box.regions }
  let get box = box.regions
end

module ABox = Box(struct type t = a end)
module BBox = Box(struct type t = b end)

(* Error: Type ABox.t is not a subtype of BBox.t *)
let join (abox : ABox.t) = BBox.get (abox :> BBox.t)