(* TEST
flags = " -w a "
* setup-ocamlc.byte-build-env
** ocamlc.byte
*** check-ocamlc.byte-output
*)

module X = struct module Y = struct module type S = sig type t end end end

(* open X  (* works! *) *)
module Y = X.Y

type 'a arg_t = 'at constraint 'a = (module Y.S with type t = 'at)
type t = (module X.Y.S with type t = unit)

let f (x : t arg_t) = ()

let () = f ()