Module subtyping

module type Big = sig
  type t

  val v : t
end

module type Small = sig
  include Big

  val v' : t
end

let get_v (type a) (smol : (module Big with type t = a)) : a =
  let module M = (val smol) in
  M.v
;;

let create () =
  ( module struct
    type t = int

    let v = 0
    let v' = 1
  end
  : Small
    with type t = int )
;;

module Sub' : Small = (val create ())

let _zero = get_v (module Sub')

let _zero =
  let module M = (val create ()) in
  get_v (module M)
;;

(** Why does the above work, but not this?: *)
let _zero = get_v (create ())