Failure of transitivity in OCaml's subtyping

nontrans.ml

(* OCaml's subtyping relation is not always transitive *)

type (_,_) eq = Refl : ('a, 'a) eq
type 'a inty = int

type s = < f : int -> int >
type t = < f : 'a . 'a inty -> 'a inty >

(* s and t are equal and subtype each other *)
let eq_s_t : (s, t) eq = Refl
let sub_s_t x = (x : s :> t)
let sub_t_s x = (x : t :> s)

(* But only one of them is a supertype of poly *)
type poly = < f : 'a . 'a -> 'a >

let sub_poly_s x = (x : poly :> s)
let sub_poly_t x = (x : poly :> t)
(* Error: Type poly = < f : 'a. 'a -> 'a > is not a subtype of
            t = < f : 'a. int -> int > 
          Type int is not a subtype of 'a *)