subtyping of polymorphic variants when using GADTs

subtype.ml

type 'r t = A : [`A | `T] t | B : [`B | `T] t;; (* the information 'r : [> `T] seems to be missing *) 
(* type _ t = A : [ `A | `T ] t | B : [ `B | `T ] t *)

(* the following works as expected: *)
let rec f : type r. r t -> r = function A -> `T | B -> `T;;
(* val f : 'r t -> 'r = <fun> *)
f A;;
(* - : [ `A | `T ] = `T *)

(* now f should be called on different modules for the same q, and the results should be combined *)
let rec g : type r. r t -> r = fun q -> let h a s = f q in List.fold_left h `T [1;2;3];;
(* Error: This expression has type [> `T ]
   but an expression was expected of type r *)