type foo = [ `A | `B ]
type _ bar =
  | A : [> `A] bar
  | B : [> `B] bar

(* This function's type is inferred to be [ `A | `B ] bar -> string
   Is it possible for it to be            [< `A | `B ] bar -> string ? *)
let to_string (t : [< `A | `B] bar) =
  match (t : [< `A | `B] bar :> [`A | `B] bar) with
  | A -> "A"
  | B -> "B"