# module A = struct type t = A | B let x = B end;;
module A : sig type t = A | B val x : t end
# let test () =
    match A.x with
    | A as a -> `A_t a
    | B when false -> `Onoes
    | B -> if Random.bool () then `Onoes else `A_t B;;
val test : unit -> [> `A_t of A.t | `Onoes ] = <fun>
# let test () =
    match A.x with
    | B when false -> `Onoes
    | A as a -> `A_t a
    | B -> if Random.bool () then `Onoes else `A_t B;;
Characters 128-129:
    | B -> if Random.bool () then `Onoes else `A_t B;;
                                                   ^
Error: Unbound constructor B