type _ op = Noop
and _ t =
    | Null : int t
    | Op : 'e t * 'e op -> 'a t

(* when -principal is enabled, this function typechecks... *)
let g : type a . a t -> unit =
    fun t ->
    let op_idx : int op * int =
        match t with
        | Null -> Noop, 1
        | Op (Null, op) -> op, 2
        | _ -> assert false in
    let op, idx = op_idx in
    ()

(* ...and this is not *)
let f : type a . a t -> unit =
    fun t ->
    let (op, idx) : int op * int =
        match t with
        | Null -> Noop, 1
        | Op (Null, op) -> op, 2
        | _ -> assert false in
    ()