type 'a t

let dog : 'this =
  let module Dog = struct
    external make
      : bark:('self -> unit)
      -> < bark : ('self -> unit) > t = "" ""
  end
  in
  Dog.make ~bark:(fun (o : 'this) -> ())

$ ./ocamlc.opt -I stdlib -stop-after typing x.ml
File "x.ml", line 4, characters 4-7:
4 | let dog : 'this =
        ^^^
Error: The type of this expression, < bark : '_this -> unit > t,
       contains the non-generalizable type variable(s): '_this.
       (see manual section 6.1.2)