We cannot spot x in the following definition:
class type t = object method m : int method n : int end class c x : t = object method m = x + 1 end
This is OCaml 4.00.1's problem. If we make the above well typed by adding method n, then at the position of the argument x, we see 2 val x str_item deifinitions. In the ill-typed version, there is only one. The missing one provides the link between the argument and the use. (You can verify it by ocamlspot --dump-tree).