No one ever said that the original question makes much sense. In my
opinion the OP (how to add x + y without using the + operator) is sort
of useless; in fact I've NEVER seen a situation in my whole IT life
(and this lasts for almost 27 years by now) when I would have needed
to add two values without the possibility to add them using the +
operator (except of course when writing a compiler in assembler). So
this sort of questions is in my personal opinion(!) futile; a waste of
time, at best an intellectually challenging game (but to no avail).

You are right, x == -2 and y == +2 is not covered by the approach
given above, but then Good Man just gave a short hint as to how to
approach the question; he did not at all claim he had given a complete
solution.