> if (x > 0) {…}. But what if you entered the body when x <= 0?

> if (false) {…}. But what if you execute the body?

> These are “impossible”. What happens when the impossible occurs is “undefined”.

By the way, that's exactly what happens with Spectre and its class of ghostly vulnerabilities! The processor speculatively executes these "impossible" paths, and discards the result once it detects they couldn't happen; but there are ways to "leak" information from that irreal world through side-effects like cache lines being discarded.