[kazinator]

If a compiler can determine that some statement is UB, it can treat that as an assertion that the code is unreachable. All other statements which reach only that code and no other are also unreachable.

A compiler's analysis can go backward in time. That is to say, the compiler can build a model of what happens in some section of code over time, and analyze it whichever way it wants.

You cannot go back in time from execution time to translation time, but the translator can follow the code as if it were executing it at translation time.