> UB problems happen only when compilers detect it, understand it is an opportunity for optimization, because they are allowed to do anything, then do that optimization.

This is a deep misconception, and not at all how most undefined behavior is related to optimization.

Trivial example: Compilers assume that your variables aren't written to randomly from other threads. Without this assumption, virtually no optimization would be possible. Therefore, data races are UB - they violate a hard assumption of the compiler. But at no point did the compiler say "oh, you wrote to this variable without synchronization! Now I'll show you, hehehe!".

This is the same for e.g. removed overflow checks. Compilers can optimize many loops only if they assume signed integer overflow never happens. So, because compilers should be able to assume this, it is UB if it happens in your code. The same logic that deduces "this loop cannot wrap around" deduces "this if condition [an overflow check] cannot ever be true".

But it's easier for a programmer who got their UB-reliant checks optimized out to attribute malice to the compiler than to understand optimization fundamentals, and thus we get people complaining to high heaven and back.