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It's allowed to reject programs that will evaluate an undefined operation on every possible execution. This is pretty much impossible, so no compiler even attempts this.

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It can replace any path with undefined behaviour to one that does std::abort.

It can, but in practice no one does it.

When you think about it, that's what -fsanitize=undefined is doing. It detects undefined behaviour at runtime and aborts the program.

It detects some, but not all UBs. AFAIK it can't detect UB due to hnsequenced access to the same variable.

There are different sanitizers that detect different types of UB.

Asan, Msan, Tsan and UBSan all detects different ones.

Which is very different from diagnosing at compile time.

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