> Undefined behavior is always undefined behavior.

This is not true; the standard explicitly allows implementations to, and I quote directly from the standard, "behave during translation or program execution in a documented manner characteristic of the environment".

This idea that undefined behavior is always undefined behavior and there's no way to reason about it is purely academic, incorrect, and not in any way justified either by source material or in practice.

As for debugging undefined behavior, UBSan [1] is an excellent tool that is well supported by GCC and clang, and MSVC is working to add support for it as well.

[1] https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html