It's surprising and potentially buggy behavior, but that's very different from undefined behavior. To such a degree that I think you might not understand what it means, and what the risks are around undefined behavior, especially in the presence of an optimizing compiler.

As a starter / refresher perhaps, both of these are perfectly permissible and happen in practice with UB, but never with "wrap or panic" / "implementation defined" behavior: https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-... This kind of thing is an example of the "time travel" stevekablanik is referring to, stuff that is literally impossible as written, that absolutely no human would consider to be a reasonable execution of the code, but occurs regularly with UB.