Comment by steveklabnik
5 hours ago
It is defined as an error. That error’s default handling is wrapping when debug_assertions is off, and panic when it’s on, but since it’s an incorrect program (though not UB) either behavior is acceptable in any mode.
If it is defined as an error, but the compiled build will continue to run with the value wrapped around, I would say that's indistinguishable from UB.
It's indistinguishable from unspecified behavior, not from undefined behavior. Unspecified behavior has to pick from a finite list of allowed behaviors. Undefined behavior can do anything.
No. An integer getting deterministically set to an unintended value is a bug. A bug is not the same thing as UB. (Even if it were non-deterministic, it would still not be anything like UB.) It's not the same ballpark, not even the same sport.