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.
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.