Suppose somebody have written above code. Then under current C++ standard it would invoke UB on overflow. But, if standard were to change and require wrapping behaviour for int, then it would be "fixed", i.e, do what programmer intended it to do. Similarly if you use some compiler specific option to ensure wrapping behaviour for signed integers like GCC -fwrapv.

Oh, you are assuming someone actually wrote that code. That is circular reasoning.

It would be much better to write proper checks or just call functions that do so.