When or where?

I would say it's always a bug in a program—except when (mis)using ints as bit-fields in very special circumstances.

Simply, as more or less every computation in the real world is build around the concept of whole numbers, and not around the idea of overflowing two-complement bit-fields.

The later is so exotic that there is afik almost no proper use for it (the only exception being as a very low level building block to enable computers do actually useful calculations).

In C/C++ it's "only" UB for signed ints.

For unsigned ints it's arithmetic modulo 2^n (where n is the number of bits in the value representation of that particular size of integer).

Such divergent definitions make the whole mess of course even bigger, that's not the point.

This would be possible only if the standard would define the behavior in question as implementation defined. But that's not the case afaik.

Signed overflow is undefined behavior as it's afaik defined to be that. But maybe I'm just wrong here?

But at least SO is confining my knowledge:

https://stackoverflow.com/questions/16188263/is-signed-integ...