Yeah, signed integer overflow is as UB as ever. I've heard the primary reason for it is to avoid the possibility of wraparound on 'for (int i = 0; i < length; i++)' loops where the 'length' is bigger than an int. (Of course, the more straightforward option would be to use proper types like size_t for all your indices, but it's a classic tradition to use nothing but char and int, and people judge compilers based on existing code.)

ptrdiff_t is also useful in this case if signed semantics are desired.

> I've heard the primary reason for it is to avoid the possibility of wraparound on

Making it UB doesn’t fix that in any way that I can think of.

What it means is that since i as the variable is monotonically increasing, an array indexing operation that is in the loop body can be replaced with an incrementing pointer instead, which eliminates quite a lot of code. An example here: https://pvs-studio.com/en/blog/posts/cpp/0374/