> If a compiler can detect UB

In the general case it can not, that is why they were made UBs in the first place.

And that is generally not how compilers see UB, they don’t look for UBs lying around, they assume UBs can’t happen and use that for constraints, which they then propagate.

For instance

    *i = 1
    if (id) { … }

And this occurs and is extremely desirable every time e.g. a function is inlined in an other one which already checked for null.

But how is your example UB? It's straightforward data flow analysis.

AFAICT, UBI is stuff like that:

  short int i;
  for (i = 0; i < 33000; i++) do_something();