I think you have a non-standard definition. An escaping pointer is an address that the compiler cannot fully track (directly or indirectly). It could be to a syscall, it could be a separately compiled function (without LTO), it could even be to a function in the same translation unit if the compiler cannot inline that function nor do sufficient intraprocedural analysis.

Again, I'm not a compiler writer, but my understanding is that non escaping variables can be optimized in SSA form, escaped variables are otherwise treated as memory and the compiler must be significantly more conservative.

In any case, whether a pointer escapes or not depends purely on the compiler capabilities and optimization level, so it would not be sane making a code well defined or UB depending on the compiler or optimization level.

edit: to be more concrete, do you think that in my example the constant folding of the return into return 1 should be allowed? And if so, which variant of this code would prevent the optimization and why?