It's true that code with UB does not have to be reached, per se, but it does have to be something your program will reach before it can hurt you.
Usually when we talk about UB, we're implicitly talking about runtime UB, since translation-time UB is generally far less subtle. If a program contains only conditional runtime UB, the compiler is not permitted to break the entire program from the very beginning, since all possible executions that do not trigger runtime UB must execute correctly as per 5.1.2.3.
But those of us who are actually writing programs mostly care about "in practical terms", and in practical terms, this doesn't happen, so we don't care. We've got enough trouble worrying about what does happen; we don't have time and energy to worry about what doesn't and won't happen.
If the reality is that any UB can invalidate the entire program (as is the interpretation taken by other standards re: C), then that's not remotely sufficient. You have to ensure the complete absence of UB.
Sure, this is perfectly fine.
Only that you're not writing any C/C++ than, but something in the "gcc 12 language with some switches", or maybe the "LLVM 15 language with some switches", or something like that.
