Comment by tekne

No, that's actually UB. The important bit here is "compiler defined" -- UB means the compiler is allowed to assume it never happens while compiling.

Consider, for example, an implementation defined function f() -- which can also diverge/crash horribly, etc.

If I write

    if p {
      print("p is true")
    } else {
      g()
    }

    if p {
      f()
    }

Then either we: - print p is true and execute f - do nothing

This is true regardless of if f immediately crashes the computer, nasal demons, whatever -- that's implementation defined.

UB means f may never happen.

And that means the compiler may optimize this to just:

    g()

Notice the difference here -- the print never happens!, and g always happens.

You can see why this is concerning when you write code like

    if dry_run {
      print("would run rm -rf /")
    } else {
      run("rm -rf /")
    }

    if dry_run {
      // oops: some_debug_string is NULL and will segfault!
      print(some_debug_string);
    }