Item — HN Reader

For:

  A; // observable
  B; // UB

and when A is reachable, the compiler can not remove A even if it sees at compile-time that B has UB when executed. This is the case in C but not in C++.

If A is not reachable, the code will never be executed at run-time, so while there is no UB at run-time this is irrelevant as the code can simply be removed anyway because it is not reachable. For "unreachable()" the question is tricky and I think this might need to be clarified specifically.

1 comments

It's not tricky though. Unreachable invokes UB. It's something like the most canonically reduced version of B that we can write.

(Because unreachable is meant to be used in certain ways, implementations can give it relevant diagnostic powers. But all it means "please make the spot in the program have no defined behavior").

If the implementation cannot remove A on grounds of B being undefined, then you need an awkward special case for when B is the unreachable gizmo.

It is tricky because it not specified simply as UB - at least in C. And the question is whether this specification is already the special case or not, and even if we want this special case or rather only the diagnostic.