It comes up from time to time, for example sometimes you'll have a big enum where most of the values can be handled in a straightforward way, and things are wei

It comes up from time to time, for example sometimes you'll have a big enum where most of the values can be handled in a straightforward way, and things are weird and complex for the other values. So you write something like int process(my_enum x, state_t state) { if (x == my_enum::COMPLICATED) return do_stuff(state); switch (x) { case my_enum::SIMPLE0: return 0; case my_enum::SIMPLE1: return 1; case my_enum::SIMPLE2: return 2; } } and your compiler complains that my_enum::COMPLICATED isn't handled, even though it clearly was. You generally like these warnings, because they do keep you safe, but in this case, you're smarter than the compiler, but it forces you to put something there. With std::unreachable, it will squelch the warning and not emit extra code. For example, you might be tempted to throw an error -- but if nothing else in the function throws, you end up emitting a bunch of unreachable error-handling code that the compiler can't remove.