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

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.