That's what everybody originally believes (me included)! constexpr is_right_answer(int x){ if (x==42) return true; else { std::cout <<"t

That's what everybody originally believes (me included)! constexpr is_right_answer(int x){ if (x==42) return true; else { std::cout <<"try again \n"; return false; } A function can be constexpr as long as at least one possible invocation can be compile time evaluated. In fact compilers can't generally prove that a function can't be constexpr. I believe consteval has the semantics you want but I've yet to study it in details. For more details see: https://www.foonathan.net/2020/10/constexpr-platform/ https://www.foonathan.net/2020/10/constexpr-platform/ .