c++ - 如何断言 constexpr if else 子句永远不会发生？

Question

如果条件为真，我想在非 constexpr 时引发编译时错误，例如:

if constexpr(condition1){
    ...
} else if constexpr (condition2) {
   ....
} else if constexpr (condition3) {
  ....
} else {
    // I want the else clause never taken. But I heard the code below is not allowed
    static_assert(false);
}

// I'd rather not repeat the conditions again like this:
static_assert(condition1 || condition2 || condition3);

Answer 1

您必须使丢弃的语句依赖于模板参数

template <class...> constexpr std::false_type always_false{};

if constexpr(condition1){
    ...
} else if constexpr (condition2) {
   ....
} else if constexpr (condition3) {
  ....
} else {       
    static_assert(always_false<T>);
}

这是因为

[temp.res]/8 - The program is ill-formed, no diagnostic required, if

no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or ...