从语言设计的层面来看,当条件在编译时无法推导时,“if constexpr”为什么不衰减为“trival if”

constexpr function compile-time ,将延迟计算 run-time non-constexpr function constexpr

if statement if constexpr compile-time if statement 容易(没有 true_type false_type

constexpr int factorial(int n)
{
    if constexpr(n == 0) return 1;
    else return n * factorial(n-1);
}

所以,上面的代码 cannot pass compilation n the function can be calculated at compile-time when input is known at compile-time