使用constexpr-if时出错:预期'(''constexpr'之前'

Question

我试图使用if-constexpr检查一些东西,但我遇到了类似的错误

预期'('之前'constexpr'

没有先前'if'的'else'

到目前为止,我检查我的代码没有任何问题

我的编译标志是g ++ -std = c ++ 17 main.cpp

#include <iostream>
template<typename T, typename Comp = std::less<T> >
struct Facility
{
template<T ... list>
struct List
{
    static void print()
    {
        std::cout<<"\""<<"Empty List"<<"\""<<"\n";
    }
};
template<T head,T ... list>
struct List<head,list...>
{
    static void print()
    {
        std::cout<<"\"" << head;
        ((std::cout << " " << list), ...);
        std::cout<<"\""<<"\n";
    }
};
template<unsigned N,typename AA>
struct RemoveFirst{};

template<unsigned N,T head,T ... Rest>
struct RemoveFirst<N,List<head,Rest...>>
{
    struct result
    {
        static void print()
        {   
            if constexpr (N == head)
            {
                std::cout<<"";
            }
            else
            {
                std::cout<<"\""<<head;
                ((std::cout << " " << Rest), ...);
                std::cout<<"\""<<"\n";
            }
        }
    };
 };
};
template<int ... intlist>
using IntList = typename Facility<int>::List<intlist...>;

int main()
{
 using IntFacility = Facility<int>;
 using List = IntList<2, 8, 2, 3, 5, 10, 8, 5>;
}

Answer 1

较旧版本的GCC(最高6.x)不支持最终版本的C++ 17会产生该错误,因为它们识别constexpr为关键字但不理解constexpr-if构造.确保您的GCC是版本7或更高版本.