类静态成员的声明冲突

Question

#include <iostream>
#include <utility>

using namespace std;

template<typename>
struct Class1;

template<size_t...Is>
struct Class1<index_sequence<Is...>> {
    template<typename T, size_t N>
    struct Holder {
        constexpr Holder(T const(&Ns)[N]) : data{T(Ns[Is] * Is)...} {
        }

        T data[N];
    };
};

template<typename T, T...Ns>
class Class2 {
public:
    static constexpr const size_t N = sizeof...(Ns);
    static constexpr const T mNs[] = {Ns...};
    static constexpr const typename Class1<make_index_sequence<N>>::template Holder<T, N> Hs{mNs}; 
    // If I replace the above N with sizeof...(Ns), the error is gone.
};

template <typename T, T...Ns>
constexpr const typename Class1<make_index_sequence<sizeof...(Ns)>>::template Holder<T, sizeof...(Ns)>
        Class2<T, Ns...>::Hs; // g++ error: conflicting declaration.

int main() {
    cout << &Class2<int, 1, 2, 3>::Hs << endl;
}

This is the simplified version of my code. It compiles in VS because VS does not comformant to this shit. But failed to compile under g++ 6.3. What exactly is the type of Class2<...>::Hs?

Basically I need to initialize an array using another array's element and the index of that element at compile time. So if there is a better way, It would be better.

Answer 1

The fact that N will hold the value of sizeof...(Ns) seems to be misleading. I think this error is legit as use of N in Hs declaration makes the type of Hs depend on Class2::N . If you use Class2< T, Ns... >::N in Hs definition error should be gone:

#include <iostream>
#include <utility>
#include <cstddef>

template< typename > struct
Class1;

template< ::std::size_t...Is > struct
Class1< ::std::index_sequence< Is... > >
{
    template< typename T, ::std::size_t N > struct
    Holder
    {
        constexpr
        Holder(T const(&Ns)[N])
        :   data{T(Ns[Is] * Is)...}
        {}

        T data[N];
    };
};

template< typename T, T...Ns > class
Class2
{
    public: static constexpr const ::std::size_t N{sizeof...(Ns)};
    public: static constexpr const int mNs[]{Ns...};
    public: static constexpr const typename Class1< ::std::make_index_sequence< N > >::
        template Holder< T, N > Hs{mNs};
};

template< typename T, T...Ns >
constexpr const typename Class1< ::std::make_index_sequence< Class2< T, Ns... >::N > >::
    template Holder< T, Class2< T, Ns... >::N > Class2< T, Ns... >::Hs;

int
main()
{
    ::std::cout << &Class2<int, 1, 2, 3>::Hs << ::std::endl;
}

working code online

Actually getting rid of N and using sizeof...(Ns) everywhere will be shorter...