msvc_template_friend_bug.cpp

//godbolt: https://godbolt.org/z/6Ffp3J
#include <cstdint>
#include <utility>

using namespace std;

template <typename T, size_t N>
class Test
{
    private:
        template <typename T_, size_t N_>
        friend class Test;

        T data[N];
        
        constexpr Test() noexcept = default;

    public:
        template <typename... U>
        constexpr Test(U &&... vals) noexcept
            : data{ std::forward<U>(vals)... }
        {

        }

        template <size_t N_>
        constexpr auto operator+ (const Test<T, N_>& rhs) const noexcept
        {
            auto output = Test<T, N+N_>{};
            for (size_t i = 0; i < N; i++)
                output.data[i] = data[i];
            for (size_t i = 0; i < N_; i++)
                output.data[N+i] = rhs.data[i];
            return output;
        }
};

inline constexpr auto foo = Test<int, 3>{1,2,3};
inline constexpr auto bar = Test<int, 5>{4,5,6,7,8};
inline constexpr auto kek = foo + bar;

int main()
{
    return 0;
}