Слияние строк времени компиляции

constexpr const char *str1 = "QWERTY";
constexpr const char *str2 = "q1w2";

constexpr const char *result = str1 + str2; // Итог: QWERTYq1w2

template<int N>
class ConstString
{
    //...
}

constexpr const char* concat(const char *str1, const char *str2)
{
    // ...
}

constexpr const char* result = concat("ABCD", "q2Sef333"); // #1
 
constexpr const char *str = "QWERTY";
constexpr const char* result = concat(str, "hello"); // #2
 
constexpr const char str[] = "QWERTY";
constexpr const char* result = concat(str, "hello"); // #3

#include <iostream>
 
template <typename CharT, CharT ...Chars>
struct ct_string
{
    static constexpr char const value[] = { Chars..., 0 };
};
 
template <typename CharT, CharT ...Chars>
constexpr char const ct_string<CharT, Chars...>::value[];
 
template <typename CharType, CharType... Chars>  // <<========= :-)
constexpr auto operator"" _S() 
{
    return ct_string<CharType, Chars...>();
}
 
template<typename CharType, CharType... LeftChars, CharType... RightChars>
constexpr auto operator+(const ct_string<CharType, LeftChars...> &, const ct_string<CharType, RightChars...> &)
{
    return ct_string<CharType, LeftChars..., RightChars...>();
}
 
int main()
{
    constexpr auto result1 = "ABCD"_S + "q2Sef333"_S; // #1
    
    std::cout << result1.value << '\n';
 
    constexpr auto str = "QWERTY"_S;
    constexpr auto result2 = str + "hello"_S; // #2
 
    std::cout << result2.value << '\n';
}

#define str1 "1234"
#define str2 "5678"
#define str3 str1 str2
int main()
{
    cout<<str3<<endl;
    return 0;
}