c++ concat with separator

#include <string>
#include <sstream>

template <typename S>
void concat(std::ostringstream& ss, S&& separator)
{
}

template <typename S, typename H, typename... T>
void concat(std::ostringstream& ss, S&& separator, H&& h, T&&... t)
{
    ss << separator << std::forward<H>(h);
    concat(ss, std::forward<S>(separator), std::forward<T>(t)...);
}

template <typename S>
std::string concat(S&& separator)
{
    return std::string();
}

template <typename S, typename H, typename... T>
std::string concat(S&& separator, H&& h, T&&... t)
{
    std::ostringstream oss;
    oss << std::forward<H>(h);
    concat(oss, std::forward<S>(separator), std::forward<T>(t)...);
    return oss.str();
}

#include <iostream> // for std::cout

int main()
{
    {
        unsigned long long u = -1;
        long long i = -1;
        char c = 100;
        std::string s = "asd";
        std::string const k = "a d";
        std::cout << concat(' ', 123, k, s, i, u, c) << std::endl;
        std::cout << concat(", ", 123, k, std::string("t"), s, i, u, c) << std::endl;
        std::cout << concat(",") << std::endl;
        std::cout << concat("", 123, k, s, i, u, c) << std::endl;
        std::cout << concat(k, 123, k, s, i, u, c) << std::endl;
        return 0;
    }
}