scratching_template.cpp

#include <iostream>
#include <string>

#define MAKE_FUNC_BODY(...) static __VA_ARGS__ { __assume(false); }
#define MAKE_TYPE_CONSTRAINT(Name, ...) struct Name { MAKE_FUNC_BODY(__VA_ARGS__) };

template<typename T>
int call(int index, T const& v)           { std::cout << "-["<<index<<"] " << v << std::endl; return index+1; }
int call(int index, std::string const& v) { std::cout << "+["<<index<<"] " << v << std::endl; return 100; }

template<auto CommandT>
struct Dispatch;

template<
    typename ReturnT,
    typename... ArgsT,
    ReturnT (*FunctionT)(ArgsT...)
>
struct Dispatch<FunctionT>
{
    template<typename... ArgsT, int... Indices>
    static auto dispatch(ArgsT const&... args, std::integer_sequence<int, Indices...>)
    {
        return (call(Indices, args) + ...);
    }

	static ReturnT with(ArgsT... args)
	{
        std::cout << "---" << std::endl;
        return dispatch<ArgsT...>(args..., std::make_integer_sequence<int, sizeof...(ArgsT)>());
	}
};

int main()
{
    MAKE_TYPE_CONSTRAINT(
        Test,
        int call(int a, double b, const std::string& c, const std::string& d)
    )

	const int res = Dispatch<Test::call>::with(1, 2.0, "hello", "world");
    std::cout<<"[RETURN] "<<res<<std::endl;
}