Variadic template function with function in argument

See more:

C++14

How Can I modify template function "sum" to have a compilable code without modifying the main function?

I've already tried almost everything.

What I have tried:

#include <utility>
#include <iostream>

template<typename T, typename... Args>
auto sum(T (*F) (T, const T &), Args&&... args)
// -> decltype(F(std::forward<Args>(args)...)) // for C++ 11
{
	return F(std::forward<Args>(args)...);
}

int main()
{
	auto func = [](int a, int const& b) { return a += b; };

	int i = 4;
	sum(func, i, 2);
	std::cout << i << std::endl;
	return 0;
}