How can std::reference_wrapper

[Solved] Thank you all, I didn't even know about user-defined conversion function and how it works.

Why it's possible to use std::reference_wrapper::operator+=, if such operator does not exist, are there some implicit conversions?

#include <iostream>
#include <functional>
#include <boost/type_index.hpp>

using boost::typeindex::type_id_with_cvr;

template <typename C>
void test(C c)
{
    c += 1;
}

int main()
{
    int a = 3;
    test(a);
    std::cout << a << std::endl;
    test(std::ref(a));
    std::cout << a << std::endl;dd
}

Output: 3 4

To check that template works perfectly fine:

void test_2(std::reference_wrapper<int> c)
{
    c += 1;
}
int main()
{

    int a = 3;
    test_2(std::ref(a));
    std::cout << a << std::endl;
}

Output: 4

Still works as before. How is that possible?

Funny thing, that in auto d = b + c. d has an integer type.

int main()
{
    auto b = std::ref(a);
    auto c = std::ref(a);
    auto d = b + c;
    std::cout << type_id_with_cvr<decltype(d)>).pretty_name() << std::endl;
}

Output: int