main.cpp

#include <iostream>
#include <memory>

#include <cxxabi.h> // <-- gcc header

template <typename type> std::unique_ptr<const char*> type_name(const type *addr)
{ return std::make_unique<const char *>(abi::__cxa_demangle(typeid(*addr).name(), 0, 0, NULL)); }

template<typename T> struct MyTest {
    MyTest(T _data) : data(_data) { }

    template<typename U> MyTest<T> operator+(MyTest<U> oth) {
        return this->data + oth.data;
    }

    friend std::ostream &operator<<(std::ostream &os, const MyTest<T> &test) {
        os << "MyTest [data: " << test.data << " of type <" << *type_name(&test.data) << ">]";
        return os;
    }

    template<typename U> friend class MyTest;

private:
    T data;
};

int main(int argc, char *argv[]) {
    MyTest a(1);
    MyTest b(1.0);

    auto c = a + b;
    std::cerr << "c: " << c << std::endl;
}