Proper forwarding

fwd.cpp

// see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0644r1.html

// C++14
template <class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return std::forward<X>(x)(std::forward<Y>(y));
}


// C++14 (?)
#define FWD(x) std::forward<decltype(x)>(x)

template <class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return FWD(x)(FWD(y));
}

auto bar = [](auto&& x, auto&& y) {
    return FWD(x)(FWD(y));
};

// C++17
auto bar = [](auto&& x, auto&& y) {
    return std::forward<decltype(x)>(x)(std::forward<decltype(y)>(y));
};

// C++20
// https://www.bfilipek.com/2020/08/lambda-syntax.html
auto bar = []<class X, class Y>(X&& x, Y&& y) {
    return std::forward<X>(x)(std::forward<Y>(y));
};