#include <cassert>
#include <iostream>
#include <type_traits>

struct Box { int value; };
decltype(auto) copy_value(Box& b) { return b.value; }
decltype(auto) borrow_value(Box& b) { return (b.value); }

int main() {
    const int source = 3;
    auto copy = source;
    auto& ref = source;
    static_assert(std::is_same<decltype(copy), int>::value, "value");
    static_assert(std::is_same<decltype(ref), const int&>::value, "borrow");
    Box b{4};
    static_assert(std::is_same<decltype(copy_value(b)), int>::value, "copy return");
    static_assert(std::is_same<decltype(borrow_value(b)), int&>::value, "reference return");
    auto detached = copy_value(b);
    borrow_value(b) = 9;
    assert(detached == 4 && b.value == 9);
    std::cout << detached << ' ' << b.value << '\n';
}