How to make copy initialization private in C++17?

Question

In the following simplification of a real-world example, the intention is to enforce that a user of class A can only obtain temporary references to its objects (in my actual example, the class A is a proxy class used to overload operators on the return values of other classes):

struct A {
    static A foo() {
        return A();
    }
private:
    A() {}
    A(const A&) = default;
};

int main() {
    A a = A::foo();
}

I therefore expect the assignment to a to fail, and this is indeed what happens in C++14 and earlier:

main.cpp:11:11: error: calling a private constructor of class 'A'
    A a = A::foo();
          ^
main.cpp:7:5: note: declared private here
    A(const A&) = default;
    ^

However, in C++17, this is not an error since, because there is no assignment happening, rather a is merely being copy initialized. In this case, how can I enforce the previous semantics? Is there a way to declare copy initialization of A to be private as well (preferably without modifying the return type of foo)?

Answer 1

You cannot. The entire point of guaranteed elision is to make it so that A a = A::foo(); doesn't do any copying. Ever. Even hypothetically. And the point of that is to be able to have factory functions even for types that cannot be copied/moved.

C++17 and above do not allow you the power to deny someone the ability to use a factory function for your type, so long as their code complies with the rules of guaranteed elision.