Which standard wording tells us that ref-to-const temporary lifetime extension only “works once”?

Question

I was shown the following example in chat:

#include 
struct foo { ~foo() { std::cout << \"destroying!\\n\"; } };
const foo& func(co         


        

Answer 1

Probably I am a bit slow but to me it did not become clear what the resolution of this question is from reading the other answers. Thus I modified the code shown and wanted to summarize for others: the answer is, you get undefined behavior if you access y!

Run this code:

struct foo {
    int id;
    foo(int id) : id(id) { std::cout << "ctor " << id << std::endl; };
    ~foo() { std::cout << "dtor " << id << std::endl; }
};
const foo& func(const foo& a, const foo&) { return a; }

int main(int argc, char** argv) {
    foo x(1);
    const foo& y = func(foo(2), x);
    std::cout << "main " << y.id << std::endl;
    return 0;
}

The output for me is:

ctor 1
ctor 2
dtor 2
main 2
dtor 1

But the line main 2 is undefined behavior.