C++17: non-const reference to temporary from method returning *this

Question

I'm sure this has been asked before, but my googling skills weren't sufficient to find my case. Simply put, is the following valid C++ code?

struct A {
    int a;

    static A foo() { return {5}; }
    A& bar() { return *this; }

    A(A&) = delete;
};

int main() {
    A& a = A::foo().bar();
    a.a = 7;
    return a.a;
}

Normally, I should only be allowed to store const references to temporaries, but I'm unsure if C++17 relaxed this. I tried to read the specification, but got confused.

clang, gcc and msvc seem to able to compile this, but gcc breaks with O2 if I remove aa = 7; : https://godbolt.org/z/n7vGT44Y5 .

Answer 1

bar() doesn't return temporary, it returns reference.

A::foo() returns a temporary, its lifetime ends at end of full expression. so a is a dangling reference.