Lifetime extension on subobject of return value?

Question

Please consider the following C++20 program:

struct S {
  S(int x) : x(x) {}

  int x;
};

S f() { return S(42); }

int main() {
    auto&& r = f().x;
    return r;
}

Is this undefined behaviour and the reference dangles? or is the int object lifetime extended?