Relying on RVO for finally function

Reading The C++ Programming Language (4th edition), in the Exception Handling chapter there's an example helper for ad hoc cleanup code:

template<typename F>
struct Final_action {
    Final_action(F f): clean{f} {}
    ~Final_action() { clean(); }
    F clean;
};

template<class F>
Final_action<F> finally(F f)
{
    return Final_action<F>(f);
}

It's used like

auto act1 = finally([&]{ delete p; });

to run the lambda code at the end of the block in which act1 is declared. I suppose this worked for Stroustrup when he tested it, due to Return Value Optimization limiting Final_action<> to a single instance - but isn't RVO just an optional optimization? If the instance is copied on return from finally, obviously ~Final_action() runs for both copies, IOW p is deleted twice. Is such behavior prevented by something in the standard, or is the code just simple enough for most compilers to optimize it?