Why is the local variable copied despite being returned by name?

In the following code snippet, the return s gives the warning local variable 's' will be copied despite being returned by name [-Wreturn-std-move]. Why is this the case?

My goal with this lambda function is to take ownership of the input string and then return it after modification through RVO or move semantics. I would really like to avoid any copying.

const auto to_upper = [](std::string&& s) {
    std::transform(s.begin(), s.end(), s.begin(), 
        [](unsigned char c){ return std::toupper(c); }
    );
    return s;
};

Returning std::move(s) or std::forward<std::string>(s) will resolve the issue, but I thought this was not necessary since the compiler can elide the use of copy constructor. Also, I think I should be using std::forward but which one is correct and why?