title: Why are std::future and std::promise not final?
body: <p>I wounder why the classes <a href="https://en.cppreference.com/w/cpp/thread/future" rel="noreferrer"><code>std::future</code></a> and <a href="https://en.cppreference.com/w/cpp/thread/promise" rel="noreferrer"><code>std::promise</code></a> are not marked with the <code>final</code> specifier. The <a href="https://en.cppreference.com/w/cpp/thread/future/~future" rel="noreferrer">destructor</a> is <strong>not</strong> virtual so why was <code>final</code> not added? What was (is) the rationale? </p>