title: std::optional specialization for reference types
body: <p>Why <code>std::optional</code> (<code>std::experimental::optional</code> in <strong>libc++</strong> at the moment) does not have specialization for reference types (compared with <code>boost::optional</code>)?</p>
  
  <p>I think it would be very useful option.</p>
  
  <p>Is there some object with <em>reference to <strong>maybe</strong> already existing object</em> semantics in <strong>STL</strong>?</p>