title: GCC NRVO/RVO warning
body: <p>Is there any <em>warning</em>, which allows us to know whether <strong>NRVO/RVO</strong> performed or not, in <strong>GCC</strong>?</p>
  
  <p>I found that <code>-fno-elide-constructors</code> turns off <strong>NRVO/RVO</strong>, but <strong>NRVO/RVO</strong> has its own conditions to occur and sometimes does not occur. There is a need to know if <strong>NRVO/RVO</strong> occurs to understand, when extra copy-construction happens.</p>
  
  <p>I am especially interested in compile-time features. It would be nice if there were some specific <code>#pragma GCC...</code> (which activates the diagnostic immediately following itself) or something using static assertion mechanism.</p>