title: When should I use decltype(expression)?
body: <p>I understand why do we need <code>decltype(auto)</code> and it's difference from <code>auto</code> , but I can't get why and when  I should/need to use <code>decltype(expression)</code>. In all examples on <a href="https://en.cppreference.com/w/cpp/language/decltype" rel="nofollow noreferrer">cppreference</a> I can just use decltype(auto), I checked it.
  Thanks in advance for your help.</p>