title: Why is pair of const trivially copyable but pair is not?
body: <pre><code>std::cout &lt;&lt; std::boolalpha;
  std::cout &lt;&lt; std::is_trivially_copyable&lt; std::pair&lt;const int,int&gt; &gt;::value;
  std::cout &lt;&lt; std::is_trivially_copyable&lt; std::pair&lt;int,int&gt; &gt;::value;
  </code></pre>
  
  <p>When I use GCC 9.2 the output is <code>true</code> <code>false</code>.</p>
  
  <p>When I use Clang 5.0 or GCC 5.2 the output is <code>false</code> <code>false</code>.</p>
  
  <p>Why the difference?</p>