title: What is the meaning of "... ..." token? i.e. double ellipsis operator on parameter pack
body: <p>While browsing through gcc's current implementation of new C++11 headers, I stumbled upon &quot;......&quot; token. You can check, that the following code <a href="https://godbolt.org/z/4GfbYYb75" rel="nofollow noreferrer">compiles fine</a> [via godbolt.org].</p>
  <pre><code>template &lt;typename T&gt;
  struct X
  { /* ... */ };
  
  template &lt;typename T, typename ... U&gt;
  struct X&lt;T(U......)&gt; // this line is the important one
  { /* ... */ };
  </code></pre>
  <p>So, what is the meaning of this token?</p>
  <p>edit: Looks like SO trimmed &quot;......&quot; in question title to &quot;...&quot;, I did really mean &quot;......&quot; . :)</p>