title: Error "C2027 use of undefined type" when use std::deque instead of vector
body: <pre><code>#include &lt;string&gt;
  #include &lt;deque&gt;
  //#include &lt;vector&gt;
  #include &lt;map&gt;
  
  namespace xmltree {
      class XMLNode {
          // other fields, methods...
          std::deque&lt;XMLNode&gt; _childs;
  ...
  </code></pre>
  
  <p>If to define <code>_childs</code> as <code>std::vector&lt;XMLNode&gt;</code> the code will compile, but with <code>std::deque</code> I'm getting error:</p>
  
  <blockquote>
    <p>C2027 use of undefined type</p>
  </blockquote>
  
  <p>What is wrong with std::deque here and how to fix it?</p>