title: incomplete type and cannot be defined
body: <p>I have been struggling with this error for more than two hours: </p>
  
  <pre><code>error: aggregate 'Ui::UpdaterProgress updaterProgress' has incomplete type and cannot be defined
  </code></pre>
  
  <p>I have a simple UI screen on Qt. Setting up with a dialog:</p>
  
  <pre><code>  #include &lt;ui_UpdaterProgress.h&gt;
  
    namespace Ui {
    class UpdaterProgress;
    }
    ....
    Ui::UpdaterProgress updaterProgress;
    QDialog updateProgressDialog;
    updaterProgress.setupUi(&amp;updateProgressDialog);
  </code></pre>
  
  <p>It's a simple UI with an icon, two labels, progress bar and a cancel button. Am I doing anything wrong here?</p>
  
  <p>I get this error if I don't do the forward declaration:</p>
  
  <pre><code>error: 'UpdaterProgress' is not a member of 'Ui'
  error: expected `;' before 'updaterProgress'
  error: 'updaterProgress' was not declared in this scope
  </code></pre>