title: Xcode 8 / Swift 3: "Expression of type UIViewController? is unused" warning
body: <p>I've got the following function which compiled cleanly previously but generates a warning with Xcode 8.</p>
  
  <pre><code>func exitViewController()
  {
      navigationController?.popViewController(animated: true)
  }
  </code></pre>
  
  <blockquote>
    <p>"Expression of type "UIViewController?" is unused".</p>
  </blockquote>
  
  <p>Why is it saying this and is there a way to remove it?</p>
  
  <p>The code executes as expected.</p>