title: The supportedInterfaceOrientations method doesn't override any method from its superclass
body: <p>In a UIViewController, this code:</p>
  
  <pre><code>public override func supportedInterfaceOrientations() -&gt; UIInterfaceOrientationMask {
      if let mainController = self.mainViewController{
          return mainController.supportedInterfaceOrientations
      }
      return UIInterfaceOrientationMask.all
  }
  </code></pre>
  
  <p>gives the error <code>Method doesn't override any method from its superclass</code></p>
  
  <p>I am using Xcode 8 beta 4 and the iOS deployment target is 9.0, and <code>Use Legacy Swift Language Version</code> is set to No in the <code>Build Settings</code></p>
  
  <p>How could I convert the code above to Swift 3?</p>