title: Neither weak nor strong or unowned will do during self dismissal?
body: <p>Is this a valid pattern to avoid being retained after the dismissal?</p>
  
  <pre><code>@objc func backButtonTapped() {
      var s: SiteViewController! = self
      navigationController!.popToRootViewController(animated: true, completion: {
          s.dismissCompletion()
          s = nil // break the strong reference to self
      })
  }
  </code></pre>