title: How do I use navigationController?.popViewController(animated: true) inside of a closure with no warnings and errors?
body: <p>I know that in swift 3 that this:</p>
  
  <pre><code>navigationController?.popViewController(animated: true)
  </code></pre>
  
  <p>will produce this warning:</p>
  
  <pre><code>"Expression of type "UIViewController?" is unused"
  </code></pre>
  
  <p>and I know the recommended way to rid myself of this warning is to do the following:</p>
  
  <pre><code>_ = navigationController?.popViewController(animated: true)
  </code></pre>
  
  <p>However when that is inside of a closure:</p>
  
  <pre><code>myFunction(myParams: ["blah", "bleh"], callback: { myResult in
  _ = navigationController?.popViewController(animated: true)
  })
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>Implicit use of 'self' in closure; use 'self.' to make capture semantics explicit
  </code></pre>
  
  <p>and if I do this:</p>
  
  <pre><code>myFunction(myParams: ["blah", "bleh"], callback: { myResult in
  self.navigationController?.popViewController(animated: true)
  })
  </code></pre>
  
  <p>I'm back to getting the warning:</p>
  
  <pre><code>"Expression of type "UIViewController?" is unused"
  </code></pre>
  
  <p>How do I go about using navigationController?.popViewController(animated: true) inside of a closure with no warnings and errors?</p>