title: Argument of `#selector` does not refer to an initializer or method
body: <p>I'm trying to perform a protocol extension method in the background:</p>
  
  <pre><code>performSelectorInBackground(#selector(retrieveCategories()), withObject: nil)
  </code></pre>
  
  <p>However I get the below error message:</p>
  
  <pre><code>Argument of `#selector` does not refer to an initializer or method
  </code></pre>
  
  <p>Here is my protocol declaration:</p>
  
  <pre><code>@objc protocol DataRetrievalOperations {
      optional func retrieveCategories()
      ...
  }
  </code></pre>
  
  <p>And my extension:</p>
  
  <pre><code>extension DataRetrievalOperations {
      func retrieveCategories() {
          ...
      }
  }
  </code></pre>
  
  <p>How can I achieve this?</p>