title: Swift 3 warning: Non-optional expression of type 'String' used in a check for optionals
body: <p>I'm updating a project to Swift 3 and came across the following warning which I can't seem to resolve.</p>
  
  <pre><code>fileprivate var filteredTitlesList: [String] = []
  
  if let filteredTitle: String = filteredTitlesList[indexPath.row] as String { // 'Non-optional expression of type 'String' used in a check for optionals'
  
    // Do something
  
  }
  </code></pre>
  
  <p>The answer to a similar question here didn't help me: <a href="https://stackoverflow.com/questions/39799471/non-optional-expression-of-type-anyobject-used-in-a-check-for-optionals">Non-optional expression of type &#39;AnyObject&#39; used in a check for optionals</a></p>
  
  <p>Thanks a lot!</p>