title: Xcode 10 convert to Swift 4.2 fails
body: <p>I am getting this error trying to migrate to Swift 4.2 using the Xcode provided conversion tool. How do I fix it? The code builds fine otherwise.</p>
  
  <p><a href="https://i.stack.imgur.com/GHGGf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/GHGGf.png" alt="enter image description here"></a></p>