Strange generic function appear in view controller after converting to swift 3 : <p>In my project, after converting to swift 3, a new function appeared before my <code>ViewController</code> class: </p>

<pre><code>fileprivate func &lt; &lt;T : Comparable&gt;(lhs: T?, rhs: T?) -&gt; Bool {
   switch (lhs, rhs) {
  case let (l?, r?):
    return l &lt; r
  case (nil, _?):
    return true
  default:
    return false
  }
}
</code></pre>

<p>What does this function do? Why do I need it? </p>
