title: No * candidates produce the expected contextual result type 'FloatingPointRoundRule'
body: <p>I get the error from the title with this code on the <code>return</code> line using Swift 3:</p>
  
  <pre><code>extension Double {
      /// Rounds the double to decimal places value
      mutating func roundToPlaces(_ places:Int) -&gt; Double {
          let divisor = pow(10.0, Double(places))
          return round(self * divisor) / divisor
      }
  }
  </code></pre>
  
  <p>Does someone know what does this mean ? From what I can see <code>round</code> takes a <code>Double</code> and returns a <code>Double</code>. I can't see what the issue is.</p>