title: Xcode 8 Beta 4 Swift 3 - "round" behaviour changed
body: <p>I have the following simple extension to <code>Double</code>, which worked fine in everything up to Xcode 8 beta 3</p>
  
  <pre><code>public extension Double {
      public func roundTo(_ decimalPlaces: Int) -&gt; Double {
          var v = self
          var divisor = 1.0
          if decimalPlaces &gt; 0 {
              for _ in 1 ... decimalPlaces {
                  v *= 10.0
                  divisor *= 0.1
              }
          }
          return round(v) * divisor
      }
  }
  </code></pre>
  
  <p>As of Beta 4, I am getting "Cannot use mutating member on immutable value: 'self' is immutable" on the <code>round</code> function in the return - has anyone got any clues?</p>