title: Swift enum does not have a member named 'rawValue' when declare a variable as optional value
body: <p>I'm looking the Swift Programming Language, and I faced an issue, I coudln't distinguish whether it's the Lauguage's issue or not(I'm using Xcode Version 6.1 (6A1052c)):</p>
  
  <pre><code>enum Rank: Int {
      case Ace = 1
      case Two, Three, Four, Five
      func simpleDescription() -&gt; String {
          switch self {
          case .Ace: return "Ace"
          default: return String(self.rawValue)
          }
      }
  }
  
  var rank: Rank?
  rank = Rank(rawValue: 2)
  println(rank.rawValue)
  </code></pre>
  
  <p>in the last line, it throws an error: <code>'Rank?' does not have a member named</code>rawValue``</p>
  
  <p>but if a declare the variable like <code>var rank: Rank</code>, and modify <code>rank = Rank(rawValue: 2)</code> to <code>rank = Rank(rawValue: 2)!</code>, it can pass and no error arised.</p>