title: Swift generate error for default return value
body: <p>The following code sample pretty well illustrates what I'm after. I have a function that is expected to return a given type. It does that based on an <em>incomplete</em> switch. But in the event the <code>default</code> case is reached, I was hoping to just generate an error. Rather than return an <code>optional</code> which someone chooses to <code>!</code> and then die at that point.</p>
  
  <pre><code>func someFunc(value:UInt8) -&gt; SomeType {
      switch value {
      case 0x00:
          return SomeType.foo()
      case 0x13:
          return SomeType.bar()
      default:
          break // BUT I WANT AN ERROR HERE
      }
  }
  </code></pre>