title: Swift 3 Compiler error: Command failed due to signal: Segmentation fault: 11
body: <p>I'm trying to extend a Firebase class but am getting a compiler error with the following:</p>
  
  <p>It appears to be related to the failable initialiser but I'm not sure. Is this a bug?</p>
  
  <pre><code>class FIRAuthMock: FIRAuth {
      var mockUser: FIRUser
  
      init?(user: FIRUser) {
          self.mockUser = user
      }   
  }
  </code></pre>
  
  <p>Xcode 8.0, Swift 3.0</p>