title: a switch bug in swift? - "Switch must be exhaustive, consider adding a default clause."
body: <p>I am positive that the following <strong>swift</strong> code has covered all possibilities, but Xcode keeps telling me that, <strong>"Switch must be exhaustive, consider adding a default clause."</strong></p>
  
  <p>Can anyone tell me what did I miss? Thanks.</p>
  
  <pre><code>let a = false
  let b = false
  let c = false
  
  func test(a: Bool, _ b: Bool, _ c: Bool) {
      switch (a, b, c) {
      case (true, false, _):
          print("Moved left!!!")
      case (true, true, _):
          print("Moved right!!!")
      case (false, _, false):
          print("Moved up!!!")
      case (false, _, true):
          print("Moved down!!!")
      // Error: Switch must be exhaustive, consider adding a default clause.
      }
  }
  
  test(false, false, false)
  test(false, false, true)
  test(false, true, false)
  test(false, true, true)
  test(true, false, false)
  test(true, false, true)
  test(true, true, false)
  test(true, true, true)
  </code></pre>