title: Swift 3.0-Switch statement without default statement getting error 'Switch must be exhaustive, consider adding default clause'
body: <p>Following is the code used for executing Switch statement in playground. I executed few switch statements without using default. My doubt is why it is optional for some and mandatory for other statements.Thanks in advance!.</p>
  
  <pre><code>let someNumber = 3.5
  
  switch someNumber {
  
  case 2 , 3 , 5 , 7 , 11 , 13 :
    print("Prime numbers")
  case 4 , 6 , 24 , 12 , 66 :
    print("Normal numbers")
  
   }
  </code></pre>
  
  <p>Counter statement executed successfully without using default</p>
  
  <pre><code>  let yetAnotherPoint = (3,1)
  
    switch yetAnotherPoint {
  
    case let (x,y) where x == y :
     print("(\(x),\(y)) is on the line x == y")
    case let (x,y) where x == -y :
     print("(\(x),\(y)) is on the line x == -y")
    case let (x,y):
     print("(\(x),\(y)) is just some arbitrary point")
  
     }
  </code></pre>