title: Closure tuple does not support destructuring in Xcode 9 Swift 4
body: <p>After the gloss project for Swift 4 in Xcode 9 </p>
  
  <p>I am getting following error which i have no idea</p>
  
  <blockquote>
    <p>Closure tuple parameter '(key: _, value: _)' does not support
    destructuring</p>
  </blockquote>
  
  <p><strong>Code:</strong></p>
  
  <pre><code>extension Dictionary
  {
      init(elements: [Element]) {
          self.init()
          for (key, value) in elements {
              self[key] = value
          }
      }
  
      func flatMap&lt;KeyPrime, ValuePrime&gt;(_ transform: (Key, Value) throws -&gt; (KeyPrime, ValuePrime)?) rethrows -&gt; [KeyPrime:ValuePrime] {
          return Dictionary&lt;KeyPrime, ValuePrime&gt;(elements: try flatMap({ (key, value) in
              return try transform(key, value)
          }))
      }
  }
  </code></pre>
  
  <p>Error comes at this point <code>try flatMap({ (key, value)in</code></p>