title: Immutable value of type `Int` only has mutating members named adjust
body: <p>I'm learning Swift language now a days.</p>
  
  <p>In Apple's documentation I saw a example for extension like:</p>
  
  <pre><code>extension Int: ExampleProtocol {
      var simpleDescription: String {
          return "The number \(self)"
      }
      mutating func adjust() {
          self += 42
      }
  }
  7.simpleDescription
  </code></pre>
  
  <p>So I just called the <code>adjust()</code> like:</p>
  
  <pre><code>7.adjust()
  </code></pre>
  
  <p>It throws an error:</p>
  
  <pre><code>Immutable value of type `Int` only has mutating members named adjust.
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/lmvZQ.png" alt="Swift Error"></p>
  
  <p>I'm not sure what causes the error ? Can anybody help me to understand the issue ?</p>