title: `Ambiguous reference to member` when creating convenience init in extension
body: <p>This is my <code>init</code>:</p>
  
  <pre><code>extension NSNumberFormatter {
  
      convenience init(digits: Int = 0) {
          self.init() //ambiguous reference to member 'NSNumberFormatter.init'
  
          groupingSeparator = " "
          decimalSeparator = "."
          numberStyle = .DecimalStyle
          roundingMode = .RoundHalfDown
  
          maximumFractionDigits = digits
          minimumFractionDigits = digits
      }
  }
  </code></pre>
  
  <p>What is the reason?</p>
  
  <p>The same problem is when I put <code>self.init()</code> at the end of my convenience initializer.</p>