title: Swift 3 NSCache Generic parameter 'KeyType' could not be inferred
body: <p>This code worked in Swift 2.x:</p>
  
  <pre><code>/// An internal in-memory cache
  private var dataCache = NSCache.init()
  </code></pre>
  
  <p>In <strong>Swift 3</strong> it causes compilation error:</p>
  
  <pre><code>Generic parameter 'KeyType' could not be inferred
  </code></pre>
  
  <p>Why is that so and how should I refactor this (Migration tool did not pick this up)?</p>