title: Mapping an array of tuples results in a strange error about types
body: <p>My method for finding the maximum y value in an array of tuples is resulting in a error that I can't understand.</p>
  
  <pre><code>let a: [(x: Float, y: Float)]? = [(x:5,y:9),(x:1,y:4),(x:4,y:3),(x:2,y:5),(x:6,y:8)]
  let maxY = a.map { $0.y }.max()
  </code></pre>
  
  <p>It throws an error like:</p>
  
  <p><a href="https://i.stack.imgur.com/uRkni.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uRkni.png" alt="error"></a></p>