title: Swift trailing closure can't compile
body: <p>I tried to turn a sequence of key-value pairs (tuples) into a dictionary:</p>
  
  <pre><code>let pairs = [("a", 1), ("b", 2), ("a", 3), ("b", 4)]
  
  let firstValues = Dictionary( pairs,
      uniquingKeysWith: { (first, _) in first }
  )
  // ["b": 2, "a": 1]
  </code></pre>
  
  <p>but when I try to use the trailing closure syntax, the code just can't compile, why?</p>
  
  <pre><code>// ⛔️ error: contextual closure type '(_, _) -&gt; _' 
  //    expects 2 arguments, but 1 was used in closure body
  let firstValues2 = Dictionary( pairs ) { $0 }
  </code></pre>