title: arithmetic within string literal in Swift
body: <p>Playing with Swift, I found something awkward error.</p>
  
  <pre><code>let cost = 82.5
  let tip = 18.0
  let str = "Your total cost will be \(cost + tip)"
  </code></pre>
  
  <p>This works fine as I expect, but</p>
  
  <pre><code>let cost = 82.5
  let tip:Float = 18
  let str = "Your total cost will be \(cost + tip)"
  </code></pre>
  
  <p>would not work with error </p>
  
  <pre><code>could not find member 'convertFromStringInterpolationSegment' 
  let str = "Your total cost will be \(cost + tip)"
  </code></pre>
  
  <p>The difference between two example is declaring tip constant to explicitly float or not. Would this be reasonable result? </p>