title: Multiplication of Integer by double and storing result as a double
body: <p>Out of plain curiosity, in swift:</p>
  
  <pre><code>var a:Double = 3.5
  var b:Int = 4
  var c = a * Double(b)
  var d:Double = a * b
  </code></pre>
  
  <p>Why is <code>d</code> not valid but <code>c</code> is? Since I'm specifying that <code>d</code> is a double, shouldn't the compiler detect that <code>b</code> should be casted to a double?</p>