title: format String with array of Strings
body: <p>I am trying to create a <code>String</code> with an array of <code>String</code>s, I would expect this to work:</p>
  
  <pre><code>let format = "%@ and %@!"
  let str1 = "bla"
  let str2 = "blob"
  
  private func formatItNicely(format: String, substrings: [String]) -&gt; String {
      return String(format: format, arguments: substrings)
  }
  
  let result = formatItNicely(format, substrings: [str1, str2])
  </code></pre>
  
  <p>but I am getting <code>fatal error: can't unsafeBitCast between types of different sizes</code>.</p>
  
  <p>I have seen <a href="https://stackoverflow.com/questions/29724932/cannot-find-an-initializer-for-type-string-that-accepts-an-argument-list-of-ty">this</a> and <a href="https://stackoverflow.com/questions/30905530/swift-something-wrong-with-string-initformat-method">this</a> and <a href="https://stackoverflow.com/questions/30905530/swift-something-wrong-with-string-initformat-method">this</a> questions (and many more), but I still don't know how to accomplish what I am trying to do. </p>