title: Cannot convert value of type Substring to expected argument type String - Swift 4
body: <p>Trying to get substring of String and append it to array of Strings:</p>
  
  <pre><code>var stringToSplit = "TEST TEXT"
  var s = [String]()
  let subStr = anotherString[0 ..&lt; 6]
  s.append(subStr) // &lt;---- HERE I GET THE ERROR
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/9Ijkp.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9Ijkp.png" alt="error desc"></a></p>