title: Migration from swift 3 to swift 4 - Cannot convert String to expected String.Element
body: <p>I am converting my code from swift 3 to swift 4 and getting this error in the following code. I am getting this even when I try to use flatmap to flatten an array </p>
  
  <blockquote>
    <p>Cannot convert value of type 'String' to expected argument type
    'String.Element' (aka 'Character')</p>
  </blockquote>
  
  <pre><code>if favoritedProducts.contains("helloWorld") {}
  </code></pre>
  
  <p>The below line of code does not return a [String] instead it is a '[String.Element]' How do I convert it to a [String]. If I try to cast it as a [String], it says it will always fail. </p>
  
  <pre><code>let productIDs = allItems.flatMap{$0.productID}
  </code></pre>