title: Double colon in function declaration
body: <p>While reading <a href="https://medium.com/@mlbors/getting-started-with-symfony-4-9169583a1710" rel="nofollow noreferrer">a post on Medium</a>, I stumbled upon a function signature I couldn't understand. </p>
  
  <pre><code>public function index(): Response
  {
      $posts = $this-&gt;getDoctrine()
                  -&gt;getRepository(Post::class)
                  -&gt;findAll();
      return $this-&gt;render('posts/list.html.twig', ['posts' =&gt; $posts]);
  }
  </code></pre>
  
  <p>What is the purpose of <code>: Response</code> in this case ? Is it specific to Symfony ? </p>