title: In PHP, what is the `ClosedGeneratorException`?
body: <p>When looking through the output of <a href="http://php.net/manual/en/function.get-declared-classes.php" rel="nofollow noreferrer"><code>get_declared_classes()</code></a> in PHP 7.0 and 7.1, I noticed a <strong><code>ClosedGeneratorException</code></strong>.</p>
  
  <p>The manual does not make <a href="http://php.net/results.php?q=ClosedGeneratorException&amp;l=en&amp;p=all" rel="nofollow noreferrer">much mention of it</a> and it does not seem to be listed on either the <a href="http://php.net/manual/en/reserved.exceptions.php" rel="nofollow noreferrer">Predefined Exceptions</a> or <a href="http://php.net/manual/en/spl.exceptions.php" rel="nofollow noreferrer">SPL exceptions</a> entries in the manual.</p>
  
  <p>Even the source-code does <a href="https://github.com/php/php-src/search?q=ClosedGeneratorException" rel="nofollow noreferrer">not hold much information about it</a>.</p>
  
  <p><strong>So, what is the <code>ClosedGeneratorException</code>?</strong> </p>
  
  <p><sup>(What is it for? When does it occur? When should it be used?)</sup></p>