title: Encoding clone $this in JsonSerializable
body: <p>This simplified case is resulting in a PHP segfault (exit 127):</p>
  
  <pre><code>class Datum implements \JsonSerializable{
    public function jsonSerialize(){
      return clone $this;
    }
  }
  echo json_encode(new Datum);
  </code></pre>
  
  <p>The last line of code results in exit(127). I'm unable to retrieve any stack in my current environment.</p>
  
  <p>Meanwhile, removing the <code>clone</code> token works.</p>
  
  <p>Is there any possible explanation why this is happening?</p>