title: Symfony 4 - Default value for parameters with a class type hint can only be NULL
body: <p>I have a similar problem than here : <a href="https://stackoverflow.com/questions/48348650/symfony-4-default-value-for-parameters-with-a-class-type-can-only-be]">Symfony 4 : Default value for parameters with a class type can only be</a> but when I launch composer commands... :</p>
  
  <pre><code>executing script security-checker security:check [KO]
   [KO]
  Script security-checker security:check returned with error code 255
  !!  PHP Fatal error:  Default value for parameters with a class type hint can only be NULL in /xxx/vendor/symfony/console/Application.php on line 83
  !!  
  Script @auto-scripts was called via post-update-cmd
  When I launched for example : php7 composer.phar update, because I had the same problem for others commands...
  </code></pre>
  
  <p>I don't understand because I use the 7.1.8 PHP version... I installed the whole project with my PHP 7.1.8... :/</p>
  
  <p>Code affected : </p>
  
  <pre><code>/**
       * @param string $name    The name of the application
       * @param string $version The version of the application
       */
      public function __construct(string $name = 'UNKNOWN', string $version = 'UNKNOWN') // Line affected HERE (line 83)
      {
          $this-&gt;name = $name;
          $this-&gt;version = $version; 
          $this-&gt;terminal = new Terminal();
          $this-&gt;defaultCommand = 'list';
      }
  </code></pre>
  
  <p>An idea for me ?</p>
  
  <p>Thank you very much !</p>