Problem extending BookNavigationBlock i have a Problem extending a BookNavigationBlock Plugin. When I extend BlockBase it works, but when trying to extend BookNavigationBlock, it fails when placing the block, without any error messages. What am I missing here? (The Code is inside modules/ifg_book_navigation/src/Plugin/Block/IfgBookNavigationBlock.php)

namespace Drupal\ifg_book_navigation\Plugin\Block;

use Drupal\book\Plugin\Block;
/**
 * Provides a fully expanded 'Book navigation' block.
 *
 * @Block(
 *   id = "ifg_book_navigation",
 *   admin_label = @Translation("Expanded Book navigation"),
 *   category = @Translation("Menus")
 * )
 */
class IfgBookNavigationBlock extends BookNavigationBlock {

  /**
   * {@inheritdoc}
   */
  public function build() {
    return array(
      '#markup' =&gt; $this-&gt;t('Hello, World!'),
    );
  }
}


Thanx!

UPDATE:
I could trace the Problem to the Plugin Factory.
In the getPluginClass method of DefaultFactory it is checked if the class exists:

if (!class_exists($class)) {
  throw new PluginException(sprintf('Plugin (%s) instance class "%s" does not exist.', $plugin_id, $class));
}


The value of $class is Drupal\ifg_book_navigation\Plugin\Block\IfgBookNavigationBlock which seems to be correct. However class_exists() itself seems to fail, not even returning false. This could have something to do with autoloading, as by default class_exists() tries this by default ...

UPDATE: It seems like the behaviour is the same for other existing blocks, just tested UserLoginBlock
The actual class has to be included in the use statement.

use Drupal\book\Plugin\Block\BookNavigationBlock;