code: protected static function hasLoader(
          /*# bool */ $search = false
      ) {
          $class = get_called_class();
  
          if (isset(self::$loaders[$class])) {
              return $class;
          } elseif (__CLASS__ === $class) {
              return false;
          } elseif ($search) {
              $parent = get_parent_class($class);
              return $parent::hasLoader(true);
          }
  
          return false;
      }
docs: Check loader for calling message class (static bind)
  
  if $search is true, search upwards in inhertiant tree for loader
  if current class has no loader set
  
  @param  bool $search search upwards
  @return false|string false or classname for which has loader
  @access protected