code: protected function mergeParams($class, $params)
      {
          if (empty($this->_params[$class])) {
              return $params;
          } elseif (empty($params)) {
              return $this->_params[$class];
          }
  
          $ps = $this->_params[$class];
          foreach ($params as $index => $value) {
              $ps[$index] = $value;
          }
  
          return $ps;
      }
docs: Merges the user-specified constructor parameters with the ones registered via [[set()]].
  @param string $class class name, interface name or alias name
  @param array $params the constructor parameters
  @return array the merged parameters