framework/di/Container.php
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 $ps;
    }
protected function mergeParams($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;
    }
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