class SystemMessagesBlock

class SystemMessagesBlock extends BlockBase implements MessagesBlockPluginInterface {
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'label_display' => FALSE,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return [
            '#type' => 'status_messages',
            '#include_fallback' => TRUE,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheMaxAge() {
        // The messages are session-specific and hence aren't cacheable, but the
        // block itself *is* cacheable because it uses a #lazy_builder callback and
        // hence the block has a globally cacheable render array.
        return Cache::PERMANENT;
    }

}