Issue #3016038 by dpi, acbramley, jibran, dww, phenaproxima, jungle, Mingsong:...

      case 'delete':

        return AccessResult::allowedIfHasPermission($account, 'administer menu')

          ->andIf(AccessResult::allowedIf(!$entity->isNew())->addCacheableDependency($entity));

      default:

        return parent::checkAccess($entity, $operation, $account);

    }

  }

<?php

namespace Drupal\Tests\menu_link_content\Unit;

use Drupal\Core\Access\AccessManagerInterface;

use Drupal\Core\Access\AccessResultInterface;

use Drupal\Core\Entity\ContentEntityInterface;

use Drupal\Core\Entity\EntityTypeInterface;

use Drupal\Core\Extension\ModuleHandlerInterface;

use Drupal\Core\Language\LanguageInterface;

use Drupal\Core\Session\AccountInterface;

use Drupal\menu_link_content\MenuLinkContentAccessControlHandler;

use Drupal\Tests\UnitTestCase;

/**

 * Tests menu link content entity access.

 *

 * @coversDefaultClass \Drupal\menu_link_content\MenuLinkContentAccessControlHandler

 * @group menu_link_content

 */

class MenuLinkContentEntityAccessTest extends UnitTestCase {

  /**

   * Tests an operation not implemented by the access control handler.

   *

   * @covers ::checkAccess

   */

  public function testUnrecognizedOperation() {

    $entityType = $this->createMock(EntityTypeInterface::class);

    $accessManager = $this->createMock(AccessManagerInterface::class);

    $moduleHandler = $this->createMock(ModuleHandlerInterface::class);

    $moduleHandler->expects($this->any())

      ->method('invokeAll')

      ->willReturn([]);

    $language = $this->createMock(LanguageInterface::class);

    $language->expects($this->any())

      ->method('getId')

      ->will($this->returnValue('de'));

    $entity = $this->createMock(ContentEntityInterface::class);

    $entity->expects($this->any())

      ->method('language')

      ->willReturn($language);

    $account = $this->createMock(AccountInterface::class);

    $accessControl = new MenuLinkContentAccessControlHandler($entityType, $accessManager);

    $accessControl->setModuleHandler($moduleHandler);

    $access = $accessControl->access($entity, 'not-an-op', $account, TRUE);

    $this->assertInstanceOf(AccessResultInterface::class, $access);

  }

}