Issue #3209453: The service "search.index" should be backend_overridable

core/modules/search/tests/src/Kernel/SearchIndexTest.php

+<?php
+
+namespace Drupal\Tests\search\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the service "search.index".
+ *
+ * @group search
+ */
+class SearchIndexTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['search'];
+
+  /**
+   * Test that the service "search.index" is backend overridable.
+   */
+  public function testSearchIndexServiceIsBackendOverridable() {
+    $definition = $this->container->getDefinition('search.index');
+    $this->assertTrue($definition->hasTag('backend_overridable'));
+  }
+
+}