diff --git a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
index 2384497..4dafc3e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
@@ -29,25 +29,28 @@ public static function getInfo() {
    * Tests services.yml in site directory.
    */
   public function testServicesYml() {
-    // A service provider class always has precedence over services.yml files.
-    // The most common swappable core services are overridden by
-    // \Drupal\simpletest\TestServiceProvider already (which proxies into
-    // \Drupal\simpletest\DrupalUnitTestBase::buildContainer()). Due to those
-    // overrides, there are not many options of swappable services left, so this
-    // test does not test an override, but simply adds a dummy service instead.
     $this->assertFalse($this->container->has('site.service.yml'));
+    // A service provider class always has precedence over services.yml files.
+    // DrupalUnitTestBase::buildContainer() swaps out many services with
+    // in-memory implementations already, so those cannot be tested.
+    $this->assertIdentical(get_class($this->container->get('cache.backend.database')), 'Drupal\Core\Cache\DatabaseBackendFactory');
 
     $class = __CLASS__;
     $doc = <<<EOD
 services:
+  # Add a new service.
   site.service.yml:
     class: $class
+  # Swap out a core service.
+  cache.backend.database:
+    class: Drupal\Core\Cache\MemoryBackendFactory
 EOD;
     file_put_contents($this->siteDirectory . '/services.yml', $doc);
 
     $this->rebuildContainer();
 
     $this->assertTrue($this->container->has('site.service.yml'));
+    $this->assertIdentical(get_class($this->container->get('cache.backend.database')), 'Drupal\Core\Cache\MemoryBackendFactory');
   }
 
 }