diff --git a/core/tests/Drupal/Tests/Component/Annotation/AnnotatedClassDiscoveryTest.php b/core/tests/Drupal/Tests/Component/Annotation/AnnotatedClassDiscoveryTest.php
index 4392b3a..6f675cc 100644
--- a/core/tests/Drupal/Tests/Component/Annotation/AnnotatedClassDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Component/Annotation/AnnotatedClassDiscoveryTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\Component\Annotation;
 
+use Drupal\Component\Annotation\Plugin;
 use Drupal\Component\Annotation\Plugin\Discovery\AnnotatedClassDiscovery;
 use Drupal\Tests\UnitTestCase;
 
@@ -34,13 +35,55 @@ public function testGetPluginNamespaces() {
    * @covers ::getAnnotationReader
    */
   public function testGetDefinitions() {
-    $discovery = new AnnotatedClassDiscovery(['com/example' => [__DIR__ . '/Fixtures']]);
+    $discovery = new AnnotatedClassDiscovery(['com\example' => [__DIR__ . '/Fixtures']]);
     $this->assertEquals([
       'discovery_test_1' => [
         'id' => 'discovery_test_1',
-        'class' => 'com/example\com\example\DiscoveryTest1',
+        'class' => 'com\example\PluginNamespace\DiscoveryTest1',
       ],
     ], $discovery->getDefinitions());
+
+    $custom_annotation_discovery = new AnnotatedClassDiscovery(['com\example' => [__DIR__ . '/Fixtures']], CustomPlugin::class, ['Drupal\Tests\Component\Annotation']);
+    $this->assertEquals([
+      'discovery_test_1' => [
+        'id' => 'discovery_test_1',
+        'class' => 'com\example\PluginNamespace\DiscoveryTest1',
+        'title' => 'Discovery test plugin',
+      ],
+    ], $custom_annotation_discovery->getDefinitions());
+
+    $empty_discovery = new AnnotatedClassDiscovery(['com\example' => [__DIR__ . '/Fixtures']], CustomPlugin2::class, ['Drupal\Tests\Component\Annotation']);
+    $this->assertEquals([], $empty_discovery->getDefinitions());
   }
 
 }
+
+/**
+ * Custom plugin annotation.
+ *
+ * @Annotation
+ */
+class CustomPlugin extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * The plugin title.
+   *
+   * @ingroup plugin_translatable
+   */
+  public $title = '';
+
+}
+
+/**
+ * Custom plugin annotation.
+ *
+ * @Annotation
+ */
+class CustomPlugin2 extends Plugin {}
diff --git a/core/tests/Drupal/Tests/Component/Annotation/AnnotationBaseTest.php b/core/tests/Drupal/Tests/Component/Annotation/AnnotationBaseTest.php
index 5e02fdd..4c5eaa1 100644
--- a/core/tests/Drupal/Tests/Component/Annotation/AnnotationBaseTest.php
+++ b/core/tests/Drupal/Tests/Component/Annotation/AnnotationBaseTest.php
@@ -43,6 +43,7 @@ public function testSetClass() {
 }
 
 class AnnotationBaseStub extends AnnotationBase {
-  public function get() {
-  }
+
+  public function get() {}
+
 }
diff --git a/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/DiscoveryTest1.php b/core/tests/Drupal/Tests/Component/Annotation/Fixtures/PluginNamespace/DiscoveryTest1.php
similarity index 40%
rename from core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/DiscoveryTest1.php
rename to core/tests/Drupal/Tests/Component/Annotation/Fixtures/PluginNamespace/DiscoveryTest1.php
index 1c3bfc6..a2cbe0f 100644
--- a/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/DiscoveryTest1.php
+++ b/core/tests/Drupal/Tests/Component/Annotation/Fixtures/PluginNamespace/DiscoveryTest1.php
@@ -1,10 +1,14 @@
 <?php
 
-namespace com\example;
+namespace com\example\PluginNamespace;
 
 /**
  * @Plugin(
  *   id = "discovery_test_1"
  * )
+ * @CustomPlugin(
+ *   id = "discovery_test_1",
+ *   title = "Discovery test plugin"
+ * )
  */
 class DiscoveryTest1 {}
diff --git a/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/discoverytest2.yml b/core/tests/Drupal/Tests/Component/Annotation/Fixtures/PluginNamespace/discoverytest2.yml
similarity index 100%
rename from core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/discoverytest2.yml
rename to core/tests/Drupal/Tests/Component/Annotation/Fixtures/PluginNamespace/discoverytest2.yml
diff --git a/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/Annotation/CustomPlugin.php b/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/Annotation/CustomPlugin.php
deleted file mode 100644
index 9e3bac6..0000000
--- a/core/tests/Drupal/Tests/Component/Annotation/Fixtures/com/example/Annotation/CustomPlugin.php
+++ /dev/null
@@ -1,12 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \com\example\Annotation\CustomPlugin.
- */
-
-namespace com\example\Annotation;
-
-class CustomPlugin {
-
-}