diff -u b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
--- b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -18,6 +18,9 @@
 
   use DiscoveryTrait;
 
+  /**
+   * Whether the parser should return only wants class annotations.
+   */
   const CLASSANNOTATIONOPTIMIZE = TRUE;
 
   /**
@@ -114,7 +117,6 @@
     AnnotationRegistry::reset();
     // Register the namespaces of classes that can be used for annotations.
     AnnotationRegistry::registerLoader('class_exists');
-    $generic_finder = $this->getFinder();
 
     // Search for classes within all PSR-0 namespace locations.
     foreach ($this->getPluginNamespaces() as $namespace => $dirs) {
@@ -140,7 +142,7 @@
               // The filename is already known, so there is no need to find the
               // file. However, StaticReflectionParser needs a finder, so use a
               // mock version.
-              $finder = $generic_finder ?: MockFileFinder::create($fileinfo->getPathName());
+              $finder = MockFileFinder::create($fileinfo->getPathName());
               $parser = new StaticReflectionParser($class, $finder, static::CLASSANNOTATIONOPTIMIZE);
 
               /** @var $annotation \Drupal\Component\Annotation\AnnotationInterface */
@@ -194,9 +196,2 @@
 
-  /**
-   * Returns a finder class for every prefix.
-   */
-  protected function getFinder() {
-    // This class uses a finder per class.
-  }
-
 }
diff -u b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
--- b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -278,7 +278,7 @@
       $this->processDefinition($definition, $plugin_id);
     }
     $this->alterDefinitions($definitions);
-    return ProviderFilterDecorator::filterDefinitions($definitions, [$this, 'providerExists']);
+    return ProviderFilterDecorator::filterDefinitions($definitions, function ($provider) { return $this->providerExists($provider); });
   }
 
   /**
@@ -299,7 +299,7 @@
    * @return bool
    *   TRUE if provider exists, FALSE otherwise.
    */
-  public function providerExists($provider) {
+  protected function providerExists($provider) {
     return $this->moduleHandler->moduleExists($provider);
   }
 
diff -u b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -87,7 +87,7 @@
     parent::prepareAnnotationDefinition($annotation, $class, $parser);
 
     if (!$annotation->getProvider()) {
-      $annotation->setProvider(static::getProviderFromNamespace($class));
+      $annotation->setProvider($this->getProviderFromNamespace($class));
     }
   }
 
@@ -97,10 +97,10 @@
    * @param string $namespace
    *   The namespace to extract the provider from.
    *
-   * @return null|string
+   * @return string|null
    *   The matching provider name, or NULL otherwise.
    */
-  static public function getProviderFromNamespace($namespace) {
+  protected function getProviderFromNamespace($namespace) {
     preg_match('|^Drupal\\\\(?<provider>[\w]+)\\\\|', $namespace, $matches);
 
     if (isset($matches['provider'])) {
diff -u b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscoveryAutomatedProviders.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscoveryAutomatedProviders.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscoveryAutomatedProviders.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscoveryAutomatedProviders.php
@@ -2,7 +2,7 @@
 
 namespace Drupal\Core\Plugin\Discovery;
 
-use Doctrine\Common\Reflection\StaticReflectionParser;
+use Doctrine\Common\Reflection\StaticReflectionParser as BaseStaticReflectionParser;
 use Drupal\Component\Annotation\AnnotationInterface;
 
 /**
@@ -12,27 +12,20 @@
 
+  /**
+   * {@inheritdoc}
+   */
   const CLASSANNOTATIONOPTIMIZE = FALSE;
 
   /**
    * {@inheritdoc}
    */
-  protected function prepareAnnotationDefinition(AnnotationInterface $annotation, $class, StaticReflectionParser $parser) {
+  protected function prepareAnnotationDefinition(AnnotationInterface $annotation, $class, BaseStaticReflectionParser $parser) {
     parent::prepareAnnotationDefinition($annotation, $class, $parser);
-    $m = (new \ReflectionClass(StaticReflectionParser::class))->getMethod('getParentStaticReflectionParser');
-    $m->setAccessible(TRUE);
+    $finder = \Drupal::service('class_loader');
     $providers = array_flip((array) $annotation->getProvider());
     do {
-      foreach ($parser->getUseStatements() as $class) {
-        if ($provider = AnnotatedClassDiscovery::getProviderFromNamespace($class)) {
-          $providers[strtolower($provider)] = TRUE;
-        }
-      }
-      $parser = $m->invoke($parser);
-    } while ($parser->getClassName());
+      $providers += array_flip(array_filter(array_map([$this, 'getProviderFromNamespace'], $parser->getUseStatements())));
+    } while ($parser = StaticReflectionParser::getParentParser($parser, $finder));
     $annotation->setProvider(array_keys($providers));
   }
 
-  protected function getFinder() {
-    return \Drupal::service("Class_loader");
-  }
-
 }
diff -u b/core/lib/Drupal/Core/Plugin/Discovery/ProviderFilterDecorator.php b/core/lib/Drupal/Core/Plugin/Discovery/ProviderFilterDecorator.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/ProviderFilterDecorator.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/ProviderFilterDecorator.php
@@ -84,3 +84,2 @@
 
-
 }
reverted:
--- b/core/modules/breakpoint/src/BreakpointManager.php
+++ a/core/modules/breakpoint/src/BreakpointManager.php
@@ -140,7 +140,7 @@
   /**
    * {@inheritdoc}
    */
+  protected function providerExists($provider) {
-  public function providerExists($provider) {
     return $this->moduleHandler->moduleExists($provider) || $this->themeHandler->themeExists($provider);
   }
 
diff -u b/core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php b/core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php
--- b/core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php
+++ b/core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php
@@ -5,8 +5,21 @@
 use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscoveryAutomatedProviders;
 use Drupal\Core\Plugin\Discovery\ContainerDerivativeDiscoveryDecorator;
 
+/**
+ * Plugin manager for migrate source plugins.
+ *
+ * @see \Drupal\migrate\Plugin\MigrateSourceInterface
+ * @see \Drupal\migrate\Plugin\source\SourcePluginBase
+ * @see \Drupal\migrate\Annotation\MigrateSource
+ * @see plugin_api
+ *
+ * @ingroup migration
+ */
 class MigrateSourcePluginManager extends MigratePluginManager {
 
+  /**
+   * {@inheritdoc}
+   */
   protected function getDiscovery() {
     if (!$this->discovery) {
       $discovery = new AnnotatedClassDiscoveryAutomatedProviders($this->subdir, $this->namespaces, $this->pluginDefinitionAnnotationName, $this->additionalAnnotationNamespaces);
diff -u b/core/modules/migrate/src/Plugin/NoSourcePluginDecorator.php b/core/modules/migrate/src/Plugin/NoSourcePluginDecorator.php
--- b/core/modules/migrate/src/Plugin/NoSourcePluginDecorator.php
+++ b/core/modules/migrate/src/Plugin/NoSourcePluginDecorator.php
@@ -5,6 +5,9 @@
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
 use Drupal\Component\Plugin\Discovery\DiscoveryTrait;
 
+/**
+ * Remove definitions which refer to a non-existing source plugin.
+ */
 class NoSourcePluginDecorator implements DiscoveryInterface {
 
   use DiscoveryTrait;
only in patch2:
unchanged:
--- a/core/lib/Drupal/Component/Annotation/AnnotationInterface.php
+++ b/core/lib/Drupal/Component/Annotation/AnnotationInterface.php
@@ -13,16 +13,16 @@
   public function get();
 
   /**
-   * Gets the name of the provider of the annotated class.
+   * Gets the name(s) of the provider(s) of the annotated class.
    *
-   * @return string
+   * @return string|array
    */
   public function getProvider();
 
   /**
-   * Sets the name of the provider of the annotated class.
+   * Sets the name(s) of the provider(s) of the annotated class.
    *
-   * @param string $provider
+   * @param string|array $provider
    */
   public function setProvider($provider);
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/lib/Drupal/Core/Plugin/Discovery/StaticReflectionParser.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Drupal\Core\Plugin\Discovery;
+
+use Doctrine\Common\Reflection\StaticReflectionParser as BaseStaticReflectionParser;
+
+/**
+ * Allows getting the reflection parser for the parent class.
+ */
+class StaticReflectionParser extends BaseStaticReflectionParser {
+
+  /**
+   * @param \Doctrine\Common\Reflection\StaticReflectionParser $parser
+   *   The current static parser.
+   *
+   * @return static|null
+   *   The static parser for the parent if there's a parent class or NULL.
+   */
+  public static function getParentParser(BaseStaticReflectionParser $parser, $finder) {
+    if ($parser->parentClassName) {
+      return new static($parser->parentClassName, $finder, $parser->classAnnotationOptimize);
+    }
+  }
+
+}