diff -u b/core/lib/Drupal/Component/Annotation/AnnotationInterface.php b/core/lib/Drupal/Component/Annotation/AnnotationInterface.php
--- b/core/lib/Drupal/Component/Annotation/AnnotationInterface.php
+++ b/core/lib/Drupal/Component/Annotation/AnnotationInterface.php
@@ -16,6 +16,7 @@
    * Gets the name(s) of the provider(s) of the annotated class.
    *
    * @return string|array
+   *   Can be str4ing or array because of backwards compatibility reasons.
    */
   public function getProvider();
 
@@ -23,6 +24,7 @@
    * Sets the name(s) of the provider(s) of the annotated class.
    *
    * @param string|array $provider
+   *   Can be str4ing or array because of backwards compatibility reasons.
    */
   public function setProvider($provider);
 
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
@@ -21,7 +21,7 @@
   /**
    * Whether the parser should return only wants class annotations.
    */
-  const CLASSANNOTATIONOPTIMIZE = TRUE;
+  const CLASS_ANNOTATION_OPTIMIZE = TRUE;
 
   /**
    * The namespaces within which to find plugin classes.
@@ -143,7 +143,7 @@
               // file. However, StaticReflectionParser needs a finder, so use a
               // mock version.
               $finder = MockFileFinder::create($fileinfo->getPathName());
-              $parser = new StaticReflectionParser($class, $finder, static::CLASSANNOTATIONOPTIMIZE);
+              $parser = new StaticReflectionParser($class, $finder, static::CLASS_ANNOTATION_OPTIMIZE);
 
               /** @var $annotation \Drupal\Component\Annotation\AnnotationInterface */
               if ($annotation = $reader->getClassAnnotation($parser->getReflectionClass(), $this->pluginDefinitionAnnotationName)) {
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
@@ -13,7 +13,7 @@
   /**
    * {@inheritdoc}
    */
-  const CLASSANNOTATIONOPTIMIZE = FALSE;
+  const CLASS_ANNOTATION_OPTIMIZE = FALSE;
 
   /**
    * {@inheritdoc}
@@ -21,11 +21,12 @@
   protected function prepareAnnotationDefinition(AnnotationInterface $annotation, $class, BaseStaticReflectionParser $parser) {
     parent::prepareAnnotationDefinition($annotation, $class, $parser);
     $finder = \Drupal::service('class_loader');
-    $providers = array_flip((array) $annotation->getProvider());
+    $providers = (array) $annotation->getProvider();
     do {
-      $providers += array_flip(array_filter(array_map([$this, 'getProviderFromNamespace'], $parser->getUseStatements())));
+      $new_providers = array_map([$this, 'getProviderFromNamespace'], $parser->getUseStatements());
+      $providers = array_merge($providers, $new_providers);
     } while ($parser = StaticReflectionParser::getParentParser($parser, $finder));
-    $annotation->setProvider(array_keys($providers));
+    $annotation->setProvider(array_unique(array_filter($providers)));
   }
 
 }
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
@@ -30,8 +30,9 @@
    *
    * @param \Drupal\Component\Plugin\Discovery\DiscoveryInterface $decorated
    *   The object implementing DiscoveryInterface that is being decorated.
-   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
-   *   The module handler.
+   * @param callable $provider_exists
+   *   A callable, gets passed a provider name, should return TRUE if the
+   *   provider exists and FALSE if not.
    */
   public function __construct(DiscoveryInterface $decorated, callable $provider_exists) {
     $this->decorated = $decorated;
@@ -39,7 +40,7 @@
   }
 
   /**
-   * Remove plugin definitions with non-existing providers.
+   * Removes plugin definitions with non-existing providers.
    *
    * @param mixed[] $definitions
    *   An array of plugin definitions (empty array if no definitions were
diff -u b/core/lib/Drupal/Core/Plugin/Discovery/StaticReflectionParser.php b/core/lib/Drupal/Core/Plugin/Discovery/StaticReflectionParser.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/StaticReflectionParser.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/StaticReflectionParser.php
@@ -10,6 +10,8 @@
 class StaticReflectionParser extends BaseStaticReflectionParser {
 
   /**
+   * If the current class extends another, get the parser for the latter.
+   *
    * @param \Doctrine\Common\Reflection\StaticReflectionParser $parser
    *   The current static parser.
    *