diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index f843f27754..dea0b8c98e 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -462,8 +462,7 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  */
 function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE) {
   @trigger_error('drupal_set_message() is deprecated Drupal 8.5.0 and will be removed before Drupal 9.0.0. Use \Drupal\Core\Messenger\MessengerInterface::addMessage() instead. See https://www.drupal.org/node/2774931', E_USER_DEPRECATED);
-  /* @var \Drupal\Core\Messenger\MessengerInterface $messenger */
-  $messenger = \Drupal::service('messenger');
+  $messenger = \Drupal::messenger();
   $messenger->addMessage($message, $type, $repeat);
   return $messenger->all();
 }
diff --git a/core/lib/Drupal/Core/Messenger/BaseMessenger.php b/core/lib/Drupal/Core/Messenger/BaseMessenger.php
index f76c295301..c71b587154 100644
--- a/core/lib/Drupal/Core/Messenger/BaseMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/BaseMessenger.php
@@ -12,6 +12,27 @@
  */
 abstract class BaseMessenger implements MessengerInterface {
 
+  /**
+   * {@inheritdoc}
+   */
+  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
+    $message = $this->convertToMarkup($message);
+    if ($this->shouldAddMessage($message, $type, $repeat)) {
+      $this->doAddMessage($message, $type);
+    }
+    return $this;
+  }
+
+  /**
+   * Add a message in an implementation-specific way.
+   *
+   * @param string|\Drupal\Component\Render\MarkupInterface $message
+   *   The message.
+   * @param string $type
+   *   The message type.
+   */
+  abstract protected function doAddMessage($message, $type);
+
   /**
    * {@inheritdoc}
    */
@@ -49,4 +70,23 @@ protected function convertToMarkup($message) {
     return $message;
   }
 
+  /**
+   * Returns whether a message should be added or not.
+   *
+   * @param string|\Drupal\Component\Render\MarkupInterface $message
+   *   The message.
+   * @param string $type
+   *   The type.
+   * @param bool $repeat
+   *   Whether to repeat or not.
+   *
+   * @return bool
+   *   TRUE if the message should be added, FALSE otherwise.
+   */
+  protected function shouldAddMessage($message, $type, $repeat) {
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    return $repeat || !in_array($message, $this->messagesByType($type));
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Messenger/ChainedMessenger.php b/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
index 21b39b691b..1356dfde26 100644
--- a/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
@@ -9,9 +9,11 @@
  * container is initialized. When initialised, it will copy over messages and
  * use that thereafter.
  *
+ * You should not use this class directly. Instead use the 'messenger' service.
+ *
  * @internal
  */
-class ChainedMessenger extends BaseMessenger {
+class ChainedMessenger implements MessengerInterface {
 
   /**
    * The memory messenger.
@@ -25,7 +27,7 @@ class ChainedMessenger extends BaseMessenger {
    *
    * @var \Drupal\Core\Messenger\MessengerInterface
    */
-  protected $serviceMessenger;
+  protected $messengerService;
 
   /**
    * ChainedMessenger constructor.
@@ -41,6 +43,27 @@ public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE)
     return $this->getMessenger()->addMessage($message, $type, $repeat);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function addStatus($message, $repeat = FALSE) {
+    return $this->getMessenger()->addStatus($message, $repeat);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addError($message, $repeat = FALSE) {
+    return $this->getMessenger()->addError($message, $repeat);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addWarning($message, $repeat = FALSE) {
+    return $this->getMessenger()->addWarning($message, $repeat);
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -77,15 +100,15 @@ public function deleteByType($type) {
    */
   protected function getMessenger() {
     // Shortcut if we have a messenger service.
-    if (isset($this->serviceMessenger)) {
-      return $this->serviceMessenger;
+    if (isset($this->messengerService)) {
+      return $this->messengerService;
     }
 
     // If the service exists, but we haven't set it yet.
     if (\Drupal::hasService('messenger')) {
-      $this->serviceMessenger = \Drupal::service('messenger');
-      $this->mergeMessages($this->memoryMessenger, $this->serviceMessenger);
-      return $this->serviceMessenger;
+      $this->messengerService = \Drupal::service('messenger');
+      $this->mergeMessages($this->memoryMessenger, $this->messengerService);
+      return $this->messengerService;
     }
 
     // Fallback to memory messenger.
diff --git a/core/lib/Drupal/Core/Messenger/MemoryMessenger.php b/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
index e82c78e21a..056b0e8a3d 100644
--- a/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
@@ -21,15 +21,8 @@ class MemoryMessenger extends BaseMessenger {
   /**
    * {@inheritdoc}
    */
-  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
-    $message = $this->convertToMarkup($message);
-    // Do not use strict type checking so that equivalent string and
-    // MarkupInterface objects are detected.
-    if ($repeat || !in_array($message, $this->messages[$type])) {
-      $this->messages[$type][] = $message;
-    }
-
-    return $this;
+  protected function doAddMessage($message, $type) {
+    $this->messages[$type][] = $message;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Messenger/Messenger.php b/core/lib/Drupal/Core/Messenger/Messenger.php
index 5dc8fa8bb7..2859070479 100644
--- a/core/lib/Drupal/Core/Messenger/Messenger.php
+++ b/core/lib/Drupal/Core/Messenger/Messenger.php
@@ -2,9 +2,7 @@
 
 namespace Drupal\Core\Messenger;
 
-use Drupal\Component\Render\MarkupInterface;
 use Drupal\Core\PageCache\ResponsePolicy\KillSwitch;
-use Drupal\Core\Render\Markup;
 use Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface;
 
 /**
@@ -45,12 +43,7 @@ public function __construct(FlashBagInterface $flash_bag, KillSwitch $killSwitch
    * {@inheritdoc}
    */
   public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
-    $message = $this->convertToMarkup($message);
-    // Do not use strict type checking so that equivalent string and
-    // MarkupInterface objects are detected.
-    if ($repeat || !in_array($message, $this->flashBag->peek($type))) {
-      $this->flashBag->add($type, $message);
-    }
+    parent::addMessage($message, $type, $repeat);
 
     // Mark this page as being uncacheable.
     $this->killSwitch->trigger();
@@ -58,6 +51,13 @@ public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE)
     return $this;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function doAddMessage($message, $type) {
+    $this->flashBag->add($type, $message);
+  }
+
   /**
    * {@inheritdoc}
    */