Introduce FlashBagAwareSessionInterface

src/Symfony/Component/HttpFoundation/Session/FlashBagAwareSessionInterface.php

@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\HttpFoundation\Session;
+
+use Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface;
+
+/**
+ * Interface for session with a flashbag.
+ */
+interface FlashBagAwareSessionInterface extends SessionInterface
+{
+    public function getFlashBag(): FlashBagInterface;
+}

src/Symfony/Component/HttpFoundation/Session/Session.php

@@ -30,7 +30,7 @@ class_exists(SessionBagProxy::class);
  *
  * @implements \IteratorAggregate<string, mixed>
  */
-class Session implements SessionInterface, \IteratorAggregate, \Countable
+class Session implements FlashBagAwareSessionInterface, \IteratorAggregate, \Countable
 {
     protected $storage;