Matthijs Van Assche
--- a/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+++ b/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+use Drupal\Core\Cache\CacheableResponseInterface;
+use Drupal\Core\PageCache\ResponsePolicyInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * A policy adding vary header to response.
+ */
+class Vary implements ResponsePolicyInterface {
+
+  /**
+   * Array of Vary: headers to add to response.
+   *
+   * @var array
+   */
+  protected $vary = [];
-  protected $vary = [];
+  protected array $vary = [];
--- a/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+++ b/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+/**
+ * A policy adding vary header to response.
+ */
+class Vary implements ResponsePolicyInterface {
+
+  /**
+   * Array of Vary: headers to add to response.
+   *
+   * @var array
+   */
+  protected $vary = [];
+
+  /**
+   * Add vary header.
+   */
+  public function add($header) {
--- a/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+++ b/core/lib/Drupal/Core/PageCache/ResponsePolicy/Vary.php 0 → 100644
+   */
+  protected $vary = [];
+
+  /**
+   * Add vary header.
+   */
+  public function add($header) {
+    if (!in_array($header, $this->vary)) {
+      $this->vary[] = $header;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function check(Response $response, Request $request) {
-  public function check(Response $response, Request $request) {
+  public function check(Response $response, Request $request): void {
--- a/core/modules/page_cache/tests/src/Functional/PageCacheVaryTest.php 0 → 100644
+++ b/core/modules/page_cache/tests/src/Functional/PageCacheVaryTest.php 0 → 100644
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->config('system.site')
+      ->set('name', 'Drupal')
+      ->set('page.front', '/test-page')
+      ->save();
+  }
+
+  /**
+   * Tests that custom vary header cached.
+   */
+  public function testPageCacheWithVary() {
-  public function testPageCacheWithVary() {
+  public function testPageCacheWithVary(): void {
--- a/core/modules/system/tests/modules/test_page_test/src/Controller/TestVaryController.php 0 → 100644
+++ b/core/modules/system/tests/modules/test_page_test/src/Controller/TestVaryController.php 0 → 100644
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('page_cache_vary')
+    );
+  }
+
+  /**
+   * Page that displays content that depends on custom request header.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request.
+   *
+   * @return array
+   *   Renderable array expected by renderer service.
+   */
+  public function pageVary(Request $request) {
-  public function pageVary(Request $request) {
+  public function pageVary(Request $request): array {
--- a/core/modules/page_cache/src/StackMiddleware/PageCache.php
+++ b/core/modules/page_cache/src/StackMiddleware/PageCache.php
  protected $responsePolicy;

  /**
-   * The cache ID for the (master) request.
+   * The cache IDs for the (main) request.
   *
-   * @var string
+   * @var string[]
   */
-  protected $cid;
+  protected $cids;
-  protected $cids;
+  protected array $cids = [];
--- a/core/modules/page_cache/src/StackMiddleware/PageCache.php
+++ b/core/modules/page_cache/src/StackMiddleware/PageCache.php
  /**
-   * The cache ID for the (master) request.
+   * The cache IDs for the (main) request.
   *
-   * @var string
+   * @var string[]
   */
-  protected $cid;
+  protected $cids;
+
+  /**
+   * The request used for generating the cache ID.
+   *
+   * @var \Symfony\Component\HttpFoundation\Request
+   */
+  protected $requestUsedForCid;