core/lib/Drupal/Core/Cache/ApcuBackend.php         |  3 ++-
 core/lib/Drupal/Core/Cache/Cache.php               | 10 +++++-----
 core/lib/Drupal/Core/Cache/DatabaseBackend.php     | 10 ++++++++--
 core/lib/Drupal/Core/Cache/MemoryBackend.php       |  6 +++++-
 core/lib/Drupal/Core/Cache/PhpBackend.php          |  3 ++-
 .../Cache/GenericCacheBackendUnitTestBase.php      | 23 ++++++++++++++++++++++
 .../toolbar/src/Tests/ToolbarAdminMenuTest.php     |  2 +-
 core/tests/Drupal/Tests/Core/Cache/CacheTest.php   | 20 +++++++++++++++++++
 8 files changed, 66 insertions(+), 11 deletions(-)

diff --git a/core/lib/Drupal/Core/Cache/ApcuBackend.php b/core/lib/Drupal/Core/Cache/ApcuBackend.php
index 199a134..50fb0fd 100644
--- a/core/lib/Drupal/Core/Cache/ApcuBackend.php
+++ b/core/lib/Drupal/Core/Cache/ApcuBackend.php
@@ -189,7 +189,8 @@ protected function prepareItem($cache, $allow_invalid) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
-    $tags = Cache::mergeTags($tags, []);
+    Cache::validateTags($tags);
+    $tags = array_unique($tags);
     $cache = new \stdClass();
     $cache->cid = $cid;
     $cache->created = round(microtime(TRUE), 3);
diff --git a/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
index a29ae57..4b475c3 100644
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -32,10 +32,10 @@ class Cache {
    * allows items to be invalidated based on all tags attached to the content
    * they're constituted from.
    *
-   * @param array …
+   * @param string[] …
    *   Arrays of cache tags to merge.
    *
-   * @return array
+   * @return string[]
    *   The merged array of cache tags.
    */
   public static function mergeTags() {
@@ -55,7 +55,7 @@ public static function mergeTags() {
    *
    * Can be called before using cache tags in operations, to ensure validity.
    *
-   * @param array $tags
+   * @param string[] $tags
    *   An array of cache tags.
    *
    * @throws \LogicException
@@ -81,7 +81,7 @@ public static function validateTags(array $tags) {
    * When deleting a given list of tags, we iterate over each cache backend, and
    * and call deleteTags() on each.
    *
-   * @param array $tags
+   * @param string[] $tags
    *   The list of tags to delete cache items for.
    */
   public static function deleteTags(array $tags) {
@@ -101,7 +101,7 @@ public static function deleteTags(array $tags) {
    * When invalidating a given list of tags, we iterate over each cache backend,
    * and call invalidateTags() on each.
    *
-   * @param array $tags
+   * @param string[] $tags
    *   The list of tags to invalidate cache items for.
    */
   public static function invalidateTags(array $tags) {
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index 2c94498..1e24968 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -147,7 +147,10 @@ protected function prepareItem($cache, $allow_invalid) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    $tags = Cache::mergeTags($tags, []);
+    Cache::validateTags($tags);
+    $tags = array_unique($tags);
+    // Sort the cache tags so that they are stored consistently in the database.
+    sort($tags);
     $try_again = FALSE;
     try {
       // The bin might not yet exist.
@@ -234,7 +237,10 @@ public function setMultiple(array $items) {
           'tags' => array(),
         );
 
-        $item['tags'] = Cache::mergeTags($item['tags'], []);
+        Cache::validateTags($item['tags']);
+        $item['tags'] = array_unique($item['tags']);
+        // Sort the cache tags so that they are stored consistently in the DB.
+        sort($item['tags']);
 
         // Remove tags that were already deleted or invalidated during this
         // request from the static caches so that another deletion or
diff --git a/core/lib/Drupal/Core/Cache/MemoryBackend.php b/core/lib/Drupal/Core/Cache/MemoryBackend.php
index ce74ff4..23fddc5 100644
--- a/core/lib/Drupal/Core/Cache/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Cache/MemoryBackend.php
@@ -107,12 +107,16 @@ protected function prepareItem($cache, $allow_invalid) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
+    Cache::validateTags($tags);
+    $tags = array_unique($tags);
+    // Sort the cache tags so that they are stored consistently in the database.
+    sort($tags);
     $this->cache[$cid] = (object) array(
       'cid' => $cid,
       'data' => serialize($data),
       'created' => REQUEST_TIME,
       'expire' => $expire,
-      'tags' => Cache::mergeTags($tags, []),
+      'tags' => $tags,
     );
   }
 
diff --git a/core/lib/Drupal/Core/Cache/PhpBackend.php b/core/lib/Drupal/Core/Cache/PhpBackend.php
index e7de3b6..e188ffb 100644
--- a/core/lib/Drupal/Core/Cache/PhpBackend.php
+++ b/core/lib/Drupal/Core/Cache/PhpBackend.php
@@ -133,12 +133,13 @@ protected function prepareItem($cache, $allow_invalid) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
+    Cache::validateTags($tags);
     $item = (object) array(
       'cid' => $cid,
       'data' => $data,
       'created' => round(microtime(TRUE), 3),
       'expire' => $expire,
-      'tags' => Cache::mergeTags($tags, []),
+      'tags' => array_unique($tags),
     );
     $this->writeItem($this->normalizeCid($cid), $item);
   }
diff --git a/core/modules/system/src/Tests/Cache/GenericCacheBackendUnitTestBase.php b/core/modules/system/src/Tests/Cache/GenericCacheBackendUnitTestBase.php
index 44e6a94..958acff 100644
--- a/core/modules/system/src/Tests/Cache/GenericCacheBackendUnitTestBase.php
+++ b/core/modules/system/src/Tests/Cache/GenericCacheBackendUnitTestBase.php
@@ -212,6 +212,15 @@ public function testSetGet() {
     $cid = str_repeat('a', 300);
     $backend->set($cid, 'test');
     $this->assertEqual('test', $backend->get($cid)->data);
+
+    // Calling ::set() with invalid cache tags.
+    try {
+      $backend->set('exception_test', 'value', Cache::PERMANENT, ['node' => [3, 5, 7]]);
+      $this->fail('::set() was called with invalid cache tags, no exception was thrown.');
+    }
+    catch (\LogicException $e) {
+      $this->pass('::set() was called with invalid cache tags, an exception was thrown.');
+    }
   }
 
   /**
@@ -398,6 +407,20 @@ public function testSetMultiple() {
 
     $this->assertEqual($cached['cid_5']->data, $items['cid_5']['data'], 'New cache item set correctly.');
     $this->assertEqual($cached['cid_5']->tags, array('test:a', 'test:b'));
+
+    // Calling ::setMultiple() with invalid cache tags.
+    try {
+      $items = [
+        'exception_test_1' => array('data' => 1, 'tags' => []),
+        'exception_test_2' => array('data' => 2, 'tags' => ['valid']),
+        'exception_test_3' => array('data' => 3, 'tags' => ['node' => [3, 5, 7]]),
+      ];
+      $backend->setMultiple($items);
+      $this->fail('::setMultiple() was called with invalid cache tags, no exception was thrown.');
+    }
+    catch (\LogicException $e) {
+      $this->pass('::setMultiple() was called with invalid cache tags, an exception was thrown.');
+    }
   }
 
   /**
diff --git a/core/modules/toolbar/src/Tests/ToolbarAdminMenuTest.php b/core/modules/toolbar/src/Tests/ToolbarAdminMenuTest.php
index 486dc20..da51771 100644
--- a/core/modules/toolbar/src/Tests/ToolbarAdminMenuTest.php
+++ b/core/modules/toolbar/src/Tests/ToolbarAdminMenuTest.php
@@ -255,7 +255,7 @@ function testCacheClearByCacheTag() {
     // Assert that a cache tag in the toolbar cache under the key "user" exists
     // for admin_user_2 against the language "en".
     $cache = $toolbarCache->get('toolbar_' . $admin_user_2_id . ':' . 'en');
-    $this->assertEqual($cache->tags[2], 'user:' . $admin_user_2_id); //, 'A cache tag in the toolbar cache under the key "user" exists for admin_user_2 against the language "en".');
+    $this->assertEqual($cache->tags[2], 'user:' . $admin_user_2_id, 'A cache tag in the toolbar cache under the key "user" exists for admin_user_2 against the language "en".');
 
     // Request a page in 'fr' to create the cache.
     $this->drupalGet('fr/test-page');
diff --git a/core/tests/Drupal/Tests/Core/Cache/CacheTest.php b/core/tests/Drupal/Tests/Core/Cache/CacheTest.php
index 612899d..e000e12 100644
--- a/core/tests/Drupal/Tests/Core/Cache/CacheTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/CacheTest.php
@@ -82,4 +82,24 @@ public function testMergeTags(array $a, array $b, array $expected) {
     $this->assertEquals($expected, Cache::mergeTags($a, $b));
   }
 
+  /**
+   * @covers deleteTags
+   *
+   * @expectedException \LogicException
+   * @expectedExceptionMessage Cache tags must be strings, array given.
+   */
+  public function testDeleteTags() {
+    Cache::deleteTags(['node' => [2, 3, 5, 8, 13]]);
+  }
+
+  /**
+   * @covers invalidateTags
+   *
+   * @expectedException \LogicException
+   * @expectedExceptionMessage Cache tags must be strings, array given.
+   */
+  public function testInvalidateTags() {
+    Cache::deleteTags(['node' => [2, 3, 5, 8, 13]]);
+  }
+
 }