Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/lib/Drupal/Core/Cache/Cache.php

@@ -148,7 +148,7 @@ class Cache {
   /**
    * Gets all cache bin services.
    *
-   * @return array
+   * @return \Drupal\Core\Cache\CacheBackendInterface[]
    *   An array of cache backend objects keyed by cache bins.
    */
   public static function getBins() {

core/lib/Drupal/Core/Cache/CacheFactory.php

@@ -64,16 +64,20 @@ class CacheFactory implements CacheFactoryInterface, ContainerAwareInterface {
    */
   public function get($bin) {
     $cache_settings = $this->settings->get('cache');
+    // First, look for a cache bin specific setting.
     if (isset($cache_settings['bins'][$bin])) {
       $service_name = $cache_settings['bins'][$bin];
     }
-    elseif (isset($cache_settings['default'])) {
-      $service_name = $cache_settings['default'];
-    }
+    // Second, use the default backend specified by the cache bin.
     elseif (isset($this->defaultBinBackends[$bin])) {
       $service_name = $this->defaultBinBackends[$bin];
     }
+    // Third, use configured default backend.
+    elseif (isset($cache_settings['default'])) {
+      $service_name = $cache_settings['default'];
+    }
     else {
+      // Fall back to the database backend if nothing else is configured.
       $service_name = 'cache.backend.database';
     }
     return $this->container->get($service_name)->get($bin);

core/lib/Drupal/Core/Cache/ChainedFastBackend.php

@@ -87,16 +87,8 @@ class ChainedFastBackend implements CacheBackendInterface, CacheTagsInvalidatorI
    *   The fast cache backend.
    * @param string $bin
    *   The cache bin for which the object is created.
-   *
-   * @throws \Exception
-   *   When the consistent cache backend and the fast cache backend are the same
-   *   service.
    */
   public function __construct(CacheBackendInterface $consistent_backend, CacheBackendInterface $fast_backend, $bin) {
-    if ($consistent_backend == $fast_backend) {
-      // @todo: should throw a proper exception. See https://www.drupal.org/node/2751847.
-      trigger_error('Consistent cache backend and fast cache backend cannot use the same service.', E_USER_ERROR);
-    }
     $this->consistentBackend = $consistent_backend;
     $this->fastBackend = $fast_backend;
     $this->bin = 'cache_' . $bin;

core/lib/Drupal/Core/Cache/ChainedFastBackendFactory.php

@@ -28,14 +28,14 @@ class ChainedFastBackendFactory implements CacheFactoryInterface {
   /**
    * Constructs ChainedFastBackendFactory object.
    *
-   * @param \Drupal\Core\Site\Settings|NULL $settings
+   * @param \Drupal\Core\Site\Settings|null $settings
    *   (optional) The settings object.
-   * @param string|NULL $consistent_service_name
+   * @param string|null $consistent_service_name
    *   (optional) The service name of the consistent backend factory. Defaults
    *   to:
    *   - $settings->get('cache')['default'] (if specified)
    *   - 'cache.backend.database' (if the above isn't specified)
-   * @param string|NULL $fast_service_name
+   * @param string|null $fast_service_name
    *   (optional) The service name of the fast backend factory. Defaults to:
    *   - 'cache.backend.apcu' (if the PHP process has APCu enabled)
    *   - NULL (if the PHP process doesn't have APCu enabled)

core/lib/Drupal/Core/Cache/Context/MenuActiveTrailsCacheContext.php

@@ -3,7 +3,8 @@
 namespace Drupal\Core\Cache\Context;
 
 use Drupal\Core\Cache\CacheableMetadata;
-use Symfony\Component\DependencyInjection\ContainerAware;
+use Symfony\Component\DependencyInjection\ContainerAwareInterface;
+use Symfony\Component\DependencyInjection\ContainerAwareTrait;
 
 /**
  * Defines the MenuActiveTrailsCacheContext service.
@@ -11,7 +12,9 @@ use Symfony\Component\DependencyInjection\ContainerAware;
  * This class is container-aware to avoid initializing the 'menu.active_trails'
  * service (and its dependencies) when it is not necessary.
  */
-class MenuActiveTrailsCacheContext extends ContainerAware implements CalculatedCacheContextInterface {
+class MenuActiveTrailsCacheContext implements CalculatedCacheContextInterface, ContainerAwareInterface {
+
+  use ContainerAwareTrait;
 
   /**
    * {@inheritdoc}

core/lib/Drupal/Core/Cache/UncacheableDependencyTrait.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\Core\Cache;
+
+/**
+ * Trait to implement CacheableDependencyInterface for uncacheable objects.
+ *
+ * Use this for objects that are never cacheable.
+ *
+ * @see \Drupal\Core\Cache\CacheableDependencyInterface
+ */
+trait UncacheableDependencyTrait {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheContexts() {
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheTags() {
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheMaxAge() {
+    return 0;
+  }
+
+}