Update to Drupal 8.1.5. For more information, see https://www.drupal.org/project/drupal/releases/8.1.5

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

@@ -149,7 +149,7 @@ class Cache {
    * Gets all cache bin services.
    *
    * @return array
-   *  An array of cache backend objects keyed by cache bins.
+   *   An array of cache backend objects keyed by cache bins.
    */
   public static function getBins() {
     $bins = array();

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

@@ -37,7 +37,7 @@ interface CacheableDependencyInterface {
    * When this object is modified, these cache tags will be invalidated.
    *
    * @return string[]
-   *  A set of cache tags.
+   *   A set of cache tags.
    */
   public function getCacheTags();
 

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

@@ -87,8 +87,16 @@ 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/Context/QueryArgsCacheContext.php

@@ -25,20 +25,22 @@ class QueryArgsCacheContext extends RequestStackCacheContextBase implements Calc
    */
   public function getContext($query_arg = NULL) {
     if ($query_arg === NULL) {
-      return $this->requestStack->getCurrentRequest()->getQueryString();
+      // All arguments requested. Use normalized query string to minimize
+      // variations.
+      $value = $this->requestStack->getCurrentRequest()->getQueryString();
+      return ($value !== NULL) ? $value : '';
     }
     elseif ($this->requestStack->getCurrentRequest()->query->has($query_arg)) {
       $value = $this->requestStack->getCurrentRequest()->query->get($query_arg);
-      if ($value !== '') {
+      if (is_array($value)) {
+        return http_build_query($value);
+      }
+      elseif ($value !== '') {
         return $value;
       }
-      else {
-        return '?valueless?';
-      }
-    }
-    else {
-      return NULL;
+      return '?valueless?';
     }
+    return '';
   }
 
   /**