diff -u b/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
--- b/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -1190,16 +1190,18 @@
  * request. When adding a template variable that depends on other context, it is
  * your responsibility to appropriately reset the static cache in
  * ThemeManager's getDefaultTemplateVariables() when needed:
- * //TODO This would be an API change. Should we stick with 'template_preprocess'?
+ * @todo Change 'template_preprocess' key for Drupal 8.1 or 9 because
+ *   template_preprocess() is removed.
  * @code
- * drupal_static_reset('theme.manager.getDefaultTemplateVariables');
+ * drupal_static_reset('template_preprocess');
  * @endcode
  *
  * See user_template_preprocess_default_variables_alter() for an example.
  *
  * @param array $variables
  *   An associative array of default template variables, as set up by
- *   _template_preprocess_default_variables(). Passed by reference.
+ *   Drupal/Core/Theme/ThemeManagerInterface::getDefaultTemplateVariables().
+ *   Passed by reference.
  * @see Drupal/Core/Theme/ThemeManagerInterface::addDefaultTemplateVariables()
  * @see Drupal/Core/Theme/ThemeManagerInterface::getDefaultTemplateVariables()
  */
diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -603,7 +603,7 @@
       // @see https://www.drupal.org/node/2457295
       if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
         $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
-        if (isset($incomplete_previous_hook['preprocess functions'])) {
+        if (!empty($incomplete_previous_hook['preprocess functions'])) {
           $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
           $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
         }
diff -u b/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
--- b/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -475,8 +475,10 @@
   public function getDefaultTemplateVariables() {
     static $drupal_static_fast;
     if (!isset($drupal_static_fast)) {
-      //TODO Should we stick with 'template_preprocess' as the cacne key?
-      $drupal_static_fast['default_variables'] = &drupal_static($this->_serviceId.'.'.__FUNCTION__);
+      // @todo Change 'template_preprocess' key for Drupal 8.1 or 9 because
+      //   template_preprocess() is removed.
+      // @see hook_template_preprocess_default_variables_alter()
+      $drupal_static_fast['default_variables'] = &drupal_static('template_preprocess');
     }
     $default_variables = &$drupal_static_fast['default_variables'];
     if(!isset($default_variables)) {
reverted:
--- b/core/modules/user/user.module
+++ a/core/modules/user/user.module
@@ -534,7 +534,7 @@
 function user_user_login($account) {
   // Reset static cache of default variables in template_preprocess() to reflect
   // the new user.
+  drupal_static_reset('template_preprocess');
-  drupal_static_reset('theme.manager.getDefaultTemplateVariables');
 }
 
 /**
@@ -543,7 +543,7 @@
 function user_user_logout($account) {
   // Reset static cache of default variables in template_preprocess() to reflect
   // the new user.
+  drupal_static_reset('template_preprocess');
-  drupal_static_reset('theme.manager.getDefaultTemplateVariables');
 }
 
 /**