Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/system/tests/themes/test_basetheme/test_basetheme.theme

@@ -4,6 +4,7 @@
  * @file
  * Add hooks for tests to use.
  */
+use Drupal\views\Plugin\views\cache\CachePluginBase;
 use Drupal\views\ViewExecutable;
 
 /**
@@ -17,7 +18,7 @@ function test_basetheme_views_pre_render(ViewExecutable $view) {
 /**
  * Implements hook_views_post_render().
  */
-function test_basetheme_views_post_render(ViewExecutable $view) {
+function test_basetheme_views_post_render(ViewExecutable $view, &$output, CachePluginBase $cache) {
   // We append the function name to the title for test to check for.
   $view->setTitle($view->getTitle() . ":" . __FUNCTION__);
 }

core/modules/system/tests/themes/test_subtheme/test_subtheme.theme

@@ -5,6 +5,7 @@
  * Add hooks for tests to use.
  */
 
+use Drupal\views\Plugin\views\cache\CachePluginBase;
 use Drupal\views\ViewExecutable;
 
 /**
@@ -18,9 +19,12 @@ function test_subtheme_views_pre_render(ViewExecutable $view) {
 /**
  * Implements hook_views_post_render().
  */
-function test_subtheme_views_post_render(ViewExecutable $view) {
+function test_subtheme_views_post_render(ViewExecutable $view, &$output, CachePluginBase $cache) {
   // We append the function name to the title for test to check for.
   $view->setTitle($view->getTitle() . ":" . __FUNCTION__);
+  if ($view->id() == 'test_page_display') {
+    $output['#rows'][0]['#title'] = t('%total_rows items found.', array('%total_rows' => $view->total_rows));
+  }
 }
 
 /**

core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--kitten--bearcat.html.twig

@@ -0,0 +1,4 @@
+<div class="suggestion">{{ foo }}</div>
+{% if bar %}
+  <div class="suggestion">{{ bar }}</div>
+{% endif %}

core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--suggestion.html.twig

@@ -0,0 +1 @@
+<div class="suggestion">{{ foo }}</div>

core/modules/system/tests/themes/test_theme/test_theme.theme

@@ -105,3 +105,47 @@ function test_theme_theme_test_function_suggestions__module_override($variables)
 function test_theme_theme_registry_alter(&$registry) {
   $registry['theme_test_template_test']['variables']['additional'] = 'value';
 }
+
+/**
+ * Tests a theme overriding a suggestion of a base theme hook.
+ */
+function test_theme_theme_test_preprocess_suggestions__kitten__meerkat($variables) {
+  return 'Theme hook implementor=test_theme_theme_test__suggestion(). Foo=' . $variables['foo'];
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ *
+ * Implements hook_preprocess_HOOK().
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions(&$variables) {
+  $variables['foo'] = 'Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions().';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__suggestion(&$variables) {
+  $variables['foo'] = 'Suggestion';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten(&$variables) {
+  $variables['foo'] = 'Kitten';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__flamingo(&$variables) {
+  $variables['bar'] = 'Flamingo';
+}
+
+/**
+ * Tests a preprocess function with suggestions.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__meerkat__tarsier__moose(&$variables) {
+  $variables['bar'] = 'Moose';
+}