diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index f8358eb..19e3bab 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1069,11 +1069,9 @@ function theme($hook, $variables = array()) {
     }
     // Add preprocess and process functions from the base hook to the $info
     // array.
-    // @todo Should this be array_merge() or NestedArray::mergeDeep() or handled
-    //   in a different way altogether?
     foreach (array('preprocess functions', 'process functions') as $processor) {
       if (isset($base_hook_info[$processor])) {
-        $info += array($processor => $base_hook_info[$processor]);
+        $info = array_merge($info, array($processor => $base_hook_info[$processor]));
       }
     }
   }
@@ -2856,9 +2854,8 @@ function template_process_html(&$variables) {
  *
  * @return
  *   An array of suggestions, suitable for adding to
- *   hook_theme_suggestions_HOOK or to
- *   $variables['attributes']['class'] if the suggestions represent extra CSS
- *   classes.
+ *   hook_theme_suggestions_HOOK_alter() or to $variables['attributes']['class']
+ *   if the suggestions represent extra CSS classes.
  */
 function theme_get_suggestions($args, $base, $delimiter = '__') {
 
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
index 51630ba..bc4ab59 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
@@ -10,7 +10,7 @@
 use Drupal\simpletest\WebTestBase;
 
 /**
- * Unit tests for template_preprocess_block().
+ * Unit tests for block_theme_suggestions_block_alter().
  */
 class BlockTemplateSuggestionsUnitTest extends WebTestBase {
 
@@ -24,13 +24,13 @@ class BlockTemplateSuggestionsUnitTest extends WebTestBase {
   public static function getInfo() {
     return array(
       'name' => 'Block template suggestions',
-      'description' => 'Test the template_preprocess_block() function.',
+      'description' => 'Test the block_theme_suggestions_block_alter() function.',
       'group' => 'Block',
     );
   }
 
   /**
-   * Test if template_preprocess_block() handles the suggestions right.
+   * Tests template suggestions from block_theme_suggestions_block_alter().
    */
   function testBlockThemeHookSuggestions() {
     // Define a block with a derivative to be preprocessed, which includes both
@@ -43,16 +43,15 @@ function testBlockThemeHookSuggestions() {
       'id' => config('system.theme')->get('default') . '.machinename',
     ));
 
+    // @todo Cast to array in suggestions alter hook signature?
+    $suggestions = array();
     $variables = array();
     $variables['elements']['#block'] = $block;
     $variables['elements']['#configuration'] = $block->getPlugin()->getConfig();
     $variables['elements']['#plugin_id'] = $block->get('plugin');
     $variables['elements']['content'] = array();
-    // Test adding a class to the block content.
-    $variables['content_attributes']['class'][] = 'test-class';
-    template_preprocess_block($variables);
-    $this->assertEqual($variables['theme_hook_suggestions'], array('block__system', 'block__system_menu_block', 'block__system_menu_block__menu_admin', 'block__machinename'));
-    $this->assertEqual($variables['content_attributes']['class'], array('test-class', 'content'), 'Default .content class added to block content_attributes');
+    block_theme_suggestions_block_alter($suggestions, $variables);
+    $this->assertEqual($suggestions, array('block__system', 'block__system_menu_block', 'block__system_menu_block__menu_admin', 'block__machinename'));
   }
 
 }
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index 8fc3b0f..c20f9cf 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -957,12 +957,12 @@ function field_page_build(&$page) {
 function field_theme_suggestions_field_alter(&$suggestions, $variables) {
   $element = $variables['element'];
 
-  $suggestions += array(
+  $suggestions = array_merge($suggestions, array(
     'field__' . $element['#field_type'],
     'field__' . $element['#field_name'],
     'field__' . $element['#bundle'],
     'field__' . $element['#field_name'] . '__' . $element['#bundle'],
-  );
+  ));
 }
 
 /**
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index 9426708..71a1435 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -1025,10 +1025,10 @@ function node_preprocess_block(&$variables) {
 function node_theme_suggestions_node_alter(&$suggestions, $variables) {
   $node = $variables['elements']['#node'];
 
-  $suggestions += array(
+  $suggestions = array_merge($suggestions, array(
     'node__' . $node->type,
     'node__' . $node->nid,
-  );
+  ));
 }
 
 /**
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 2b7b2a0..f004902 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1049,14 +1049,14 @@ function system_menu() {
  * Implements hook_theme_suggestions_HOOK_alter().
  */
 function system_theme_suggestions_html_alter(&$suggestions, $variables) {
-  $suggestions += theme_get_suggestions(arg(), 'html');
+  $suggestions = array_merge($suggestions, theme_get_suggestions(arg(), 'html'));
 }
 
 /**
  * Implements hook_theme_suggestions_HOOK_alter().
  */
 function system_theme_suggestions_page_alter(&$suggestions, $variables) {
-  $suggestions += theme_get_suggestions(arg(), 'page');
+  $suggestions = array_merge($suggestions, theme_get_suggestions(arg(), 'page'));
 }
 
 /**
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index c7077e9..e0da079 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -238,17 +238,13 @@ function views_plugin_list() {
 function views_preprocess_node(&$vars) {
   Drupal::moduleHandler()->loadInclude('node', 'views.inc');
   // The 'view' attribute of the node is added in views_preprocess_node()
+  // @todo The comment above doesn't make any sense to me - it's saying the
+  //   attribute is added in *this* function?
   if (!empty($vars['node']->view) && $vars['node']->view->storage->id()) {
-    $vars['view'] = $vars['node']->view;
-    $vars['theme_hook_suggestions'][] = 'node__view__' . $vars['node']->view->storage->id();
-    if (!empty($vars['node']->view->current_display)) {
-      $vars['theme_hook_suggestions'][] = 'node__view__' . $vars['node']->view->storage->id() . '__' . $vars['node']->view->current_display;
-
-      // If a node is being rendered in a view, and the view does not have a path,
-      // prevent drupal from accidentally setting the $page variable:
-      if ($vars['page'] && $vars['view_mode'] == 'full' && !$vars['view']->display_handler->hasPath()) {
-        $vars['page'] = FALSE;
-      }
+    // If a node is being rendered in a view, and the view does not have a path,
+    // prevent drupal from accidentally setting the $page variable:
+    if ($vars['page'] && $vars['view_mode'] == 'full' && !$vars['view']->display_handler->hasPath()) {
+      $vars['page'] = FALSE;
     }
   }
 
@@ -259,16 +255,40 @@ function views_preprocess_node(&$vars) {
 }
 
 /**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function views_theme_suggestions_node_alter(&$suggestions, $variables) {
+  if (!empty($variables['node']->view) && $variables['node']->view->storage->id()) {
+    $variables['view'] = $variables['node']->view;
+    $suggestions[] = 'node__view__' . $variables['node']->view->storage->id();
+    if (!empty($variables['node']->view->current_display)) {
+      $suggestions[] = 'node__view__' . $variables['node']->view->storage->id() . '__' . $variables['node']->view->current_display;
+    }
+  }
+}
+
+/**
  * A theme preprocess function to automatically allow view-based node
  * templates if called from a view.
  */
 function views_preprocess_comment(&$vars) {
   // The 'view' attribute of the node is added in template_preprocess_views_view_row_comment()
+  // @todo I can't find the function mentioned in the above comment.
   if (!empty($vars['comment']->view) && $vars['comment']->view->storage->id()) {
     $vars['view'] = &$vars['comment']->view;
-    $vars['theme_hook_suggestions'][] = 'comment__view__' . $vars['comment']->view->storage->id();
-    if (!empty($vars['node']->view->current_display)) {
-      $vars['theme_hook_suggestions'][] = 'comment__view__' . $vars['comment']->view->storage->id() . '__' . $vars['comment']->view->current_display;
+  }
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function views_theme_suggestions_comment_alter(&$suggestions, $variables) {
+  if (!empty($variables['comment']->view) && $variables['comment']->view->storage->id()) {
+    $suggestions[] = 'comment__view__' . $variables['comment']->view->storage->id();
+    // @todo Shouldn't the below be checking $variables['comment'] instead of
+    //   $variables['node']?
+    if (!empty($variables['node']->view->current_display)) {
+      $suggestions[] = 'comment__view__' . $variables['comment']->view->storage->id() . '__' . $variables['comment']->view->current_display;
     }
   }
 }
diff --git a/core/modules/views_ui/views_ui.theme.inc b/core/modules/views_ui/views_ui.theme.inc
index 8e503fc..48eb6d1 100644
--- a/core/modules/views_ui/views_ui.theme.inc
+++ b/core/modules/views_ui/views_ui.theme.inc
@@ -450,7 +450,13 @@ function template_preprocess_views_ui_view_preview_section(&$vars) {
     );
     $vars['links'] = drupal_render($build);
   }
-  $vars['theme_hook_suggestions'][] = 'views_ui_view_preview_section__' . $vars['section'];
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function views_ui_theme_suggestions_views_ui_view_preview_section_alter(&$suggestions, $variables) {
+  $suggestions[] = 'views_ui_view_preview_section__' . $variables['section'];
 }
 
 /**