diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index b349918..1f2694f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1514,7 +1514,6 @@ function template_preprocess_feed_icon(&$variables) {
     '#height' => 16,
     '#alt' => $text,
   );
-  $variables['attributes']['class'] = array('feed-icon');
   // Stripping tags because that's what l() used to do.
   $variables['attributes']['title'] = strip_tags($text);
 }
@@ -2262,39 +2261,30 @@ function drupal_common_theme() {
     // From theme.inc.
     'html' => array(
       'variables' => array('page_object' => NULL),
-      'template' => 'html',
     ),
     'page' => array(
       'render element' => 'page',
-      'template' => 'page',
     ),
     'region' => array(
       'render element' => 'elements',
-      'template' => 'region',
     ),
     'time' => array(
       'variables' => array('timestamp' => NULL, 'text' => NULL, 'attributes' => array(), 'html' => FALSE),
-      'template' => 'time',
     ),
     'datetime_form' => array(
-      'template' => 'datetime-form',
       'render element' => 'element',
     ),
     'datetime_wrapper' => array(
-      'template' => 'datetime-wrapper',
       'render element' => 'element',
     ),
     'status_messages' => array(
       'variables' => array('display' => NULL),
-      'template' => 'status-messages',
     ),
     'links' => array(
       'variables' => array('links' => array(), 'attributes' => array('class' => array('links')), 'heading' => array(), 'set_active_class' => FALSE),
-      'template' => 'links',
     ),
     'dropbutton_wrapper' => array(
       'variables' => array('children' => NULL),
-      'template' => 'dropbutton-wrapper',
     ),
     'image' => array(
       // HTML 4 and XHTML 1.0 always require an alt attribute. The HTML 5 draft
@@ -2310,140 +2300,114 @@ function drupal_common_theme() {
       // The title attribute is optional in all cases, so it is omitted by
       // default.
       'variables' => array('uri' => NULL, 'width' => NULL, 'height' => NULL, 'alt' => '', 'title' => NULL, 'attributes' => array()),
-      'template' => 'image',
     ),
     'breadcrumb' => array(
       'variables' => array('links' => array()),
-      'template' => 'breadcrumb',
     ),
     'table' => array(
       'variables' => array('header' => NULL, 'rows' => NULL, 'footer' => NULL, 'attributes' => array(), 'caption' => NULL, 'colgroups' => array(), 'sticky' => FALSE, 'responsive' => TRUE, 'empty' => ''),
-      'template' => 'table',
     ),
     'tablesort_indicator' => array(
       'variables' => array('style' => NULL),
-      'template' => 'tablesort-indicator',
     ),
     'mark' => array(
       'variables' => array('status' => MARK_NEW),
-      'template' => 'mark',
     ),
     'item_list' => array(
       'variables' => array('items' => array(), 'title' => '', 'list_type' => 'ul', 'attributes' => array(), 'empty' => NULL),
-      'template' => 'item-list',
     ),
     'feed_icon' => array(
       'variables' => array('url' => NULL, 'title' => NULL),
-      'template' => 'feed-icon',
     ),
     'progress_bar' => array(
       'variables' => array('label' => NULL, 'percent' => NULL, 'message' => NULL),
-      'template' => 'progress-bar',
     ),
     'indentation' => array(
       'variables' => array('size' => 1),
+      'function' => 'theme_indentation',
     ),
     // From theme.maintenance.inc.
     'maintenance_page' => array(
       'render element' => 'page',
-      'template' => 'maintenance-page',
     ),
     'install_page' => array(
       'render element' => 'page',
-      'template' => 'install-page',
     ),
     'task_list' => array(
       'variables' => array('items' => NULL, 'active' => NULL,  'variant' => NULL),
-      'template' => 'task-list',
     ),
     'authorize_message' => array(
       'variables' => array('message' => NULL, 'success' => TRUE),
+      'function' => 'theme_authorize_message',
     ),
     'authorize_report' => array(
       'variables' => array('messages' => array()),
+      'function' => 'theme_authozie_report',
     ),
     // From pager.inc.
     'pager' => array(
       'variables' => array('tags' => array(), 'element' => 0, 'parameters' => array(), 'quantity' => 9),
-      'template' => 'pager',
     ),
     // From menu.inc.
     'menu_link' => array(
       'render element' => 'element',
+      'function' => 'theme_menu_link',
     ),
     'menu_tree' => array(
       'render element' => 'tree',
-      'template' => 'menu-tree',
     ),
     'menu_local_task' => array(
       'render element' => 'element',
-      'template' => 'menu-local-task',
     ),
     'menu_local_action' => array(
       'render element' => 'element',
-      'template' => 'menu-local-action',
     ),
     'menu_local_tasks' => array(
       'variables' => array('primary' => array(), 'secondary' => array()),
-      'template' => 'menu-local-tasks',
     ),
     // From form.inc.
     'input' => array(
       'render element' => 'element',
-      'template' => 'input',
     ),
     'select' => array(
       'render element' => 'element',
-      'template' => 'select',
     ),
     'fieldset' => array(
       'render element' => 'element',
-      'template' => 'fieldset',
     ),
     'details' => array(
       'render element' => 'element',
-      'template' => 'details',
     ),
     'radios' => array(
       'render element' => 'element',
-      'template' => 'radios',
     ),
     'checkboxes' => array(
       'render element' => 'element',
-      'template' => 'checkboxes',
     ),
     'form' => array(
       'render element' => 'element',
-      'template' => 'form',
     ),
     'textarea' => array(
       'render element' => 'element',
-      'template' => 'textarea',
     ),
     'form_element' => array(
       'render element' => 'element',
-      'template' => 'form-element',
     ),
     'form_element_label' => array(
       'render element' => 'element',
-      'template' => 'form-element-label',
     ),
     'vertical_tabs' => array(
       'render element' => 'element',
-      'template' => 'vertical-tabs',
     ),
     'container' => array(
       'render element' => 'element',
-      'template' => 'container',
     ),
     // From field system.
     'field' => array(
       'render element' => 'element',
-      'template' => 'field',
     ),
     'field_multiple_value_form' => array(
       'render element' => 'element',
-      'template' => 'field-multiple-value-form',
     ),
   );
 }
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 0ad005d..ab28c9b 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -401,6 +401,9 @@ protected function build() {
    * @see _theme()
    * @see hook_theme()
    * @see list_themes()
+   * @see twig_render_template()
+   *
+   * @throws \BadFunctionCallException
    */
   protected function processExtension(&$cache, $name, $type, $theme, $path) {
     $result = array();
@@ -430,12 +433,6 @@ protected function processExtension(&$cache, $name, $type, $theme, $path) {
         $result[$hook]['type'] = $type;
         $result[$hook]['theme path'] = $path;
 
-        // If function and file are omitted, default to standard naming
-        // conventions.
-        if (!isset($info['template']) && !isset($info['function'])) {
-          $result[$hook]['function'] = ($type == 'module' ? 'theme_' : $name . '_') . $hook;
-        }
-
         if (isset($cache[$hook]['includes'])) {
           $result[$hook]['includes'] = $cache[$hook]['includes'];
         }
@@ -451,20 +448,32 @@ protected function processExtension(&$cache, $name, $type, $theme, $path) {
           $result[$hook]['includes'][] = $include_file;
         }
 
+        // Templates are the default implementation for a theme hook. If a
+        // theme hook provides a theme function callback, check to ensure that
+        // it actually exists.
+        if (isset($info['function']) && !function_exists($info['function'])) {
+          continue;
+        }
+        // Provide a default naming convention for 'template' based on the
+        // hook used. If the template does not exist, the theme engine used
+        // should throw an exception at runtime when attempting to include
+        // the template file.
+        elseif (!isset($info['template'])) {
+          $result[$hook]['template'] = strtr($hook, '_', '-');
+        }
+
+        // Prepend the current theming path when none is set. This is required
+        // for the default theme engine to know where the template lives.
+        if (isset($result[$hook]['template']) && !isset($info['path'])) {
+          $result[$hook]['path'] = $path . '/templates';
+        }
+
         // If the default keys are not set, use the default values registered
         // by the module.
         if (isset($cache[$hook])) {
           $result[$hook] += array_intersect_key($cache[$hook], $hook_defaults);
         }
 
-        // The following apply only to theming hooks implemented as templates.
-        if (isset($info['template'])) {
-          // Prepend the current theming path when none is set.
-          if (!isset($info['path'])) {
-            $result[$hook]['template'] = $path . '/templates/' . $info['template'];
-          }
-        }
-
         // Preprocess variables for all theming hooks, whether the hook is
         // implemented as a template or as a function. Ensure they are arrays.
         if (!isset($info['preprocess functions']) || !is_array($info['preprocess functions'])) {
diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index 64eb275..7bf900a 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -59,35 +59,27 @@ function aggregator_theme() {
     'aggregator_feed_source' => array(
       'variables' => array('aggregator_feed' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.theme.inc',
-      'template' => 'aggregator-feed-source',
     ),
     'aggregator_block_item' => array(
       'variables' => array('item' => NULL, 'feed' => 0),
       'file' => 'aggregator.theme.inc',
-      'template' => 'aggregator-block-item',
     ),
     'aggregator_summary_items' => array(
       'variables' => array('summary_items' => NULL, 'source' => NULL),
       'file' => 'aggregator.theme.inc',
-      'template' => 'aggregator-summary-items',
     ),
     'aggregator_summary_item' => array(
       'variables' => array('aggregator_item' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.theme.inc',
-      'template' => 'aggregator-summary-item',
     ),
     'aggregator_item' => array(
       'variables' => array('aggregator_item' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.theme.inc',
-      'template' => 'aggregator-item',
     ),
     'aggregator_page_opml' => array(
       'variables' => array('feeds' => NULL),
       'file' => 'aggregator.theme.inc',
-    ),
-    'aggregator_page_rss' => array(
-      'variables' => array('feeds' => NULL),
-      'file' => 'aggregator.theme.inc',
+      'function' => 'theme_aggregator_page_opml',
     ),
   );
 }
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 339d163..4e77e24 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -53,11 +53,9 @@ function block_theme() {
   return array(
     'block' => array(
       'render element' => 'elements',
-      'template' => 'block',
     ),
     'block_list' => array(
       'render element' => 'form',
-      'template' => 'block-list',
     ),
   );
 }
diff --git a/core/modules/block_content/block_content.module b/core/modules/block_content/block_content.module
index dadcf4b..63c8fb3 100644
--- a/core/modules/block_content/block_content.module
+++ b/core/modules/block_content/block_content.module
@@ -46,7 +46,6 @@ function block_content_theme($existing, $type, $theme, $path) {
     'block_content_add_list' => array(
       'variables' => array('content' => NULL),
       'file' => 'block_content.pages.inc',
-      'template' => 'block-content-add-list',
     ),
   );
 }
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index e67b279..4dbd3c4 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -54,30 +54,27 @@ function book_theme() {
   return array(
     'book_navigation' => array(
       'variables' => array('book_link' => NULL),
-      'template' => 'book-navigation',
     ),
     'book_tree' => array(
       'render element' => 'tree',
-      'template' => 'book-tree',
     ),
     'book_link' => array(
       'render element' => 'element',
+      'function' => 'theme_book_link',
     ),
     'book_export_html' => array(
       'variables' => array('title' => NULL, 'contents' => NULL, 'depth' => NULL),
-      'template' => 'book-export-html',
     ),
     'book_admin_table' => array(
       'render element' => 'form',
       'file' => 'book.admin.inc',
+      'function' => 'theme_book_admin_table',
     ),
     'book_all_books_block' => array(
       'render element' => 'book_menus',
-      'template' => 'book-all-books-block',
     ),
     'book_node_export_html' => array(
       'variables' => array('node' => NULL, 'children' => NULL),
-      'template' => 'book-node-export-html',
     ),
   );
 }
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 5cb8ef3..d1476d9 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -40,7 +40,6 @@ function ckeditor_theme() {
     'ckeditor_settings_toolbar' => array(
       'file' => 'ckeditor.admin.inc',
       'variables' => array('editor' => NULL, 'plugins' => NULL),
-      'template' => 'ckeditor-settings-toolbar',
     ),
   );
 }
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index a0c0896..3aa2f68 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -38,7 +38,6 @@ function color_theme() {
   return array(
     'color_scheme_form' => array(
       'render element' => 'form',
-      'template' => 'color-scheme-form',
     ),
   );
 }
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index dbe3bee..6f67392 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -129,11 +129,9 @@ function comment_theme() {
   return array(
     'comment' => array(
       'render element' => 'elements',
-      'template' => 'comment',
     ),
     'field__comment' => array(
       'base hook' => 'field',
-      'template' => 'field--comment',
     ),
   );
 }
diff --git a/core/modules/config_translation/config_translation.module b/core/modules/config_translation/config_translation.module
index 43aa754..c756394 100644
--- a/core/modules/config_translation/config_translation.module
+++ b/core/modules/config_translation/config_translation.module
@@ -55,7 +55,6 @@ function config_translation_theme() {
   return array(
     'config_translation_manage_form_element' => array(
       'render element' => 'element',
-      'template' => 'config_translation_manage_form_element',
     ),
   );
 }
diff --git a/core/modules/field_ui/field_ui.module b/core/modules/field_ui/field_ui.module
index 9dbcbd2..b349527 100644
--- a/core/modules/field_ui/field_ui.module
+++ b/core/modules/field_ui/field_ui.module
@@ -96,6 +96,7 @@ function field_ui_theme() {
   return array(
     'field_ui_table' => array(
       'render element' => 'elements',
+      'function' => 'theme_field_ui_table',
     ),
   );
 }
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 03e10bf..ffd5fc0 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -577,27 +577,22 @@ function file_theme() {
     // From file.module.
     'file_link' => array(
       'variables' => array('file' => NULL, 'icon_directory' => NULL, 'description' => NULL, 'attributes' => array()),
-      'template' => 'file-link',
     ),
     'file_managed_file' => array(
       'render element' => 'element',
-      'template' => 'file-managed-file',
     ),
 
     // From file.field.inc.
     'file_widget' => array(
       'render element' => 'element',
-      'template' => 'file-widget',
       'file' => 'file.field.inc',
     ),
     'file_widget_multiple' => array(
       'render element' => 'element',
-      'template' => 'file-widget-multiple',
       'file' => 'file.field.inc',
     ),
     'file_upload_help' => array(
       'variables' => array('description' => NULL, 'upload_validators' => NULL, 'cardinality' => NULL),
-      'template' => 'file-upload-help',
       'file' => 'file.field.inc',
     ),
   );
diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index d31cca1..d17329a 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -61,7 +61,6 @@ function filter_theme() {
   return array(
     'filter_tips' => array(
       'variables' => array('tips' => NULL, 'long' => FALSE),
-      'template' => 'filter-tips',
     ),
     'text_format_wrapper' => array(
       'variables' => array(
@@ -69,11 +68,9 @@ function filter_theme() {
         'description' => NULL,
         'attributes' => array(),
       ),
-      'template' => 'text-format-wrapper',
     ),
     'filter_guidelines' => array(
       'variables' => array('format' => NULL),
-      'template' => 'filter-guidelines',
     ),
     'filter_caption' => array(
       'variables' => array(
@@ -82,7 +79,6 @@ function filter_theme() {
         'caption' => NULL,
         'classes' => NULL,
       ),
-      'template' => 'filter-caption',
     )
   );
 }
diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index 039c305..8b6aea2 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -82,19 +82,15 @@ function forum_help($route_name, RouteMatchInterface $route_match) {
 function forum_theme() {
   return array(
     'forums' => array(
-      'template' => 'forums',
       'variables' => array('forums' => array(), 'topics' => array(), 'topics_pager' => array(), 'parents' => NULL, 'term' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL, 'header' => array()),
     ),
     'forum_list' => array(
-      'template' => 'forum-list',
       'variables' => array('forums' => NULL, 'parents' => NULL, 'tid' => NULL),
     ),
     'forum_icon' => array(
-      'template' => 'forum-icon',
       'variables' => array('new_posts' => NULL, 'num_posts' => 0, 'comment_mode' => 0, 'sticky' => 0, 'first_new' => FALSE),
     ),
     'forum_submitted' => array(
-      'template' => 'forum-submitted',
       'variables' => array('topic' => NULL),
     ),
   );
diff --git a/core/modules/image/image.module b/core/modules/image/image.module
index a6ce171..8a7ad86 100644
--- a/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -107,47 +107,38 @@ function image_theme() {
         'title' => NULL,
         'attributes' => array(),
       ),
-      'template' => 'image-style',
     ),
 
     // Theme functions in image.admin.inc.
     'image_style_preview' => array(
       'variables' => array('style' => NULL),
       'file' => 'image.admin.inc',
-      'template' => 'image-style-preview',
     ),
     'image_anchor' => array(
       'render element' => 'element',
       'file' => 'image.admin.inc',
-      'template' => 'image-anchor',
     ),
     'image_resize_summary' => array(
       'variables' => array('data' => NULL, 'effect' => array()),
-      'template' => 'image-resize-summary',
     ),
     'image_scale_summary' => array(
       'variables' => array('data' => NULL, 'effect' => array()),
-      'template' => 'image-scale-summary',
     ),
     'image_crop_summary' => array(
       'variables' => array('data' => NULL, 'effect' => array()),
-      'template' => 'image-crop-summary',
     ),
     'image_rotate_summary' => array(
       'variables' => array('data' => NULL, 'effect' => array()),
-      'template' => 'image-rotate-summary',
     ),
 
     // Theme functions in image.field.inc.
     'image_widget' => array(
       'render element' => 'element',
       'file' => 'image.field.inc',
-      'template' => 'image-widget',
     ),
     'image_formatter' => array(
       'variables' => array('item' => NULL, 'item_attributes' => NULL, 'path' => NULL, 'image_style' => NULL),
       'file' => 'image.field.inc',
-      'template' => 'image-formatter',
     ),
   );
 }
diff --git a/core/modules/language/language.module b/core/modules/language/language.module
index 0c37638..ad98e35 100644
--- a/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -106,15 +106,16 @@ function language_theme() {
     'language_negotiation_configure_form' => array(
       'render element' => 'form',
       'file' => 'language.admin.inc',
-      'template' => 'language-negotiation-configure-form',
     ),
     'language_negotiation_configure_browser_form_table' => array(
       'render element' => 'form',
       'file' => 'language.admin.inc',
+      'function' => 'theme_language_negotiation_configure_browser_form_table',
     ),
     'language_content_settings_table' => array(
       'render element' => 'element',
       'file' => 'language.admin.inc',
+      'function' => 'theme_language_content_settings_table',
     ),
   );
 }
diff --git a/core/modules/link/link.module b/core/modules/link/link.module
index d33181e..ba3c3a1 100644
--- a/core/modules/link/link.module
+++ b/core/modules/link/link.module
@@ -41,7 +41,6 @@ function link_theme() {
   return array(
     'link_formatter_link_separate' => array(
       'variables' => array('title' => NULL, 'url_title' => NULL, 'url' => NULL),
-      'template' => 'link-formatter-link-separate',
     ),
   );
 }
diff --git a/core/modules/locale/locale.module b/core/modules/locale/locale.module
index 3122f59..67a9b79 100644
--- a/core/modules/locale/locale.module
+++ b/core/modules/locale/locale.module
@@ -193,16 +193,15 @@ function locale_theme() {
     'locale_translate_edit_form_strings' => array(
       'render element' => 'form',
       'file' => 'locale.pages.inc',
+      'function' => 'theme_locale_translate_edit_form_strings',
     ),
     'locale_translation_last_check' => array(
       'variables' => array('last' => NULL),
       'file' => 'locale.pages.inc',
-      'template' => 'locale-translation-last-check',
     ),
     'locale_translation_update_info' => array(
       'variables' => array('updates' => array(), 'not_found' => array()),
       'file' => 'locale.pages.inc',
-      'template' => 'locale-translation-update-info',
     ),
   );
 }
diff --git a/core/modules/menu_ui/menu_ui.module b/core/modules/menu_ui/menu_ui.module
index dcedc00..b709865 100644
--- a/core/modules/menu_ui/menu_ui.module
+++ b/core/modules/menu_ui/menu_ui.module
@@ -89,6 +89,7 @@ function menu_ui_theme() {
     'menu_overview_form' => array(
       'file' => 'menu_ui.admin.inc',
       'render element' => 'form',
+      'function' => 'theme_menu_overview_form',
     ),
   );
 }
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index 030e62a..2bef3f6 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -159,31 +159,26 @@ function node_theme() {
   return array(
     'node' => array(
       'render element' => 'elements',
-      'template' => 'node',
     ),
     'node_search_admin' => array(
       'render element' => 'form',
+      'function' => 'theme_node_search_admin',
     ),
     'node_add_list' => array(
       'variables' => array('content' => NULL),
       'file' => 'node.pages.inc',
-      'template' => 'node-add-list',
     ),
     'node_edit_form' => array(
       'render element' => 'form',
-      'template' => 'node-edit-form',
     ),
     'field__node__title' => array(
       'base hook' => 'field',
-      'template' => 'field--node--title',
     ),
     'field__node__uid' => array(
       'base hook' => 'field',
-      'template' => 'field--node--uid',
     ),
     'field__node__created' => array(
       'base hook' => 'field',
-      'template' => 'field--node--created',
     ),
   );
 }
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index 061dd03..576ae84 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -250,7 +250,6 @@ function rdf_theme() {
   return array(
     'rdf_metadata' => array(
       'variables' => array('metadata' => array()),
-      'template' => 'rdf-metadata',
     ),
   );
 }
diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index d8985c9..42b9926 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -91,6 +91,7 @@ function responsive_image_theme() {
         'attributes' => array(),
         'mapping_id' => array(),
       ),
+      'function' => 'theme_responsive_image',
     ),
     'responsive_image_formatter' => array(
       'variables' => array(
@@ -99,6 +100,7 @@ function responsive_image_theme() {
         'image_style' => NULL,
         'mapping_id' => array(),
       ),
+      'function' => 'theme_responsive_image_formatter',
     ),
     'responsive_image_source' => array(
       'variables' => array(
@@ -107,6 +109,7 @@ function responsive_image_theme() {
         'dimensions' => NULL,
         'media' => NULL,
       ),
+      'function' => 'theme_responsive_image_source',
     ),
   );
 }
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index 7cbe706..fa7d1cf 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -104,7 +104,6 @@ function search_theme() {
     'search_result' => array(
       'variables' => array('result' => NULL, 'plugin_id' => NULL),
       'file' => 'search.pages.inc',
-      'template' => 'search-result',
     ),
   );
 }
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 24665c7..8abb35d 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -56,7 +56,6 @@ function simpletest_theme() {
   return array(
     'simpletest_result_summary' => array(
       'variables' => array('label' => NULL, 'items' => array(), 'pass' => 0, 'fail' => 0, 'exception' => 0, 'debug' => 0),
-      'template' => 'simpletest-result-summary',
     ),
   );
 }
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index f3b65a2..bc5868d 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -159,7 +159,6 @@ function system_theme() {
     // is picked up.
     'block__system_branding_block' => array(
       'base hook' => 'block',
-      'template' => 'block--system-branding-block',
     ),
     'system_themes_page' => array(
       'variables' => array(
@@ -167,51 +166,46 @@ function system_theme() {
         'theme_group_titles' => array(),
       ),
       'file' => 'system.admin.inc',
-      'template' => 'system-themes-page',
     ),
     'system_config_form' => array(
       'render element' => 'form',
-      'template' => 'system-config-form',
     ),
     'confirm_form' => array(
       'render element' => 'form',
-      'template' => 'confirm-form',
     ),
     'system_modules_details' => array(
       'render element' => 'form',
       'file' => 'system.admin.inc',
+      'function' => 'theme_system_modules_details',
     ),
     'system_modules_uninstall' => array(
       'render element' => 'form',
       'file' => 'system.admin.inc',
+      'function' => 'theme_system_modules_uninstall',
     ),
     'status_report' => array(
       'variables' => array('requirements' => NULL),
       'file' => 'system.admin.inc',
-      'template' => 'status-report',
     ),
     'admin_page' => array(
       'variables' => array('blocks' => NULL),
       'file' => 'system.admin.inc',
-      'template' => 'admin-page',
     ),
     'admin_block' => array(
       'variables' => array('block' => NULL),
       'file' => 'system.admin.inc',
-      'template' => 'admin-block',
     ),
     'admin_block_content' => array(
       'variables' => array('content' => NULL),
       'file' => 'system.admin.inc',
-      'template' => 'admin-block-content',
     ),
     'system_admin_index' => array(
       'variables' => array('menu_items' => NULL),
       'file' => 'system.admin.inc',
-      'template' => 'system-admin-index',
     ),
     'system_compact_link' => array(
       'variables' => array(),
+      'function' => 'theme_system_compact_link',
     ),
   ));
 }
diff --git a/core/modules/system/tests/modules/common_test/common_test.module b/core/modules/system/tests/modules/common_test/common_test.module
index bd17563..28c5be3 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -115,14 +115,13 @@ function common_test_theme() {
   return array(
     'common_test_foo' => array(
       'variables' => array('foo' => 'foo', 'bar' => 'bar'),
-      'template' => 'common-test-foo',
     ),
     'common_test_render_element' => array(
       'render element' => 'foo',
-      'template' => 'common-test-render-element',
     ),
     'common_test_empty' => array(
       'variables' => array('foo' => 'foo'),
+      'function' => 'theme_common_test_empty',
     ),
   );
 }
diff --git a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
index e963c2f..974856a 100644
--- a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
+++ b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
@@ -12,6 +12,7 @@ function theme_suggestions_test_theme() {
   $items['theme_suggestions_test_include'] = array(
     'file' => 'theme_suggestions_test.inc',
     'variables' => array(),
+    'function' => 'theme_theme_suggestions_test_include',
   );
   return $items;
 }
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index dd71fb7..e2378fb 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -9,6 +9,7 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test'] = array(
     'file' => 'theme_test.inc',
     'variables' => array('foo' => ''),
+    'function' => 'theme_theme_test',
   );
   $items['theme_test_template_test'] = array(
     'template' => 'theme_test.template_test',
@@ -17,39 +18,39 @@ function theme_test_theme($existing, $type, $theme, $path) {
     'template' => 'theme_test.template_test',
   );
   $items['theme_test_suggestion_provided'] = array(
-    'template' => 'theme-test-suggestion-provided',
     'variables' => array(),
   );
   $items['theme_test_specific_suggestions'] = array(
-    'template' => 'theme-test-specific-suggestions',
     'variables' => array(),
   );
   $items['theme_test_suggestions'] = array(
-    'template' => 'theme-test-suggestions',
     'variables' => array(),
   );
   $items['theme_test_general_suggestions'] = array(
-    'template' => 'theme-test-general-suggestions',
     'variables' => array(),
   );
   $items['theme_test_function_suggestions'] = array(
     'variables' => array(),
+    'function' => 'theme_theme_test_function_suggestions',
   );
   $items['theme_test_suggestions_include'] = array(
     'variables' => array(),
+    'function' => 'theme_theme_test_suggestions_include',
   );
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
+    'function' => 'theme_theme_test_foo',
   );
   $items['theme_test_render_element'] = array(
     'render element' => 'elements',
-    'template' => 'theme-test-render-element',
   );
   $items['theme_test_render_element_children'] = array(
     'render element' => 'element',
+    'function' => 'theme_theme_test_render_element_children',
   );
   $items['theme_test_function_template_override'] = array(
     'variables' => array(),
+    'function' => 'theme_theme_test_function_template_override',
   );
   $info['test_theme_not_existing_function'] = array(
     'function' => 'test_theme_not_existing_function',
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index c8db9d6..fb8f857 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -17,11 +17,9 @@ function twig_theme_test_theme($existing, $type, $theme, $path) {
   );
   $items['twig_namespace_test'] = array(
     'variables' => array(),
-    'template' => 'twig_namespace_test',
   );
   $items['twig_raw_test'] = array(
     'variables' => array('script' => ''),
-    'template' => 'twig-raw-test',
   );
   $items['twig_autoescape_test'] = array(
     'variables' => array('script' => ''),
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 089334f..18a2cb7 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -49,7 +49,6 @@
  *       'plugin_id' => NULL,
  *     ),
  *    'file' => 'search.pages.inc',
- *    'template' => 'search-result',
  *   ),
  * );
  * @endcode
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index 7b26e7b..75b3b81 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -219,7 +219,6 @@ function taxonomy_theme() {
   return array(
     'taxonomy_term' => array(
       'render element' => 'elements',
-      'template' => 'taxonomy-term',
     ),
   );
 }
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 49e2530..11bade8 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -50,10 +50,6 @@ function toolbar_permission() {
 function toolbar_theme($existing, $type, $theme, $path) {
   $items['toolbar'] = array(
     'render element' => 'element',
-    'template' => 'toolbar',
-  );
-  $items['toolbar_item'] = array(
-    'render element' => 'element',
   );
 
   return $items;
@@ -96,7 +92,6 @@ function toolbar_element_info() {
   // property contains a renderable array.
   $elements['toolbar_item'] = array(
     '#pre_render' => array('toolbar_pre_render_item'),
-    '#theme' => 'toolbar_item',
     'tab' => array(
       '#type' => 'link',
       '#title' => NULL,
diff --git a/core/modules/update/update.module b/core/modules/update/update.module
index 6529a48..09ce0fc 100644
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -176,24 +176,20 @@ function update_theme() {
   return array(
     'update_last_check' => array(
       'variables' => array('last' => 0),
-      'template' => 'update-last-check',
     ),
     'update_report' => array(
       'variables' => array('data' => NULL),
       'file' => 'update.report.inc',
-      'template' => 'update-report',
     ),
     'update_project_status' => array(
       'variables' => array('project' => array(), 'includes_status' => array()),
       'file' => 'update.report.inc',
-      'template' => 'update-project-status',
     ),
+    // We are using template instead of '#type' => 'table' here to keep markup
+    // out of preprocess and allow for easier changes to markup.
     'update_version' => array(
       'variables' => array('version' => NULL, 'title' => NULL, 'attributes' => array()),
       'file' => 'update.report.inc',
-      // We are using template instead of '#type' => 'table' here to keep markup
-      // out of preprocess and allow for easier changes to markup.
-      'template' => 'update-version',
     ),
   );
 }
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index eb3d5ae..e8580e7 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -100,11 +100,9 @@ function user_theme() {
     'user' => array(
       'render element' => 'elements',
       'file' => 'user.pages.inc',
-      'template' => 'user',
     ),
     'username' => array(
       'variables' => array('account' => NULL, 'attributes' => array(), 'link_options' => array()),
-      'template' => 'username',
     ),
   );
 }
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index 3ba93f5..6d796eb 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -121,7 +121,6 @@ function views_theme($existing, $type, $theme, $path) {
   // Our extra version of pager from pager.inc
   $hooks['views_mini_pager'] = $base + array(
     'variables' => array('tags' => array(), 'quantity' => 9, 'element' => 0, 'parameters' => array()),
-    'template' => 'views-mini-pager',
   );
 
   $variables = array(
@@ -155,10 +154,10 @@ function views_theme($existing, $type, $theme, $path) {
   // Default view themes
   $hooks['views_view_field'] = $base + array(
     'variables' => array('view' => NULL, 'field' => NULL, 'row' => NULL),
+    'function' => 'theme_views_view_field',
   );
   $hooks['views_view_grouping'] = $base + array(
     'variables' => array('view' => NULL, 'grouping' => NULL, 'grouping_level' => NULL, 'rows' => NULL, 'title' => NULL),
-    'template' => 'views-view-grouping',
   );
 
   $plugins = Views::getPluginDefinitions();
@@ -187,35 +186,44 @@ function views_theme($existing, $type, $theme, $path) {
         'variables' => $variables[$type],
       );
 
-      // For the views module we ensure views.theme.inc is included.
-      if ($def['provider'] == 'views') {
-        $def['theme_file'] = 'views.theme.inc';
-      }
       // We always use the module directory as base dir.
       $module_dir = drupal_get_path('module', $def['provider']);
+      $hooks[$def['theme']]['path'] = $module_dir;
 
+      // For the views module we ensure views.theme.inc is included.
+      if ($def['provider'] == 'views') {
+        if (!isset($hooks[$def['theme']]['includes'])) {
+          $hooks[$def['theme']]['includes'] = array();
+        }
+        if (!in_array('views.theme.inc', $hooks[$def['theme']]['includes'])) {
+          $hooks[$def['theme']]['includes'][] = $module_dir . '/views.theme.inc';
+        }
+      }
       // The theme_file definition is always relative to the modules directory.
-      if (isset($def['theme_file'])) {
-        $hooks[$def['theme']]['path'] = $module_dir;
+      elseif (!empty($def['theme_file'])) {
         $hooks[$def['theme']]['file'] = $def['theme_file'];
       }
+
       // Whenever we got a theme file, we include it directly so we can
       // auto-detect the theme function.
       if (isset($def['theme_file'])) {
-        $include = DRUPAL_ROOT . '/' . $module_dir. '/' . $def['theme_file'];
+        $include = DRUPAL_ROOT . '/' . $module_dir . '/' . $def['theme_file'];
         if (is_file($include)) {
           require_once $include;
         }
       }
-     // If there is no theme function for the given theme definition, we assume
-     // a template file shall be used. By default this file is located in the
-     // /templates directory of the module's folder.
-     // If a module wants to define its own location it has to set
-     // register_theme of the plugin to FALSE and implement hook_theme() by
-     // itself.
+
+      // If there is no theme function for the given theme definition, it must
+      // be a template file. By default this file is located in the /templates
+      // directory of the module's folder. If a module wants to define its own
+      // location it has to set register_theme of the plugin to FALSE and
+      // implement hook_theme() by itself.
       if (!function_exists('theme_' . $def['theme'])) {
-        $hooks[$def['theme']]['path'] = $module_dir;
-        $hooks[$def['theme']]['template'] = 'templates/' . drupal_clean_css_identifier($def['theme']);
+        $hooks[$def['theme']]['path'] .= '/templates';
+        $hooks[$def['theme']]['template'] = drupal_clean_css_identifier($def['theme']);
+      }
+      else {
+        $hooks[$def['theme']]['function'] = 'theme_' . $def['theme'];
       }
     }
   }
@@ -225,13 +233,11 @@ function views_theme($existing, $type, $theme, $path) {
   );
 
   $hooks['views_exposed_form'] = $base + array(
-    'template' => 'views-exposed-form',
     'render element' => 'form',
   );
 
   $hooks['views_more'] = $base + array(
     'variables' => array('more_url' => NULL, 'link_text' => 'more', 'view' => NULL),
-    'template' => 'views-more',
   );
 
   return $hooks;
diff --git a/core/modules/views_ui/views_ui.module b/core/modules/views_ui/views_ui.module
index 3960294..f083adc 100644
--- a/core/modules/views_ui/views_ui.module
+++ b/core/modules/views_ui/views_ui.module
@@ -70,57 +70,50 @@ function views_ui_theme() {
     // edit a view
     'views_ui_display_tab_setting' => array(
       'variables' => array('description' => '', 'link' => '', 'settings_links' => array(), 'overridden' => FALSE, 'defaulted' => FALSE, 'description_separator' => TRUE, 'class' => array()),
-      'template' => 'views-ui-display-tab-setting',
       'file' => 'views_ui.theme.inc',
     ),
     'views_ui_display_tab_bucket' => array(
       'render element' => 'element',
-      'template' => 'views-ui-display-tab-bucket',
       'file' => 'views_ui.theme.inc',
     ),
     'views_ui_rearrange_filter_form' => array(
       'render element' => 'form',
       'file' => 'views_ui.theme.inc',
-      'template' => 'views-ui-rearrange-filter-form',
     ),
     'views_ui_expose_filter_form' => array(
       'render element' => 'form',
       'file' => 'views_ui.theme.inc',
-      'template' => 'views-ui-expose-filter-form',
     ),
 
     // list views
     'views_ui_view_info' => array(
       'variables' => array('view' => NULL, 'displays' => NULL),
       'file' => 'views_ui.theme.inc',
-      'template' => 'views-ui-view-info',
     ),
 
     // Group of filters.
     'views_ui_build_group_filter_form' => array(
       'render element' => 'form',
       'file' => 'views_ui.theme.inc',
+      'function' => 'theme_views_ui_build_group_filter_form',
     ),
 
     // On behalf of a plugin
     'views_ui_style_plugin_table' => array(
       'render element' => 'form',
       'file' => 'views_ui.theme.inc',
-      'template' => 'views-ui-style-plugin-table',
     ),
 
     // When previewing a view.
     'views_ui_view_preview_section' => array(
       'variables' => array('view' => NULL, 'section' => NULL, 'content' => NULL, 'links' => ''),
       'file' => 'views_ui.theme.inc',
-      'template' => 'views-ui-view-preview-section',
     ),
 
     // Generic container wrapper, to use instead of theme_container when an id
     // is not desired.
     'views_ui_container' => array(
       'variables' => array('children' => NULL, 'attributes' => array()),
-      'template' => 'views-ui-container',
       'file' => 'views_ui.theme.inc',
     ),
   );
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index e0cfcc2..06ec07c 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -38,23 +38,28 @@ function twig_init(Extension $theme) {
  * If the Twig debug setting is enabled, HTML comments including _theme() call
  * and template file name suggestions will surround the template markup.
  *
- * @param $template_file
+ * @param string $template_file
  *   The file name of the template to render.
- * @param $variables
+ * @param array $variables
  *   A keyed array of variables that will appear in the output.
  *
- * @return
+ * @return string
  *   The output generated by the template, plus any debug information.
  */
 function twig_render_template($template_file, $variables) {
   /** @var \Twig_Environment $twig_service */
   $twig_service = \Drupal::service('twig');
-  $output = array(
-    'debug_prefix'    => '',
-    'debug_info'      => '',
-    'rendered_markup' => $twig_service->loadTemplate($template_file)->render($variables),
-    'debug_suffix'    => '',
-  );
+  try {
+    $output = array(
+      'debug_prefix'    => '',
+      'debug_info'      => '',
+      'rendered_markup' => $twig_service->loadTemplate($template_file)->render($variables),
+      'debug_suffix'    => '',
+    );
+  }
+  catch (\Twig_Error_Loader $e) {
+    drupal_set_message($e->getMessage(), 'error');
+  }
   if ($twig_service->isDebug()) {
     $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
     $output['debug_prefix'] .= "\n<!-- CALL: _theme('{$variables['theme_hook_original']}') -->";