diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 3be853b..f2fb8a9 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2603,6 +2603,8 @@ function drupal_common_theme() {
     'task_list' => array(
       'variables' => array('items' => NULL, 'active' => NULL,  'variant' => NULL),
       'function' => 'theme_task_list',
+      'path' => 'core/includes',
+      'file' => 'theme.maintenance.inc',
     ),
     'authorize_message' => array(
       'variables' => array('message' => NULL, 'success' => TRUE),
@@ -2621,6 +2623,8 @@ function drupal_common_theme() {
     'menu_link' => array(
       'render element' => 'element',
       'function' => 'theme_menu_link',
+      'path' => 'core/includes',
+      'file' => 'menu.inc',
     ),
     'menu_tree' => array(
       'render element' => 'tree',
diff --git a/core/lib/Drupal/Core/Template/TwigEnvironment.php b/core/lib/Drupal/Core/Template/TwigEnvironment.php
index c671899..ae7179a 100644
--- a/core/lib/Drupal/Core/Template/TwigEnvironment.php
+++ b/core/lib/Drupal/Core/Template/TwigEnvironment.php
@@ -40,17 +40,19 @@ public function __construct(\Twig_LoaderInterface $loader = NULL, $options = arr
 
     // Set twig path namespace for themes and modules.
     $namespaces = array();
-    foreach ($module_handler->getModuleList() as $name => $extension) {
-      $namespaces[$name] = $extension->getPath();
+    foreach ($module_handler->getModuleList() as $extension) {
+      $namespaces[] = $extension->getPath();
     }
-    foreach ($theme_handler->listInfo() as $name => $extension) {
-      $namespaces[$name] = $extension->getPath();
+    foreach ($theme_handler->listInfo() as $extension) {
+      $namespaces[] = $extension->getPath();
     }
 
-    foreach ($namespaces as $name => $path) {
-      $templatesDirectory = $path . '/templates';
-      if (file_exists($templatesDirectory)) {
-        $loader->addPath($templatesDirectory, $name);
+    foreach ($namespaces as $path) {
+      $templates_dir = DRUPAL_ROOT . '/' . $path . '/templates';
+      if (file_exists($templates_dir)) {
+        // We currently do not use namespaces in Twig. We must add all possible
+        // template directory paths to the __main__ namespace.
+        $loader->addPath($templates_dir);
       }
     }
 
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 04e5882..abaad13 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -389,6 +389,9 @@ 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) {
     $result = array();
@@ -418,12 +421,6 @@ protected function processExtension(&$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]['template'] = strtr($hook, '_', '-');
-        }
-
         if (isset($cache[$hook]['includes'])) {
           $result[$hook]['includes'] = $cache[$hook]['includes'];
         }
@@ -439,20 +436,36 @@ 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, it should only
+        // override existing template implementations.
+        if (isset($info['function'])) {
+          // Ensure theming function exists and remove any provided template.
+          if (function_exists($info['function'])) {
+            unset($result[$hook]['template']);
+          }
+          else {
+            throw new \BadFunctionCallException(sprintf(
+              'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
+              $hook,
+              $info['function']
+            ));
+          }
+        }
+        // Provide a default naming convention for 'template' based on the
+        // hook used. If the template does not exist, the template engine used
+        // (Twig) will throw throw an exception at runtime when attempting to
+        // include the template file (\Twig_Error_Loader).
+        elseif (!isset($info['template'])) {
+          $result[$hook]['template'] = strtr($hook, '_', '-');
+        }
+
         // If the default keys are not set, use the default values registered
         // by the module.
         if (isset($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($info['preprocess functions'])) {
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 2b838d6..db2d953 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -50,11 +50,9 @@ function toolbar_permission() {
 function toolbar_theme($existing, $type, $theme, $path) {
   $items['toolbar'] = array(
     'render element' => 'element',
-    'template' => 'toolbar',
   );
   $items['toolbar_item'] = array(
     'render element' => 'element',
-    'function' => 'theme_toolbar_item',
   );
 
   return $items;
@@ -96,7 +94,7 @@ function toolbar_element_info() {
   // property contains a renderable array.
   $elements['toolbar_item'] = array(
     '#pre_render' => array('toolbar_pre_render_item'),
-    '#theme' => 'toolbar_item',
+    '#theme' => 'theme_toolbar_item',
     'tab' => array(
       '#type' => 'link',
       '#title' => NULL,
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index 1acf4b5..59c5be9 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -121,8 +121,6 @@ function views_theme($existing, $type, $theme, $path) {
   // Our extra version of pager from pager.inc
   $hooks['views_mini_pager'] = $base + array(
     'variables' => array('tags' => array(), 'quantity' => 10, 'element' => 0, 'parameters' => array()),
-    'template' => 'views-mini-pager',
-    'function' => 'theme_views_mini_pager',
   );
 
   $variables = array(
@@ -146,7 +144,6 @@ function views_theme($existing, $type, $theme, $path) {
   );
   $hooks['views_view_grouping'] = $base + array(
     'variables' => array('view' => NULL, 'grouping' => NULL, 'grouping_level' => NULL, 'rows' => NULL, 'title' => NULL),
-    'template' => 'views-view-grouping',
   );
 
   $plugins = Views::getPluginDefinitions();
@@ -202,8 +199,8 @@ function views_theme($existing, $type, $theme, $path) {
      // 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'] = $module_dir . '/templates';
+        $hooks[$def['theme']]['template'] = drupal_clean_css_identifier($def['theme']);
       }
       else {
         $hooks[$def['theme']]['function'] = 'theme_' . $def['theme'];
@@ -213,17 +210,14 @@ function views_theme($existing, $type, $theme, $path) {
 
   $hooks['views_form_views_form'] = $base + array(
     'render element' => 'form',
-    'function' => 'theme_views_form_views_form',
   );
 
   $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/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 1595bf8..b6d8fdb 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -36,22 +36,27 @@ 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) {
   $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']}') -->";