diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 72b2b88..dac8591 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2317,10 +2317,12 @@ function drupal_common_theme() {
     'authorize_message' => array(
       'variables' => array('message' => NULL, 'success' => TRUE),
       'function' => 'theme_authorize_message',
+      'file' => 'theme.maintenance.inc',
     ),
     'authorize_report' => array(
       'variables' => array('messages' => array()),
       'function' => 'theme_authorize_report',
+      'file' => 'theme.maintenance.inc',
     ),
     // From pager.inc.
     'pager' => array(
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 1c883d5..e56f944 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -452,7 +452,11 @@ protected function processExtension(&$cache, $name, $type, $theme, $path) {
         // theme hook provides a theme function callback, check to ensure that
         // it actually exists.
         if (isset($info['function']) && !function_exists($info['function'])) {
-          continue;
+          throw new \BadFunctionCallException(sprintf(
+            'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
+            $hook,
+            $info['function']
+          ));
         }
         // Provide a default naming convention for 'template' based on the
         // hook used. If the template does not exist, the theme engine used
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 6364e90..88aecfc 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1100,14 +1100,17 @@ function hook_help($route_name, \Drupal\Core\Routing\RouteMatchInterface $route_
  *   - template: If specified, this theme implementation is a template, and
  *     this is the template file without an extension. If neither 'template'
  *     nor 'function' is specified, a default template name will be assumed.
- *     For example, if a module registers the 'node' theme hook, 'node'
- *     will be assigned to its template. Do not put .html.twig on this file;
- *     that extension will be added automatically by the default rendering
- *     engine (which is Twig). If 'path' above is specified, the template should
- *     also be in this path.
- *   - function: If specified, this will be the function name to invoke for
- *     this implementation. If the chameleon theme registers the node hook,
- *     it will be assigned 'chameleon_node' as its function.
+ *     For example, if a module registers the 'search_result' theme hook,
+ *     'search-result' will be assigned to its template. Do not put .html.twig
+ *     on this file, that extension will be added automatically by the default
+ *     rendering engine (which is Twig). If 'path' above is specified, the
+ *     template should also be in this path.
+ *   - function: If the theme implementation is not a template but a theme
+ *     function, you must specify the function name. For example, a theme hook
+ *     of 'search_result' would, by convention, use a theme function of
+ *     'theme_search_result'. So, if you are building a theme called 'chameleon'
+ *     , that registers a new 'search_result' hook and theme function, then you
+ *     would usually use 'chameleon_search_result' as the theme function name.
  *   - base hook: Used for _theme() suggestions only: the base theme hook name.
  *     Instead of this suggestion's implementation being used directly, the base
  *     hook will be invoked with this implementation as its first suggestion.