From c281a1279548ad7d16caf504ee3ad2346eaa0d7d Mon Sep 17 00:00:00 2001
From: David Meister <thedavidmeister@gmail.com>
Date: Sun, 9 Jun 2013 20:13:56 +1000
Subject: [PATCH] 2012812-20

---
 core/includes/common.inc                           | 37 +++++++++++++---------
 core/includes/theme.inc                            |  5 ++-
 .../lib/Drupal/system/Tests/Theme/ThemeTest.php    |  2 +-
 3 files changed, 27 insertions(+), 17 deletions(-)

diff --git a/core/includes/common.inc b/core/includes/common.inc
index 00decfa..cb7d4ac 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4800,31 +4800,38 @@ function drupal_render(&$elements) {
   // Initialize this element's #children, unless a #pre_render callback already
   // preset #children.
   if (!isset($elements['#children'])) {
-    $elements['#children'] = '';
+    $elements['#children'] = NULL;
   }
   // Call the element's #theme function if it is set. Then any children of the
   // element have to be rendered there. If the internal #render_children
   // property is set, do not call the #theme function to prevent infinite
   // recursion.
+  $theme_is_implemented = FALSE;
   if (isset($elements['#theme']) && !isset($elements['#render_children'])) {
     $elements['#children'] = theme($elements['#theme'], $elements);
-  }
-  // If #theme was not set and the element has children, render them now.
-  // This is the same process as drupal_render_children() but is inlined
-  // for speed.
-  if ($elements['#children'] === '') {
+    // If theme() returns NULL this means that the hook in #theme was not found
+    // in the registry. This is common for theme suggestions.
+    $theme_is_implemented = isset($elements['#children']);
+  }
+  // #theme is either not set or does not exist in the registry.
+  if (!$theme_is_implemented) {
+    // If #theme is not implemented and the element has children, render them
+    // now. This is the same process as drupal_render_children() but is inlined
+    // for speed.
     foreach ($children as $key) {
       $elements['#children'] .= drupal_render($elements[$key]);
     }
-  }
-  // If #theme was not set, but the element has raw #markup, prepend the content
-  // in #markup to #children. #children may contain the rendered content
-  // supplied by #theme, or the rendered child elements, as processed above. If
-  // both #theme and #markup are set, then #theme is responsible for rendering
-  // the element. Eventually assigned #theme_wrappers will expect both the
-  // element's #markup and the rendered content of child elements in #children.
-  if (!isset($elements['#theme']) && isset($elements['#markup'])) {
-    $elements['#children'] = $elements['#markup'] . $elements['#children'];
+
+    // If #theme is not implemented and the element has raw #markup as a
+    // fallback, prepend the content in #markup to #children. In this case
+    // #children will contain whatever is provided by #pre_render prepended to
+    // what is rendered recursively above. If #theme is implemented then it is
+    // the responsibility of that theme implementation to render #markup if
+    // required. Eventually #theme_wrappers will expect both #markup and
+    // #children to be a single string as #children.
+    if (isset($elements['#markup'])) {
+      $elements['#children'] = $elements['#markup'] . $elements['#children'];
+    }
   }
 
   // Add any JavaScript state information associated with the element.
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index a3f967a..35269f0 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -978,7 +978,10 @@ function theme($hook, $variables = array()) {
       if (!isset($candidate)) {
         watchdog('theme', 'Theme hook %hook not found.', array('%hook' => $hook), WATCHDOG_WARNING);
       }
-      return '';
+      // There is no theme implementation for the hook passed. Return NULL so
+      // the function calling theme() can differentiate between a hook that
+      // exists and renders an empty string and a hook that is not implemented.
+      return NULL;
     }
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 5054555..e3a9a9d 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -182,7 +182,7 @@ function testRegistryRebuild() {
     // throws an exception.
     $this->rebuildContainer();
     $this->container->get('module_handler')->loadAll();
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), '', 'The theme registry does not contain theme_test_foo, because the module is disabled.');
+    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), NULL, 'The theme registry does not contain theme_test_foo, because the module is disabled.');
 
     module_enable(array('theme_test'), FALSE);
     // After enabling/disabling a module during a test, we need to rebuild the
-- 
1.7.11.1