.../lib/Drupal/system/Tests/Theme/ThemeTest.php    |   22 ++++++++++++++++++++
 .../templates/theme-test-render-element.tpl.php    |    1 +
 .../tests/modules/theme_test/theme_test.module     |   10 +++++++++
 3 files changed, 33 insertions(+)

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 20ccc2b..cdf5d7b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -36,6 +36,28 @@ function setUp() {
   }
 
   /**
+   * Test attribute merging.
+   *
+   * Render arrays that use a render element and templates (and hence call
+   * template_preprocess()) must ensure the attributes at different occassions
+   * are all merged correctly:
+   *   - $variables['attributes'] as passed in to theme()
+   *   - the render element's #attributes
+   *   - any attributes set in the template's preprocessing function
+   */
+  function testAttributeMerging() {
+    $output = theme('theme_test_render_element', array(
+      'elements' => array(
+        '#attributes' => array('data-foo' => 'bar'),
+      ),
+      'attributes' => array(
+        'id' => 'bazinga',
+      ),
+    ));
+    $this->assertIdentical($output, '<div id="bazinga" data-foo="bar" class="theme-test-render-element" data-variables-are-preprocessed></div>' . "\n");
+  }
+
+  /**
    * Test function theme_get_suggestions() for SA-CORE-2009-003.
    */
   function testThemeSuggestions() {
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-render-element.tpl.php b/core/modules/system/tests/modules/theme_test/templates/theme-test-render-element.tpl.php
new file mode 100644
index 0000000..f301d31
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-render-element.tpl.php
@@ -0,0 +1 @@
+<div<?php print $attributes; ?>></div>
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 137fdfc..c51eb0c 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -17,6 +17,10 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
   );
+  $items['theme_test_render_element'] = array(
+    'render element' => 'elements',
+    'template' => 'theme-test-render-element',
+  );
   return $items;
 }
 
@@ -172,3 +176,9 @@ function theme_theme_test_foo($variables) {
   return $variables['foo'];
 }
 
+/**
+ * Process variables for theme-test-render-element.tpl.php.
+ */
+function template_preprocess_theme_test_render_element(&$variables) {
+  $variables['attributes']['data-variables-are-preprocessed'] = TRUE;
+}