Issue #2061835 by thedavidmeister, Cottser: Fixed theme() doesn't enforce that...

Issue #2061835 by thedavidmeister, Cottser: Fixed theme() doesn't enforce that what it returns is a string or FALSE.

core/includes/theme.inc

@@ -1123,7 +1123,7 @@ function theme($hook, $variables = array()) {
 
   // restore path_to_theme()
   $theme_path = $temp;
-  return $output;
+  return (string) $output;
 }
 
 /**

core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php

@@ -57,6 +57,24 @@ function testAttributeMerging() {
     $this->assertIdentical($output, '<div id="bazinga" data-foo="bar" data-variables-are-preprocessed></div>' . "\n");
   }
 
+  /**
+   * Test that theme() returns expected data types.
+   */
+  function testThemeDataTypes() {
+    // theme_test_false is an implemented theme hook so theme() should return a
+    // string, even though the theme function itself can return anything.
+    $foos = array('null' => NULL, 'false' => FALSE, 'integer' => 1, 'string' => 'foo');
+    foreach ($foos as $type => $example) {
+      $output = theme('theme_test_foo', array('foo' => $example));
+      $this->assertTrue(is_string($output), format_string('theme() returns a string for data type !type.', array('!type' => $type)));
+    }
+
+    // suggestionnotimplemented is not an implemented theme hook so theme()
+    // should return FALSE instead of a string.
+    $output = theme(array('suggestionnotimplemented'));
+    $this->assertIdentical($output, FALSE, 'theme() returns FALSE when a hook suggestion is not implemented.');
+  }
+
   /**
    * Test function theme_get_suggestions() for SA-CORE-2009-003.
    */