Issue #2837013 by benjifisher, leslieg, alexpott: clean_class Twig filter does not work with Views rewriting

core/lib/Drupal/Component/Utility/Html.php

@@ -61,13 +61,15 @@ class Html {
    * Do not pass one string containing multiple classes as they will be
    * incorrectly concatenated with dashes, i.e. "one two" will become "one-two".
    *
-   * @param string $class
-   *   The class name to clean.
+   * @param mixed $class
+   *   The class name to clean. It can be a string or anything that can be cast
+   *   to string.
    *
    * @return string
    *   The cleaned class name.
    */
   public static function getClass($class) {
+    $class = (string) $class;
     if (!isset(static::$classes[$class])) {
       static::$classes[$class] = static::cleanCssIdentifier(Unicode::strtolower($class));
     }

core/tests/Drupal/Tests/Component/Utility/HtmlTest.php

@@ -2,6 +2,8 @@
 
 namespace Drupal\Tests\Component\Utility;
 
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Render\MarkupTrait;
 use Drupal\Component\Utility\Html;
 use Drupal\Tests\UnitTestCase;
 
@@ -87,7 +89,12 @@ class HtmlTest extends UnitTestCase {
    */
   public function testHtmlClass() {
     // Verify Drupal coding standards are enforced.
-    $this->assertSame(Html::getClass('CLASS NAME_[Ü]'), 'class-name--ü', 'Enforce Drupal coding standards.');
+    $this->assertSame('class-name--ü', Html::getClass('CLASS NAME_[Ü]'), 'Enforce Drupal coding standards.');
+
+    // Test Html::getClass() handles Drupal\Component\Render\MarkupInterface
+    // input.
+    $markup = HtmlTestMarkup::create('CLASS_FROM_OBJECT');
+    $this->assertSame('class-from-object', Html::getClass($markup), 'Markup object is converted to CSS class.');
   }
 
   /**
@@ -390,3 +397,11 @@ class HtmlTest extends UnitTestCase {
   }
 
 }
+
+/**
+ * Marks an object's __toString() method as returning markup.
+ */
+class HtmlTestMarkup implements MarkupInterface {
+  use MarkupTrait;
+
+}