Issue #3334045 by yash.rode, joachim, smustgrave, quietone:...

Issue #3334045 by yash.rode, joachim, smustgrave, quietone: Drupal\Core\Template\Attribute doesn't support adding attributes with array syntax if attribute name not already initialised

core/lib/Drupal/Core/Template/Attribute.php

@@ -95,6 +95,13 @@ public function offsetGet($name) {
     if (isset($this->storage[$name])) {
       return $this->storage[$name];
     }
+    // The 'class' array key is expected to be itself an array, and therefore
+    // can be accessed using array append syntax before it has been initialized.
+    if ($name === 'class') {
+      // Initialize the class attribute as an empty array if not set.
+      $this->offsetSet('class', []);
+      return $this->storage['class'];
+    }
   }
 
   /**

core/tests/Drupal/Tests/Core/Template/AttributeTest.php

@@ -148,12 +148,16 @@ public function testRemoveAttribute(): void {
    * @covers ::addClass
    */
   public function testAddClasses(): void {
-    // Add empty Attribute object with no classes.
+    // Add a class with the array syntax without first initializing the 'class'
+    // attribute.
     $attribute = new Attribute();
+    $attribute['class'][] = 'test-class';
+    $this->assertEquals(new AttributeArray('class', ['test-class']), $attribute['class']);
 
+    $attribute = new Attribute();
     // Add no class on empty attribute.
     $attribute->addClass();
-    $this->assertEmpty($attribute['class']);
+    $this->assertEmpty($attribute['class']->value());
 
     // Test various permutations of adding values to empty Attribute objects.
     foreach ([NULL, FALSE, '', []] as $value) {