Link render element overwrites CSS class

Problem/Motivation

The Link render element supports supplying attributes both in $element['#attributes'] and $element['#options']['attributes']. These are combined with an array + operator, which does not combine classes when they are present in both array elements.

Steps to reproduce

As a site builder:

The Twig extension add_class() sets the class in the #attributes. For a link, there may already be a class in the #options. This happens in radix v6 for local tasks and is exhibited when viewing a page which is a secondary local task. The primary local task has a class of "is-active" and radix adds classes "nav-link" and "active", which fails.

1. Install Radix v6.
2. Create a page with primary and secondary local tasks.
3. Visit the page with the secondary local task.
4. Notice that the primary task tab is display incorrectly and contains only the "is-active" class.

As a developer:

1. Create a custom module with a controller.
2. Make the controller render this element:
   

   [
     '#type' => 'link',
     '#title' => 'title',
     '#url' => Url::fromUri('https://www.drupal.org')->setOption('class', ['url-option-class']),
     '#attributes' => [
       'class' => ['attributes-class'],
     ],
     '#options' => [
       'attributes' => [
         'class' => ['options-attributes-class'],
       ],
     ],
   ]
   

3. Visit the controller page, inspect the link and check that only attributes-class class is added to the link.

Proposed resolution

Merge the classes from ['#attributes']['class'] and ['#options']['attributes']['class'] when rendering a link.

Remaining tasks

• Fix. Done
• Tests. Done

User interface changes

None

Introduced terminology

None

API changes

Now link elements render both ['#attributes']['class'] and ['#options']['attributes']['class'] classes.

Data model changes

None

Release notes snippet

Now link render elements render both ['#attributes']['class'] and ['#options']['attributes']['class'] classes. Some unexpected visual changes may arise due to new classes possibly being added to links.