Drupal\Core\Template\Attribute doesn't support adding attributes with array syntax if attribute name not already initialised

I tried to reproduce what's mentioned in the issue but i was not able to do that.I will write some steps that i took trying to reproduce the issue.
Steps to reproduce:-
1) Install the ajax form test module.
2) go to /ajax_forms_test_get_form url.
3) In the buildform method I modified the following select element as below:-

$form['select'] = [
      '#title' => $this->t('Color'),
      '#type' => 'select',
      '#options' => [
        'red' => 'red',
        'green' => 'green',
        'blue' => 'blue',
      ],
      '#attributes' => $attributes->toArray(),
      '#ajax' => [
        'callback' => [$object, 'selectCallback'],
      ],
      '#suffix' => '<div id="ajax_selected_color">No color yet selected</div>',
    ];
    $form['select']['#attributes']['class'][] = 'TestingAttributes';

With the above changes applied I should get the error mentioned above, but i was able to get the class rendered on the page.
Attaching the SS for the same.

I also tried the same steps on system.admin.inc file and modified the way attributes is added on line 285 and 294 but there also i was able to get the class rendered.

It would be great if someone who can reproduce this, add some steps to reproduce in the issue summary.

The steps to reproduce are already in the issue summary.

yash.rode → made their first commit to this issue’s fork.

Thanks for working on this!

Tests are failing however.

Also, I don't think it's a good idea to put a special case for a particular data provider in a data-provided test method. Better to add a dedicated test method for this case. It's a unit test anyway, so it's fast.

This is ready for another round of review.

Sorry can the issue summary be updated to include a proposed solution. Not sure if this is an api change or not (didn't look deeply).

Thanks.

This isn't an API change, it's fixing a PHP error you get if you try to use Attribute with the array access as documented.

The proposed solution is 'make it work properly'.

Not sure that's a proper summary lol so will just leave here for the next reviewer. Seen enough issues kicked back for not having proper issue summary.

I'll remove the 'TBD' because that doesn't look good, but honestly I don't see that there's anything relevant to put there.

Doing $attributes['class'][] = 'black-white-cat'; produces an error. Make it not produce an error.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Rebased.

Believe feedback on this one has been addressed.

Left some comments in the MR asking for more comments and some simplification.

I think I've addressed everything -- the MR is not letting me mark things are resolved?!?!

Appears to have a cspell error

And only the original author of the MR can close threads. Which is a bummer

Believe feedback has been addressed.

Committed/pushed to 11.x and cherry-picked to 11.0.x. I think this is worth backporting to 10.4.x but it doesn't cherry-pick cleanly so moving to 'to be ported'.

catch → closed merge request !8310