Undefined array key warning in PhpUnitTestDiscovery

Problem/Motivation

Currently, running the phpunit job in at least some contrib modules will print the following warning several times:

PHP Warning: Undefined array key 0 in /builds/project/search_api/web/core/lib/Drupal/Core/Test/PhpUnitTestDiscovery.php on line 410

(Line number varies by Drupal version, of course, but that’s the one in 11.4.x dev.)

It seems that in \Drupal\Core\Test\PhpUnitTestDiscovery::getTestClassInfo() even though there is an explicit check whether $groups is empty, it can then happen that $groups[0] is not set. My conclusion would be that $groups is not necessarily a list but could be an associative array.

Examples can be seen here and here.

Steps to reproduce

Run the phpunit job of a contrib module. It might only produce the error when test classes use the @group annotation instead of the Group attribute but that is simply a guess.

Proposed resolution

Maybe use array_first($groups) instead of $groups[0]? (It’s part of symfony/polyfill-php85 so should be fine to use even though we don’t require PHP 8.5 yet.)

Remaining tasks

• Create MR.
• Discuss solution.
• Review MR.

User interface changes

N/A

Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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