Drupal\Core\Controller\TitleResolver::getTitle() can result in PHP 8.1 deprecation notices due to NULL being set as default

Suggested patch

Another solution could be to modify hasDefault to use !empty or isset instead of array_key_exists but that seems more intrusive than the suggested patch.

Don't think there is an issue with the current solution. But a test case showing the problem will be needed.

AFAICS the current implementation is just wrong: it uses strlen on the result of a getDefault, which may be NULL because it only checks that the key exists, not that the resulting value is not empty; and strlen(NULL) is deprecated since PHP 8.1, so any configuration where that function fetches something which can be NULL will return NULL, and any place where that result is used as the argument for strlen can now trigger that deprecation warning.

fgm, thank you for the patch.
It fixed 2 warnings on the node edit page with Drupal 9.5.9.

It's hard to define the reason of these warnings. Site uses a lot of contrib modules and custom alterations. Even on this node edit page.
One of them because of token replacements:

Deprecated function: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\Core\Controller\TitleResolver->getTitle() (line 60 of /var/www/docroot/core/lib/Drupal/Core/Controller/TitleResolver.php)
#0 /var/www/docroot/core/includes/bootstrap.inc(347): _drupal_error_handler_real(8192, 'strlen(): Passi...', '/var/www/docroo...', 60)
#1 [internal function]: _drupal_error_handler(8192, 'strlen(): Passi...', '/var/www/docroo...', 60)
#2 /var/www/docroot/core/lib/Drupal/Core/Controller/TitleResolver.php(60): strlen(NULL)
#3 /var/www/docroot/modules/contrib/token/token.tokens.inc(843): Drupal\Core\Controller\TitleResolver->getTitle(Object(Symfony\Component\HttpFoundation\Request), Object(Symfony\Component\Routing\Route))
#4 [internal function]: token_tokens('current-page', Array, Array, Array, Object(Drupal\Core\Render\BubbleableMetadata))
#5 /var/www/docroot/core/lib/Drupal/Core/Extension/ModuleHandler.php(426): call_user_func_array(Object(Closure), Array)
#6 /var/www/docroot/core/lib/Drupal/Core/Extension/ModuleHandler.php(405): Drupal\Core\Extension\ModuleHandler->Drupal\Core\Extension\{closure}(Object(Closure), 'token')
#7 /var/www/docroot/core/lib/Drupal/Core/Extension/ModuleHandler.php(433): Drupal\Core\Extension\ModuleHandler->invokeAllWith('tokens', Object(Closure))
#8 /var/www/docroot/core/lib/Drupal/Core/Utility/Token.php(359): Drupal\Core\Extension\ModuleHandler->invokeAll('tokens', Array)
#9 /var/www/docroot/core/lib/Drupal/Core/Utility/Token.php(241): Drupal\Core\Utility\Token->generate('current-page', Array, Array, Array, Object(Drupal\Core\Render\BubbleableMetadata))
#10 /var/www/docroot/core/lib/Drupal/Core/Utility/Token.php(191): Drupal\Core\Utility\Token->doReplace(true, '[current-page:t...', Array, Array, Object(Drupal\Core\Render\BubbleableMetadata))
#11 /var/www/docroot/modules/contrib/metatag/src/MetatagToken.php(66): Drupal\Core\Utility\Token->replace('[current-page:t...', Array, Array, NULL)
#12 /var/www/docroot/modules/contrib/metatag/src/MetatagManager.php(771): Drupal\metatag\MetatagToken->replace('[current-page:t...', Array, Array)
#13 /var/www/docroot/modules/contrib/metatag/src/MetatagManager.php(617): Drupal\metatag\MetatagManager->processTagValue(Object(Drupal\metatag\Plugin\metatag\Tag\Title), Array, Array, false, 'en')
#14 /var/www/docroot/modules/contrib/metatag/src/MetatagManager.php(549): Drupal\metatag\MetatagManager->generateRawElements(Array, NULL)
#15 /var/www/docroot/modules/contrib/metatag/metatag.module(521): Drupal\metatag\MetatagManager->generateElements(Array, NULL)
#16 /var/www/docroot/modules/contrib/metatag/metatag.module(130): metatag_get_tags_from_route()
#17 /var/www/docroot/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(315): metatag_page_attachments(Array)
#18 /var/www/docroot/core/lib/Drupal/Core/Extension/ModuleHandler.php(405): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}(Object(Closure), 'metatag')
#19 /var/www/docroot/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(316): Drupal\Core\Extension\ModuleHandler->invokeAllWith('page_attachment...', Object(Closure))
#20 /var/www/docroot/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(289): Drupal\Core\Render\MainContent\HtmlRenderer->invokePageAttachmentHooks(Array)
#21 /var/www/docroot/core/lib/Drupal/Core/Render/Renderer.php(580): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
#22 /var/www/docroot/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(290): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#23 /var/www/docroot/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(132): Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))

Glad to see I'm not alone in this. Since you identified the problem as token-related, do you think you could create a test showing the problem so that a patch can be accepted ?

I converted the patch in #3 to an MR and added a unit test.

Still have a hard time seeing the use case of setting the title to NULL but does have test coverage so lets see what the committers think.