[PHP8.1] automatic conversion of false to array is deprecated

What are the steps to reproduce this? In many similar cases contributed or custom code is misusing an API.

Thank you for reporting the issue.

#5 tags for steps to reproduce which is 100% needed. This could be a symptom of a bigger issue.

Will need a test case showing the issue.

I also found a similar error in another file:

Deprecated function: Automatic conversion of false to array is deprecated en Drupal\Core\Form\FormBuilder->prepareForm() (línea 766 de /var/www/html/web/core/lib/Drupal/Core/Form/FormBuilder.php)

Maybe it's a good idea to fix all of this conversions in the core's codebase.

In my case the error was caused by a custom form which had...

$form['#cache'] = false;

... which was working on PHP 7.4. Once I replaced the code with...

$form['#cache'] = ['max-age' => 0];

...problem solved under PHP 8.1. The stack trace returned by core shows the form which causes the problem but there is no clue about what's really causing the problem.

Deprecated function: Automatic conversion of false to array is deprecated en Drupal\Core\Form\FormBuilder->prepareForm() (línea 766 de /var/www/html/web/core/lib/Drupal/Core/Form/FormBuilder.php)
#0 /var/www/html/web/core/includes/bootstrap.inc(164): _drupal_error_handler_real(8192, 'Automatic conve...', '/var/www/html/w...', 766)
#1 /var/www/html/web/core/lib/Drupal/Core/Form/FormBuilder.php(766): _drupal_error_handler(8192, 'Automatic conve...', '/var/www/html/w...', 766)
#2 /var/www/html/web/core/lib/Drupal/Core/Form/FormBuilder.php(284): Drupal\Core\Form\FormBuilder->prepareForm('form_user_topic', Array, Object(Drupal\Core\Form\FormState))
#3 /var/www/html/web/core/lib/Drupal/Core/Form/FormBuilder.php(224): Drupal\Core\Form\FormBuilder->buildForm('Drupal\\base_mod...', Object(Drupal\Core\Form\FormState))
#4 [internal function]: Drupal\Core\Form\FormBuilder->getForm('Drupal\\base_mod...')
#5 /var/www/html/web/modules/contrib/twig_tweak/src/TwigTweakExtension.php(227): call_user_func_array(Array, Array)
#6 /var/www/html/web/sites/default/php/twig/64f59266490ac__topic.html.twig_FR_dj-f2tL4K0hPltoFUigyFe/wiF8uRKa803v5phYtUm1zVpd1CJKKxIoXVpKlk9xkB8.php(92): Drupal\twig_tweak\TwigTweakExtension::drupalForm('Drupal\\base_mod...')
#7 /var/www/html/vendor/twig/twig/src/Template.php(171): __TwigTemplate_01dff3be3cd64f367b2a2d0eb06da18c->block_heading(Array, Array)
#8 /var/www/html/web/sites/default/php/twig/64f59266490ac__node.html.twig_f6TdNAT4_mIYxydzcOX1aDUrv/ziKa-dkAAwUZCoWBS0-PoZQsWatEn7BRO9aawgteSlc.php(61): Twig\Template->displayBlock('heading', Array, Array)
#9 /var/www/html/vendor/twig/twig/src/Template.php(394): __TwigTemplate_1c017e6f63c46e6daae8a308e5878ff0->doDisplay(Array, Array)
#10 /var/www/html/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling(Array, Array)
#11 /var/www/html/web/sites/default/php/twig/64f59266490ac__topic.html.twig_FR_dj-f2tL4K0hPltoFUigyFe/wiF8uRKa803v5phYtUm1zVpd1CJKKxIoXVpKlk9xkB8.php(73): Twig\Template->display(Array, Array)
#12 /var/www/html/vendor/twig/twig/src/Template.php(394): __TwigTemplate_01dff3be3cd64f367b2a2d0eb06da18c->doDisplay(Array, Array)
#13 /var/www/html/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling(Array, Array)
#14 /var/www/html/web/sites/default/php/twig/64f59266490ac_node--topic.html.twig_oIEnjrh4qrIs1auGJz3vJLOIv/Oz6RNR9XYL3fQzAPzUSTCQYuRRD3AguVPaZEwLROWKM.php(42): Twig\Template->display(Array, Array)
#15 /var/www/html/vendor/twig/twig/src/Template.php(394): __TwigTemplate_d403d801796cf3878857db63562749fd->doDisplay(Array, Array)
#16 /var/www/html/vendor/twig/twig/src/Template.php(367): Twig\Template->displayWithErrorHandling(Array, Array)
#17 /var/www/html/vendor/twig/twig/src/Template.php(379): Twig\Template->display(Array)
#18 /var/www/html/vendor/twig/twig/src/TemplateWrapper.php(40): Twig\Template->render(Array, Array)
#19 /var/www/html/web/core/themes/engines/twig/twig.engine(53): Twig\TemplateWrapper->render(Array)
#20 /var/www/html/web/core/lib/Drupal/Core/Theme/ThemeManager.php(372): twig_render_template('themes/custom/s...', Array)
#21 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(436): Drupal\Core\Theme\ThemeManager->render('node', Array)
#22 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender(Array, false)
#23 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(238): Drupal\Core\Render\Renderer->render(Array, false)
#24 /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php(583): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
#25 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(239): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#26 /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(128): Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#27 /var/www/html/web/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php(90): Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\CurrentRouteMatch))
#28 [internal function]: Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#29 /var/www/html/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(111): call_user_func(Array, Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#30 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(187): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object(Symfony\Component\HttpKernel\Event\ViewEvent), 'kernel.view')
#31 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#32 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#33 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#34 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#35 /var/www/html/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#36 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#37 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#38 /var/www/html/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#39 /var/www/html/web/core/lib/Drupal/Core/DrupalKernel.php(704): Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#40 /var/www/html/web/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#41 {main}