Fatal error when generating form for multivalued base field

Problem/Motivation

In core/includes/theme.inc, function template_preprocess_field_multiple_value_form():

    $header = array(
      array(
        'data' => array(
          '#prefix' => '<h4' . $header_attributes . '>',
          'title' => array(
            '#markup' => t($element['#title']),
          ),
          '#suffix' => '</h4>',
         ...

But $element['#title'] is already translated either as TranslatableMarkup when the field is a base field, either as translated string when the field is a config field. In the first case the page crashes with InvalidArgumentException because t() expects a string. In the second case it only translates an already translated string.

Proposed resolution

Remove the superfluous t(). Remove also the un-needed nesting of #markup.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Original report, by @kevin.dutra

Not sure exactly where to slot this one, component-wise, but putting it under 'field system' for now.

Steps to reproduce

1. Create an entity that has a base field that is multi-valued (e.g. CARDINALITY_UNLIMITED), like
   

   $fields['multi_field'] = BaseFieldDefinition::create('entity_reference')
         ->setLabel(t('Multi field label'))
         ->setDescription(t('Some description text.'))
         ->setSetting('target_type', 'user')
         ->setSetting('handler', 'default')
         ->setCardinality(BaseFieldDefinition::CARDINALITY_UNLIMITED)
         ->setDisplayOptions('form', array(
           'type' => 'entity_reference_autocomplete',
           'weight' => 0,
           'settings' => array(
             'match_operator' => 'CONTAINS',
             'size' => '60',
             'placeholder' => '',
           ),
         ))
         ->setDisplayOptions('view', array(
           'label' => 'above',
           'type' => 'entity_reference_label',
           'weight' => 0,
         ));
   

2. Visit the add page
3. Cry

More info

It looks like the underlying problem is that a call is made to t() for the title in template_preprocess_field_multiple_value_form(), but the title is a TranslatableMarkup object, not a string, so it barfs with the error:

InvalidArgumentException: $string ("Multi field label") must be a string. in Drupal\Core\StringTranslation\TranslatableMarkup->__construct() (line 83 of core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php).

Drupal\Core\StringTranslation\TranslationManager->translate(Object, Array, Array)
t(Object)
template_preprocess_field_multiple_value_form(Array, 'field_multiple_value_form', Array)
Drupal\Core\Theme\ThemeManager->render('field_multiple_value_form', Array)
Drupal\Core\Render\Renderer->doRender(Array)
Drupal\Core\Render\Renderer->doRender(Array)
Drupal\Core\Render\Renderer->doRender(Array, )
Drupal\Core\Render\Renderer->render(Array, )
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare(Array, Object, Object)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

It's probably worth noting that this doesn't happen for configured fields, just base fields.

You can sidestep the error by forcibly casting the label to a string when you're creating the base field definition, but that's a bit hokey since that translated string is then going to be translated again when it hits the preprocessor.

Also attaching a simple sample module to demonstrate. The listing page is at /admin/test.