Fatal error occurs

kishan@kk → made their first commit to this issue’s fork.

I'm not sure if I should modify the FieldDiffBuilderInterface like this.
I deleted the mixed return.

Thanks for patch but it didn't work for me because I also have two similar errors from two separate modules.
I'm not sure if a separate integration is needed for each relevant module.

Dynamic Entity Reference Module

Fatal error: Declaration of Drupal\dynamic_entity_reference\Plugin\diff\Field\DynamicEntityReferenceFieldBuilder::build(Drupal\Core\Field\FieldItemListInterface $field_items) must be compatible with Drupal\diff\Plugin\diff\Field\EntityReferenceFieldBuilder::build(Drupal\Core\Field\FieldItemListInterface $field_items): array in /var/www/html/d10site/web/modules/contrib/dynamic_entity_reference/src/Plugin/diff/Field/DynamicEntityReferenceFieldBuilder.php on line 25

Smart Date Module

Fatal error: Declaration of Drupal\smart_date\Plugin\diff\Field\SmartdateFieldBuilder::buildConfigurationForm(array $form, Drupal\Core\Form\FormStateInterface $form_state) must be compatible with Drupal\diff\FieldDiffBuilderBase::buildConfigurationForm(array $form, Drupal\Core\Form\FormStateInterface $form_state): array in /var/www/html/d10site/web/modules/contrib/smart_date/src/Plugin/diff/Field/SmartdateFieldBuilder.php on line 63

There's a solution for smart date here. Fix diff 2.x integration 🐛 Fix diff 2.x integration Needs review
But if there is a possibility of getting this similar error in different modules, it would be better to approach the situation on a diff basis.

When 2.x was published I created issues for every module with the fixes. All modules seem to have committed these.

Please open issues in the relevant module if they still need fixing.