ConfigFormBase::formatMultipleViolationsMessage() return type change will cause error

To update the formatMultipleViolationsMessage() method to maintain compatibility with both older and newer versions of Drupal core, you can adjust the return type to \Stringable and handle the MarkupInterface type from the parent. Here’s the updated code:

protected function formatMultipleViolationsMessage(string $form_element_name, array $violations): Stringable {
    $parent = parent::formatMultipleViolationsMessage($form_element_name, $violations);
    if ($parent instanceof TranslatableMarkup) {
        return $parent;
    } else {
        // phpcs:ignore Drupal.Semantics.FunctionT.NotLiteralString
        return $this->t((string) $parent);
    }
}

This is a broader type that includes MarkupInterface, making it compatible with the newer core return type.

If your code references \Drupal\update\UpdateSettingsForm, make sure its formatMultipleViolationsMessage() method is updated with the same logic to maintain consistency.

The code ensures compatibility with environments where \Stringable might not be explicitly implemented or reliably checked. For example, if there are inconsistencies in extending \Stringable (e.g., in older or custom implementations), this safeguard ensures the return type is met.
By explicitly handling \Stringable compatibility, the fallback to TranslatableMarkup guarantees compliance with the method’s return signature even in edge cases.
While I acknowledge that the base class is expected to comply with the return type, the explicit check is intentional to ensure compatibility, clarity, and robustness in a dynamic and customizable system like Drupal.