Constraint violations are not triggered for Roles on a user's account form

Problem/Motivation

Im trying to deny certain combination of user roles while creating or editing a user.

For this purpose I created a Constraint that should check whether the entered combination of roles is allowed. After implementing this, I experienced that the ConstraintValidator is executed, but the violation is not honoured by the underlying routines.

Steps to reproduce

To verify that my Contraint implemantation is correct, I added the same Constraint to the name and mail field in the same form. On this fields the Constraint works as expected. Im not sure whether I missed something, but my implementation seems to be correct as it's working on the other fields.

---- htdocs/modules/my_module/src/Plugin/Validation/Constraint/UserRolesConstraint.php ----
<?php

namespace Drupal\my_module\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;

/**
 * Class UserRolesConstraint
 *
 * @Constraint(
 *   id = "UserRoles",
 *   label = @Translation("User roles restrictions",context = "Validation")
 * )
 *
 */
class UserRolesConstraint extends Constraint{
  public $ClientMustHaveOnlyOneGroup = 'Invalid roles selection.';
}

---- htdocs/modules/my_module/src/Plugin/Validation/Constraint/UserRolesConstraintValidator.php ----
<?php

namespace Drupal\my_module\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Class UserGroupConstraintValidator
 *
 * @package Drupal\my_module\Plugin\Validation\Constraint
 */
class UserRolesConstraintValidator extends ConstraintValidator {

    /**
   * @inheritdoc
   */
  public function validate($value, Constraint $constraint) {
    $this->context->addViolation( $constraint->ClientMustHaveOnlyOneGroup);
  }
}

---- htdocs/modules/my_module/my_module.module ----
/**
 * Implements hook_entity_base_field_info_alter().
 */
function my_module_entity_base_field_info_alter(&$fields, EntityTypeInterface $entityType) {
  if ($entityType->id() == 'user') {
    $fields['roles']->addConstraint('UserRoles');
    $fields['name']->addConstraint('UserRoles');
    $fields['mail']->addConstraint('UserRoles');
  }
}

Attached is a screen shot of the result, I would have expected the the 'Roles' were marked as error as well.

Proposed resolution

Trigger Constraint violations for Roles on a user's account form

Remaining tasks

1. Write a merge request
2. Review
3. Commit

User interface changes

Errors are now shown when there are constraint violations that apply to the assigned roles for a user.

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

None