code: public function validate($value, Constraint $constraint)
      {
          if ($constraint instanceof ChoiceUnwrapper) {
              if ((null === $value || '' === $value) && $constraint->allowBlank) {
                  return;
              }
              $resolvedChoices = [];
              if (\is_array($constraint->choices)) {
                  /** @noinspection ForeachSourceInspection */
                  foreach ($constraint->choices as $choice) {
                      if (\is_array($choice)) {
                          /** @var array $choice */
                          foreach ($choice as $subChoice) {
                              $resolvedChoices[] = $subChoice;
                          }
                      } else {
                          $resolvedChoices[] = $choice;
                      }
                  }
  
                  $constraint->choices = $resolvedChoices;
              }
          }
  
          parent::validate($value, $constraint);
      }
docs: {@inheritdoc}