core-bundle/src/Security/User/UserChecker.php
private function checkIfLoginIsAllowed(User $user): void
    {
        if (!$user instanceof FrontendUser || $user->login) {
            return;
        }

        $ex = new DisabledException(sprintf('User "%s" is not allowed to log in', $user->username));
        $ex->setUser($user);

        throw $ex;
    }
private function checkIfLoginIsAllowed(User $user): void
    {
        if (!$user instanceof FrontendUser || $user->login) {
            return;
        }

        $ex = new DisabledException(sprintf('User "%s" is not allowed to log in', $user->username));
        $ex->setUser($user);

        throw $ex;
    }
Checks wether login is allowed (front end only).