I have a custom provider which uses a custom User class with DOES NOT extend eZ\Publish\Core\MVC\Symfony\Security\User class, but it DOES implement eZ\Publish\Core\MVC\Symfony\Security\UserInterface.

Since I want to use eZ repository to check for user authentication, this combination is not possible since eZ Publish RepositoryAuthenticationProvider explicitly checks if user is an instance of eZ\Publish\Core\MVC\Symfony\Security\User class instead of checking for eZ\Publish\Core\MVC\Symfony\Security\UserInterface.