I want to decorate the Symfony UrlGenerator class.
UrlGenerator
Symfony\\Component\\Routing\\Generator\\UrlGenerator: ~ my.url_generator: class: AppBund
I believe you have to decorate the Symfony\Component\Routing\Generator\UrlGeneratorInterface, because the services should depend on the interface and not on a specific implementation (class).
Symfony\Component\Routing\Generator\UrlGeneratorInterface