E.g. https://symfony.com/doc/current/service_container/tags.html

Talks about

    Swift_SmtpTransport:
        arguments: ['%mailer_host%']
        tags:
            - { name: app.mail_transport, alias: smtp }

# ...
    Swift_SendmailTransport:
        class: \Swift_SendmailTransport
        tags: [app.mail_transport]

The first-like variants wont work, and i also think we should avoid leading \ in class attr. This might break any normalization check. Standard is no leading slash.

See also symfony/symfony#24145

basically using poor-man namespacing by example makes your life hard.