Using the standard Twig Extensions library, the following definitions work as expected:

services:
    _defaults:
        public: false
        tags: [ twig.extension ]

    random.name:
        class: Twig_Extensions_Extension_Intl

While the following doesn't:

services:
    _defaults:
        public: false
        tags: [ twig.extension ]

    Twig_Extensions_Extension_Intl: ~

It throws:

The definition for "Twig_Extensions_Extension_Intl" has no class. If you intend to inject this service dynamically at runtime, please mark it as synthetic=true. If this is an abstract definition solely used by child definitions, please add abstract=true, otherwise specify a class to get rid of this error.

It seems to be related to the fact that it's a class in the global namespace.