[kangaru 5] Figure out a better syntax for tag

[gracicot]

Is your feature request related to a problem? Please describe.
The tag syntax is quite unsatisfactory. It's quite hard to get right, hard to diagnose, and hard to discover. A better syntax needs to be drafted

Describe the solution you'd like
It should be easy to annotate a type and make it easy to tag types for most purposes

Describe alternatives you've considered

• Make injectable_reference_source forward some tags to avoid having to use multiple tags
• Add tag shortcuts that adds multiple tags, depending on specific user needs

[gracicot]

Seems like the problem has been partially solved as services cannot specify which source the need to be initialized in, as the container has that knowledge now. Most of the time, a simple friend auto tag(kangaru::tag_for<my_service&>) -> kangaru::cached; is sufficient.

However, the syntax is not solved for polymorphic services and overrides and all of that.

[gracicot]

Should we completely drop ADL based tags? Should we just use specialization?

[gracicot]

ADL was useful when defining a service_map, mapping user types to user types, because it meant we could define the map without including kangaru. It was also done to support value category conversions for the mapping, which in the end I think were confusing. I don't think those are a goal anymore. Maybe just using specialization is good enough.