I'm in favour.
For the bikeshedding question, the mathematician in me believes that using `WARNING [partial]` is more canonical. Except that there is the `deprectations` category which is suppressed with `-Wno-deprecations` (the proposal solves this in saying that `-Wno-deprecations` is actually an alias for `-Wno-x-deprecations`, why not). Pragmatically, though, the more I think about it, the more I'm favouring enforcing that the warning author write `WARNING [x-partial]` (it is also permitted to write `[deprecations]` without an x, which would avoid introducing `x-deprecations` gratuitously). The explicitness here, I think, helps. The warning category would _always_ be called `x-partial`. It's easier to process, easier to google. The cost is tiny.
As to the other question: we probably shouldn't permit warnings to join other warning categories: they are documented in the user guide, and having one of these warnings trigger for another reason can be potentially quite confusing. Not every category anyway. Maybe we will find some that can be sensibly extended. Forcing `WARNING [x-partial]` allows us to whitelist categories that can be extended with custom warnings.
I don't think that the other alternatives in the proposal need to be addressed at this point (if anything, I think they are better addressed when we have more information on how people actually use these new categories, instead of just guessing what the pain points will be).
/Arnaud