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It is worked on under the term "pattern types" mainly by Oli oli-obk Scherer I think, who has an Ada background.

Can't tell you what the current state is but this should give you the keywords to find out.

Also, here is a talk Oli gave in the Ada track at FOSDEM this year: https://hachyderm.io/@oli/113970047617836816

AFAIK the current status is that it's internal to std (used to implement `NonNull` and friends) and not planned to be exposed.

There were some talks about general pattern type, but it's not even approved as an experiment, not to talk about RFC or stabilization.