I'll reply with comments on the proposal itself separately, but I wanted to mention two things. First, this proposal has an implementation that one can look at. @concurrent function types are already in the Swift compiler from main (behind the same "experimental concurrency" flag). The rest of this proposal is implemented* in this pull request behind another compiler flag, so one can see the effects of this checking. The same pull request also implements the actor-isolation checking based on ConcurrentValue.
Second, ConcurrentValue and @concurrent are incorporated into the third pitch on actors.
Doug
[*] The implementation differs from the proposal here in a few ways that I'll get back to as comments here.