But that's a pitch about Unicode availability domain specifically, at that done by other means – you mention this thread's pitch in alternatives considered (and not taken). If there any inflexibilities stopping using this thread's pitch I wonder if they could be addressed, otherwise we may end up having a generic feature and still resort to custom means because generic feature is not flexible enough.
(ok, fair to notice that when you said "this idea" you meant not this thread's pitch idea (as I originally thought) but the idea expressed in your other message upthread, fair enough).
As for this thread's pitch - I found it quite interesting and useful. It feels more Swifty and less noisy than #if #endif brackets approach. More safe as well (frequent cause of errors are misspellings in #if xxx checks).