A good chunk of the debate here centers around whether existentials are a separate, concrete, surface-visible box type of their own, or an implementation detail which is subsumed into the more abstract logic of the type system.
In other words, given this:
let x: any P = T()
…would we say that…
- the value
xhas aP(i.e. is a box that contains aT), or - the value
xis aP?
This has been a point of tension in the language, and even within the Swift team itself, at least since I filed this bug in 2016.
The Any<P> counterproposal would seem to lean toward codifying (1): existentials are a programmer-visible box type. The proposed any P syntax certainly leans toward (2), and perhaps I like the proposal because (2) is my preferred mental model. But (2) certainly does raise questions like the ones @jackmarch is asking — and I find myself hard-pressed to come up with answers to his questions that don’t lean at least a little on the “visible box” model of (1).
I really like the feature idea Holly mentions here:
I remember we discussed it here, and IIRC Joe Groff mentioned it at some point before that. I hope that idea will come up for consideration soon! It seems like it could help close the loop on making (2) a workable mental model.
(I should clarify that my +1 for this proposal is not contingent on the future adoption or non-adoption of that future direction.)