[swift-evolution] Learning from SE-0025, a breeding group for Swift proposals

[David Hart]

Hello community,

I'm happy to see that SE-0169 got accepted and that we've patched the issues of SE-0025. But it's been a difficult process. And I can't stop asking myself if it could have been avoided. The crux of the problem is that source-compatibility is now becoming a very strong requirement. But at the same time, Swift is still missing some very big systems: reflection, property behaviours, a concurrency paradigm. How can we continue to push Swift boldly forward with very little leeway to correct our mistakes?

Then I listened to the latest episode of the excellent [Swift Unwrapped podcast](https://spec.fm/podcasts/swift-unwrapped) where they talk about the access control "saga" and ask themselves the same questions as above. One interesting idea got my attention: JavaScript has a natural breeding ground for future language features with Babel. For those who don't know, it's a transcompiler that compiles bleeding-edge JavaScript into versions supported in browsers. Perhaps Swift could also benefit from a similar experimentation time for each new proposal.

Here's my rough idea:
The Swift compiler gains a new off-by-default `next` version triggerable with the `-swift-version next` flag.
All controversial proposals start their implementation in that version.
Once one of the poposals feels stable enough, it is brought into an official version.
Developers would be encouraged to try the `next` features while being warned that source compatibility on that version will *not* be garanteed.
As the vast majority of the Swift user base are still Apple platform developers, I think it would be important for the success of that strategy that the applications compiled with the `next` flag be accepted on the Apple stores or it will reduce the group of developers ready to play in this "breeding-group".

Any comments?

David.
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