How to polyfill for older systems

[tkrajacic (Thomas Krajacic)]

@AlexanderM

I don't think conditional compilation would work here

Yeah, it needs to be a runtime check for Apple systems. But that runtime check can be compile-time conditionally added for Apple systems only I guess.

I brought related topics up before:

Which of Swift 5.1 features are backwards deployable?

Does swift 5.1 runtime not have support for ios12? Is it a technical limitation that 5.1 can’t be included with the app for back deployment?

and here

Best way to implement new feature for old SDKs

Heh. I did put that in there deliberately, but no, not that I know of. There's nothing incredibly technically difficult in making this work: @dynamicallyAvailableBefore(macOS 10.12) var temporaryDirectory: URL { … } and having the compiler check that the signature matches up and everything before generating the code you wrote above. But it'd be a proposal that needs a lot of careful thought—how does this affect how people use framework code? What happens on new platforms? What if two people do…

but I would like to reiterate on this issue as it is really pressing now with ABI stability, not only for SDK features anymore but actually standard library features.