[swift-evolution] A case for postponing ABI stability

[Tino Heth]

>> - runtime libraries for Swift 4
>> - all system frameworks will need to contain two variants - one compatible with Swift 4 and one with Swift 5. This is IMHO absolutely unmaintainable in the long run. For how long would you need to keep several versions of the framework around? What happens when Swift 6 comes along with another breaking changes? Would each system framework have 3 versions embedded?
> 
> That's right. If the OS frameworks use Swift then either (1) you have to clone the framework stack for each Swift version, or (2) you have only one copy of the frameworks but frameworks and apps can't share their Swift objects or publish Swift API.
> 
> The framework structure that Apple inherited from NeXT supports framework versioning, but *no frameworks use it*. It doesn't scale. 


sure, it's preferable to have a single version that works with all apps — but if it's technically possible to have one clone installed with the OS, isn't that better than one version for each app?
Managing several versions shouldn't be that hard (have a look at https://nixos.org/nix/ <https://nixos.org/nix/>).
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