[swift-evolution] [Review] SE-0026 Abstract classes and methods

[Brent Royal-Gordon]

> 	abstract classes may have impact on ABI : that’s the reason we may add this in release 3 if we want to be able to introduction binary interrop in Swift 4 (such as dynamic code loading,etc…).

If making an existing public concrete class into an abstract class would be a nonresilient change anyway—and I cannot *possibly* imagine how it would not be—then any abstract classes would have to be new, post-Swift 4 code, and there would be no existing code to break ABI compatibility with.

Swift 3 is not last call for features which require additions to the ABI; it's last call for features which might *change* existing ABI.

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Brent Royal-Gordon
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