I think the fact that the type checker permits ‘final’ to appear inside
protocol extensions is an oversight and this probably does not even warrant a
proposal. I don’t think allowing this was ever part of the conceptual model of
protocol extensions at any point in time (if you recall they were introduced
‘recently’, in Swift 2). If someone put together a PR which makes ‘final’ in
protocol extensions an error in Swift 4 mode (and a warning in 3), I would
merge it.
FWIW that there is one restriction around the direct dispatch here we want to
lift, but it’s not related to this proposal.
If you have a base class conforming to a protocol using default requirements, eg
protocol Proto { func f() }
extension Proto { func f() { } }
class Base : Proto {}
Currently the witness table for Base : Proto directly references the extension
method Proto.f.
We want to allow this, at least inside the module:
class Derived {
override func f() {} // does not work currently
}
This will mean that ‘Proto.f’ will appear in the vtable of ‘Base’, pointing at
the extension method. The conformance will dispatch through the vtable instead
of directly calling the extension method.
Slava
> On Mar 7, 2017, at 7:23 PM, Brian King via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Hey Folks, This draft proposal addresses starter bug SR-1762. I believe this
> is in scope for Swift 4 since it impacts source compatibility. It's not a
> very exciting proposal, but I think it will help make Swift a little more
> consistent.
>
> https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7>
> https://bugs.swift.org/browse/SR-1762 <https://bugs.swift.org/browse/SR-1762>
>
> Thanks
>
> Brian
> Introduction
>
> This proposal suggests removing support for the final keyword when declaring
> a function in a protocol extension. The presence of the final keyword does
> not currently generate an error message, and it does not actually modify the
> dispatch behavior in any way.
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#motivation>Motivation
>
> In the original protocol model of Swift, a developer could use the final
> keyword when declaring a function in a protocol extension to ensure the
> function could not be overridden. This keyword has no use in Swift's current
> protocol model, since functions in protocol extensions can not be overridden
> and will always use direct dispatch.
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#detailed-design>Detailed
> design
>
> The compiler should generate an error or warning when the final keyword is
> used on a function declaration inside of a protocol extension. This is
> consistent with the use of final in structs and enumerations.
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#source-compatibility>Source
> compatibility
>
> This change will impact source compatibility. To maintain compatibility with
> Swift 3, a warning will be generated in Swift 3 mode instead of an error
> message.
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#effect-on-abi-stability>Effect
> on ABI stability
>
> This has no effect on ABI stability
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#effect-on-api-resilience>Effect
> on API resilience
>
> This has no effect on API resilience
>
>
>
> <https://gist.github.com/KingOfBrian/6f20c566114ac0ef54c8092d80e54ee7#alternatives-considered>Alternatives
> considered
>
> The only alternative would be to not fix this bug
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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