[swift-evolution] Ownership on protocol property requirements

[David Hart]

LGTM! I'd go ahead with opening a PR. Time is running out!

> On 8 May 2017, at 14:40, Greg Spiers <gspiers at gmail.com> wrote:
> 
>> On Mon, May 8, 2017 at 7:57 AM, David Hart <david at hartbit.com> wrote:
>> Sounds great! It should be an easy one to get through,
> 
> Thanks David, appreciate it :) I've created a draft of the proposal.
> Any feedback would be very welcome. I wasn't sure if there was an
> effect on ABI stability or API resilience. I don't think there would
> be as this is only correcting syntax.
> 
> Also, just adding the link again where I had a try at implementing the
> warning/error. Hopefully that can be helpful in getting feedback about
> the proposal as well.
> https://github.com/gspiers/swift/commit/ecde3ec5f61f259f8a396618e9973bac04536fd0
> 
> 
> # Remove ownership keyword support in protocols
> 
> * Proposal: [SE-NNNN](NNNN-remove-ownership-keyword-support-in-protocols.md)
> * Authors: [Greg Spiers](https://github.com/gspiers)
> * Review Manager: TBD
> * Status:
> * Bug: [SR-479](https://bugs.swift.org/browse/SR-479)
> 
> ## Introduction
> 
> This proposal removes support for the keywords `weak` and `unowned`
> for property declarations in a protocol.
> 
> Swift-evolution thread: [Ownership on protocol property
> requirements](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170501/036495.html)
> thread. (TODO: Add link to rest of discussion)
> 
> ## Motivation
> 
> Currently it's possible to use the weak/unowned keywords for a
> property requirement in a protocol. This can lead to confusion as
> specifying one of these keywords does not enforce or raise any
> warnings in the adopting type of that protocol:
> 
> ```swift
> 
> class A {}
> 
> protocol P {
>    weak var weakVar: A? { get set }
> }
> 
> class B: P {
>    var weakVar: A? // Not declared weak, no compiler warning/error
> }
> 
> ```
> 
> This can lead to unexpected and surprising behaviour from the point of
> view of users. They keywords themselves are currently meaningless
> inside of a protocol but look like they would have an effect when the
> protocol is adopted.
> 
> ## Proposed solution
> 
> Although the case could be made that the keywords should have meaning
> in a protocol, as they are currently implemented today they don't have
> an effect. This proposal aims to cleanup the misleading syntax and
> isn't meant to remove functionality only correct to existing
> behaviour.
> 
> This proposal suggests removing support for `weak` and `unowned` in a protocol.
> 
> ## Detailed design
> 
> The compiler will flag the use of `weak` and `unowned` in a protocol
> and suggest a fix to remove the keyword.
> 
> ## Source compatibility
> 
> This is a source breaking change but one that would only correct code
> that is already has broken assumptions. For existing use the compiler
> will raise a compilation error. When running in Swift 3 mode a warning
> can be generated instead of an error. It could be possible to address
> source compatibility through source migration as well.
> 
> ## Effect on ABI stability
> 
> This proposal does not affect ABI stability.
> 
> ## Effect on API resilience
> 
> This proposal should not effect API resilience.
> 
> ## Alternatives considered
> 
> There is an argument in making `weak` and `unowned` have meaning in a
> protocol but this does open up other questions and is probably better
> as a topic of a separate discussion/proposal. As this would be
> additive it can be addressed at a later point when we have a clearer
> understanding.