[swift-evolution] [swift-evolution-announce] [Review] SE-0153: Compensate for the inconsistency of @NSCopying's behaviour

Torin Kwok torin at kwok.im
Sat Feb 18 05:57:53 CST 2017

Proposal link:

Reply text:

However, occasionally a constructor might be non-copying on purpose, so we'll need a way to disable the warning.
How about the determination method mentioned in the proposal:

... if developers are performing an assignment operation from within an initializer between a property declared as @NSCopyingand an instance of a protocol conforming class.
That is, there are two key factors determining whether a fixable error or warning should be emitted:

If the destination property has been declared as @NSCopying in class definition
If the source value conforms to <NSCopying> protocol
Once both are true, then what compiler can be sure about is that developer definitely missed the required manual copy invocation and it would emit either an error or a warning correspondingly. Otherwise, compiler would not complain of anything and would just leave the decision up to developers.

> On 18 Feb 2017, at 18:16, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>> On Feb 17, 2017, at 3:18 PM, Douglas Gregor <dgregor at apple.com> wrote:
>> https://github.com/apple/swift-evolution/blob/master/proposals/0153-compensate-for-the-inconsistency-of-nscopyings-behaviour.md
>> 	• What is your evaluation of the proposal?
> I think a warning is very appropriate.
> However, occasionally a constructor might be non-copying on purpose, so we'll need a way to disable the warning. A few possibilities:
> * Use .self on the value
> * Parenthesize the value
> * Parenthesize the variable
> I don't favor using compiler magic. Too spooky.
>> 	• Is the problem being addressed significant enough to warrant a change to Swift?
> Yeah, that's a pretty easy mistake to make.
>> 	• Does this proposal fit well with the feel and direction of Swift?
> Yes. Swift usually tries to help you avoid mistakes.
>> 	• If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
> N/A.
>> 	• How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
> Quick reading.
> -- 
> Brent Royal-Gordon
> Architechies
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> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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