[swift-evolution] Fix "private extension" (was "Public Access Modifier Respected in Type Definition")

[Jose Cheyo Jimenez]

> On Oct 2, 2017, at 5:33 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
>> On Oct 2, 2017, at 03:25, Vladimir.S via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> On 01.10.2017 1:18, Chris Lattner wrote:
>>>> On Sep 29, 2017, at 10:42 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> Vladimir, I agree with you on that change, but it’s a separate topic from this one.
>>>> 
>>>> Tony is absolutely correct that this topic has already been discussed. It is a deliberate design decision that public types do not automatically expose members without explicit access modifiers; this has been brought up on this list, and it is clearly not in scope for discussion as no new insight can arise this late in the game. The inconsistency with public extensions was brought up, the proposed solution was to remove modifiers for extensions, but this proposal was rejected. So, the final design is what we have.
>>> Agreed.  The core team would only consider a refinement or change to access control if there were something actively broken that mattered for ABI stability.
>> 
>> So we have to live with *protected* extension inconsistency for very long time just because core team don't want to even discuss _this particular_ inconsistency(when access level in *private extension* must be private, not fileprivate)?
>> 
>> Yes, we decided that access level for extension will mean a default and top most access level for nested methods, OK. But even in this rule, which already differ from access modifiers for types, we have another one special case for 'private extension'.
>> 
>> Don't you think this is not normal situation and actually there IMO can't be any reason to keep this bug-producing inconsistency in Swift? (especially given Swift 5 seems like is a last moment to fix this)
> 
> I hate to say it but I'm inclined to agree with Vladimir on this. "private extension" has a useful meaning now distinct from "fileprivate extension", and it was an oversight that SE-0169 <https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md> didn't include a fix here. On this very narrow, very specific access control issue I think it would still be worth discussing; like Xiaodi said it's not related to James' original thread-starter.
> 
> (I maintain that the current model does not include a special case; it simply means the 'private' is resolved at the level of the extension rather than the level of its members. But that isn't what people expect and it's not as useful.)
> 
> 
> I agree that changing the behavior of all access modifiers on extensions is out of scope. (I also agree that it is a bad idea. Sorry, James, but wanting 'pubic' here indicates that your mental model of extensions does not match what Swift is actually doing, and that could get you into trouble.)
> 
> Jordan


I am not in favor of including a special case for “private extension” because like Jordan said, private is resolved at the level of the extension which is always top level currently. A change would make sense if we start allowing extensions that are not top level. Anything private that is top level is equivalent to fileprivate so why should “private extension” be any different?

This is only confusing if people start mixing “fileprivate extension” and “private extension” in the same code base. 

We have a SwiftLint rule for this situation:
https://github.com/realm/SwiftLint/blame/master/Source/SwiftLintFramework/Rules/PrivateOverFilePrivateRule.swift <https://github.com/realm/SwiftLint/blame/master/Source/SwiftLintFramework/Rules/PrivateOverFilePrivateRule.swift>

Alternatively you can enforce no extensions :)
https://github.com/realm/SwiftLint/blob/master/Source/SwiftLintFramework/Rules/NoExtensionAccessModifierRule.swift <https://github.com/realm/SwiftLint/blob/master/Source/SwiftLintFramework/Rules/NoExtensionAccessModifierRule.swift>

“"There is no solution that will make everyone happy: maintaining the status quo makes “fileprivate” too common and therefore not meaningful when it occurs in source;”"
https://lists.swift.org/pipermail/swift-evolution-announce/2017-April/000357.html <https://lists.swift.org/pipermail/swift-evolution-announce/2017-April/000357.html>










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