[swift-evolution] [swift-evolution-announce] [Accepted with Revision] SE-0177: Allow distinguishing between public access and public overridability

[David Owens II]

While arguably true, that's a philosophical debate. There are plenty of reasons to use @testable, and if that's what people are using, then I think there is a valid concern here. 

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> On Jul 27, 2016, at 5:22 PM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Jul 27, 2016, at 4:41 PM, David Owens II via swift-evolution <swift-evolution at swift.org> wrote:
>> I brought this up in review, but there seems to be a serious testability problem here because of how special @testable is.
>> 
>> // This class is not subclassable outside of ModuleA.
>> public class NonSubclassableParentClass {
>>     // This method is not overridable outside of ModuleA.
>>     public func foo() {}
>> 
>>     // This method is not overridable outside of ModuleA because
>>     // its class restricts its access level.
>>     // It is not invalid to declare it as `open`.
>>     open func bar() {}
>> 
>>     // The behavior of `final` methods remains unchanged.
>>     public final func baz() {}
>> }
>> 
>> In a unit test, I *can* subclass `NonSubclassableParentClass`, the access level of `NonSubclassableParentClass` is irrelevant. There’s now no programatic way to ensure that I’m actually testing the contract that I had intended to provide to the consumers of my framework (that my class is subclassable). Is this really the intention?
> 
> A "black box" unit test emulating consumer behavior has no business using a @testable import.  It should just use the external API of the library.
> 
> John.
> 
>> 
>> The “fix”, on the authors end, is to create another target that consumes the output framework to ensure my contract is actually what I wanted (and make sure that it’s not a special test target!). No one is going to do this.
>> 
>> Sure, maybe a code review might catch it. Or I can write a custom linter to validate for this. Do we really want `open` members in non `open` types? The issue with `public` members on `internal` types is much less concerning as the `internal` type isn’t being exposed to others to begin with.
>> 
>> -David
>> 
>> 
>>> On Jul 27, 2016, at 3:18 PM, Scott James Remnant via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I realize that there’s no review needed, but I actually wanted to give a hearty 👏 to the authors and commenters of this proposal, because I genuinely think we’ve reached something good in the result.
>>> 
>>> The selling point for me is this:
>>> 
>>> // This is allowed since the superclass is `open`.
>>> class SubclassB : SubclassableParentClass {
>>>     // This is invalid because it overrides a method that is
>>>     // defined outside of the current module but is not `open'.
>>>     override func foo() { }
>>> 
>>>     // This is allowed since the superclass's method is overridable.
>>>     // It does not need to be marked `open` because it is defined on
>>>     // an `internal` class.
>>>     override func bar() { }
>>> }
>>> 
>>> This feels super-clean; it gives Library developers `open` for their APIs, without confusing app developers, and still requires that sub-classing Library developers think about `open`.
>>> 
>>> Good job, everyone!
>>> 
>>> Scott
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>> 
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