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

John McCall rjmccall at apple.com
Wed Jul 27 19:22:34 CDT 2016

> 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.


> 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 <mailto: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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