[swift-evolution] Protocol conformance error

[Paul Cantrell]

I remember discussions on this list of curious situations in which a protocol doesn’t even conform to itself. This may be related.

I’ll let others who better understand the type system and its limitations weigh in.

P

> On Jan 17, 2018, at 1:53 PM, Saagar Jha <saagar at saagarjha.com> wrote:
> 
> Interestingly, it seems like Swift allows for contravariance for subclassing, in that this is valid code:
> 
> protocol A {}
> protocol B: A {}
> 
> class C { // Notice this is a class, not a protocol
>    func test(x: B) {}
> }
> 
> class M: C {
>    func test(x: A) {}
> }
> 
> Is it an oversight that this doesn’t apply to protocols?
> 
> Saagar Jha
> 
>> On Jan 17, 2018, at 11:38, Paul Cantrell <cantrell at pobox.com <mailto:cantrell at pobox.com>> wrote:
>> 
>> Note that it would be sound for the language to allow the opposite, which is called “contravariance” (the more specific type takes a more general input):
>> 
>>     protocol A {}
>>     protocol B: A {}
>> 
>>     protocol C {
>>        func test(x: B)    // was A
>>     }
>> 
>>     class M: C {
>>        func test(x: A) {} // was B
>>     }
>> 
>> It could also allow covariant return types (the more specific type returns a more specific output):
>> 
>>     protocol C {
>>        func test() -> A
>>     }
>> 
>>     class M: C {
>>        func test() -> B {
>>           fatalError("just a stub")
>>        }
>>     }
>> 
>> Some languages support this, and Swift certainly could — though I don’t know that it’s a frequently request feature.
>> 
>> It would also be interesting if associated type constraints allowed this, which I don’t think they currently do:
>> 
>>     protocol C {
>>        associatedtype TestInput where B: TestInput   // error here
>> 
>>        func test(x: TestInput)
>>     }
>> 
>> Curiously, the following does not work, although it seems like it should:
>> 
>>     protocol A {}
>>     protocol B: A {}
>> 
>>     protocol C {
>>        associatedtype TestOutput: A
>> 
>>        func test() -> TestOutput
>>     }
>> 
>>     class M: C {
>>        func test() -> B {
>>           fatalError("just a stub")
>>        }
>>     }
>> 
>> It gives the error “inferred type 'B' (by matching requirement 'test()') is invalid: does not conform to ‘A’” even though B does conform to A. Huh.
>> 
>> Cheers, P
>> 
>> 
>>> On Jan 17, 2018, at 2:43 AM, Saagar Jha via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> If we have:
>>> 
>>> class N: A {}
>>> 
>>> you can pass an N into C’s test(x:), since N is an A, but not M’s test(x:), since N is not a B. Thus, it’s not a valid conformance.
>>> 
>>> Saagar Jha
>>> 
>>>> On Jan 17, 2018, at 00:04, Roshan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> Cross posting from swift-users in case this behaviour isn't part of
>>>> the language and might be interesting to you folks.
>>>> 
>>>> Here is some sample code that gives a protocol conformance error in a
>>>> playground:
>>>> 
>>>> protocol A {}
>>>> protocol B: A {}
>>>> 
>>>> protocol C {
>>>>    func test(x: A)
>>>> }
>>>> 
>>>> class M: C {
>>>>    func test(x: B) {}
>>>> }
>>>> 
>>>> Is there a reason why the compiler doesn't infer that ((B) -> ())
>>>> matches ((A) -> ()) because of inheritance?
>>>> 
>>>> -- 
>>>> Warm regards
>>>> Roshan
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>>> 
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>> 
> 

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