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@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@swift.org <mailto:swift-evolution@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
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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