[swift-users] Covariance in protocol adoption

[David Turnbull]

Swift 3.0 will bring many changes to protocols. I expect these issues will
be discussed a lot more once 2.2 is shipped.

-david

On Tue, Feb 16, 2016 at 11:54 AM, David Sweeris via swift-users <
swift-users at swift.org> wrote:

> I think it’s something to do with subclasses overriding the method and
> potentially returning a type that’s different that what the compiler is
> expecting. Or something like that.
>
> I hope it gets addressed at some point, though, because it drives me nuts,
> too.
>
> On Feb 16, 2016, at 1:47 PM, Diego Sánchez via swift-users <
> swift-users at swift.org> wrote:
>
> Hi all,
>
> Covariance works properly with classes:
>
> class MyType {}
> class MySubtype: MyType {}
> class MyClass {
>     func createMyType() -> MyType {
>         return MyType()
>     }
> }
>
> class MySubClass: MyClass {
>     override func createMyType() -> MySubtype {
>         return MySubtype()
>     }
> }
>
> However it doesn't work for protocol conformance:
>
> protocol MyProtocol {
>     func createMyType() -> MyType
> }
>
> class MyConformingClass: MyProtocol {
>     func createMyType() -> MySubtype {
>         return MySubtype()
>     }
> }
>
> Compiler error for this case:
> error: type 'MyConformingClass' does not conform to protocol 'MyProtocol'
> note: protocol requires function 'createMyType()' with type '() -> MyType'
> note: candidate has non-matching type '() -> MySubtype'
>
> I know I can fix it by using associated objects in MyProtocol definition,
> but then if I want to declare a property of type MyProtocol I have to go
> dancing with generics everywhere.
>
> Is this a known limitation/bug? Any plans to improve this?
>
> Cheers,
> Diego
>
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