[swift-users] How to use a protocol to infer a generic type with constraints

[Edward Connell]

Does anyone have an idea when this is going to be fixed? Or when support
will be implemented?
My app heavily uses collections of heterogenous plug-ins, so it's a real
pain in the neck to work around.


On Tue, May 2, 2017 at 2:08 AM, Satoshi Nakagawa via swift-users <
swift-users at swift.org> wrote:

> Hi Slava,
>
> Thanks for your quick response!
>
> I see. That's in line with what I can read from the Type Constraint Syntax
> section in the official Swift book.
>
> Satoshi
>
>
> On Tue, May 2, 2017 at 1:59 AM, Slava Pestov <spestov at apple.com> wrote:
>
>> Hi Satoshi,
>>
>> Protocols do not conform to themselves. Only concrete types can conform
>> to protocols in the current implementation of Swift.
>>
>> Slava
>>
>> > On May 2, 2017, at 1:57 AM, Satoshi Nakagawa via swift-users <
>> swift-users at swift.org> wrote:
>> >
>> > Hi,
>> >
>> > I got a build error "Generic parameter 'T' could not be inferred" for
>> the following code.
>> >
>> > Can anyone explain why we can't use a protocol to infer the generic
>> type T?
>> >
>> > class Emitter {
>> >     func emit<T: Emittable>() -> T {
>> >         ...
>> >     }
>> > }
>> >
>> > protocol Emittable {}
>> > protocol Subemittable: Emitable {}
>> >
>> > class ConcreteEmittable: Subemittable {}
>> >
>> > func testCode() {
>> >     let emitter = Emitter()
>> >
>> >     // Error: Generic parameter 'T' could not be inferred
>> >     let _: Emittable = emitter.emit()
>> >
>> >     // Error: Generic parameter 'T' could not be inferred
>> >     let _: Subemittable = emitter.emit()
>> >
>> >     // This works
>> >     let _: ConcreteEmittable = emitter.emit()
>> > }
>> >
>> > Thanks,
>> > Satoshi
>> >
>> > _______________________________________________
>> > swift-users mailing list
>> > swift-users at swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
>
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