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

[Slava Pestov]

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
> 
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