Question regarding Clients conforming to Sendable Protocol

[inwoodev]

While going throuch TCA practice apps, I had a question in my mind regarding conformance to Sendable Protocol.

One could see that the WeatherClient in SearchApp does conform to Sendable Protocol:

struct WeatherClient: Sendable {
  var forecast: @Sendable (GeocodingSearch.Result) async throws -> Forecast
  var search: @Sendable (String) async throws -> GeocodingSearch
}

However, other clients of sample apps do not conform to Sendable

struct AudioPlayerClient {
  var play: @Sendable (URL) async throws -> Bool
}

struct SpeechClient {
  var finishTask: @Sendable () async -> Void
  var requestAuthorization: @Sendable () async -> SFSpeechRecognizerAuthorizationStatus
  var startTask:
    @Sendable (SFSpeechAudioBufferRecognitionRequest) async -> AsyncThrowingStream<
      SpeechRecognitionResult, Error
    >

  enum Failure: Error, Equatable {
    case taskError
    case couldntStartAudioEngine
    case couldntConfigureAudioSession
  }
}

According to my understanding of Sendable, all structs are implicitly Sendable if it has sendable members only(maybe I misunderstood this part). If my understanding is correct, wouldn't it be alright to discard Sendable protocol from WeatherClient?:

struct WeatherClient {
  var forecast: @Sendable (GeocodingSearch.Result) async throws -> Forecast
  var search: @Sendable (String) async throws -> GeocodingSearch
}

Since all the properties of WeatherClient are sendable, I thought that WeatherClient would implicitly be sendable as well and there would be no need of explicit conformance to Sendable.

If I am mistaken, and that there is a valid reason of differentiating WeatherClient from other clients, I would like to know why...

[tgrapperon]

Hey @inwoodev, you're right, this is mostly an oversight. It could probably be removed, and you're not missing anything special.
Implicit conformance to Sendable is not possible when the value type is generic, or of course when it hosts non Sendable properties. This is not the case here, and WeatherClient is implicitly Sendable.

[inwoodev]

Thank your! Everything is clear now!! :)

[tgrapperon]

In any case someone stumbles upon this discussion in the future, implicit sendability is only valid at the module's scope (that is internal). You may want to explicitly conform to Sendable if you intend to define a dependency in its own dedicated module for example, as @mbrandonw pointed in #1535.

[inwoodev]

Good point to include!!