Warning: "Converting non-sendable function value to ..."

[wojciech-kulik]

Hi,

I use the approach with multiple presentation destination:

@Reducer
enum Destination {
     case fooFeature(FooFeature)
     ...
}

But during build I get these warnings:

10AppFeatureV11Destination7ReducerfMm_.swift:66:34 Converting non-Sendable function value to '@Sendable (StoreOf<MapFeature>) -> AppFeature.Destination.CaseScope' (aka '@Sendable (Store<MapFeature.State, MapFeature.Action>) -> AppFeature.Destination.CaseScope') may introduce data races

Is it possible to fix it / suppress?

[stephencelis]

@wojciech-kulik Are you on default main actor isolation? We do have a recent contribution that may fix if you'd like to verify: #3888 We plan on merging it soon.

If this branch doesn't fix things for you, can you provide a project that reproduces the issue?

[pyrtsa]

Our project started also producing the above warnings from an @Reducer enum Destination, when upgrading TCA from 1.24.1 to 1.25.2 and compiling with Xcode 26.4 and Swift 6.3:

Converting non-Sendable function value to
'@Sendable (StoreOf<PageReducer>) -> TabsReducer.Destination.CaseScope' (aka 
'@Sendable (Store<PageReducer.State, PageReducer.Action>) -> TabsReducer.Destination.CaseScope')
may introduce data races

In our case, the Destination enum is nested inside another reducer and both defined in a module of a Swift package (part of which still built under Swift 5 language mode, if that matters). We are not using default MainActor isolation¹.

I gave #3888 a try. It appears to silence this class of compiler warnings, but introduces a new one:

warning: '@preconcurrency' on conformance to 'CaseReducerState' has no effect
    public enum State: @preconcurrency ComposableArchitecture.CaseReducerState {
                ^
note: in expansion of macro 'Reducer' on enum 'Destination' here
    @Reducer public enum Destination {
    ^~~~~~~~

…All the while leaving #3768 not fixed, AFAICT.

Footnotes

1. And I'm happy to note that new projects created in Xcode 26.4 seem to no longer default to MainActor isolation any longer! 🎉 ↩

[wojciech-kulik]

Are you on default main actor isolation

No, I'm using the default - nonisolated setting in Build Settings.
I'm also using Swift 5.

I think I found the solution. Enabling "Approachable Concurrency" removed those warnings :). Not sure why, but it works :D.

[pyrtsa]

Thanks for this insight! It seems to be indeed related to Swift 5 language mode without approachable concurrency. I managed to silence the warning(s) by simply enabling the upcoming feature InferSendableFromCaptures.

I was just about to open a PR with this change, which also appeared to fix the warning.

But now I'm thinking it may be better to just adopt the upcoming features and, ultimately, Swift 6 language mode.