Reducer<Self> make build error when Default Actor Isolation is MainActor in Xcode26-beta

[Ryota-Kurokawa]

Description

Hi, I find build error using Xcode26-beta that is not occurred in Xcode16.x
if you set Default Actor Isolation, MainActor

The point of error is here

var body: some ReducerOf<Self>

error message:

Type 'xxFeature' does not conform to protocol 'Reducer'

if you code this, the error does not occurs. Reducer<State, Action>

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

Does not occurred build error using ReducerOf<Self>

Actual behavior

if you use ReducerOf<Self>, build error is occurred, when Default Actor Isolation is MainActor

Reproducing project

1. set Default Actor Isolation, MainActor
2. check using of ReducerOf<Self>
3. build your app

The Composable Architecture version information

Latest

Destination operating system

iOS 18.4, iOS 26.0

Xcode version information

Xcode 26.0-beta

Swift Compiler version information

unspecified

[mbrandonw]

Hi @Ryota-Kurokawa, the default main actor isolation features of Swift 6.2 are really not ready for primetime. There are many bugs we are waiting to be fixed before we can see what changes need to be made to support it. And this really affects all 3rd party libraries, not just TCA. In the meantime you should be able to switch to Swift 5 mode if you want to continue exploring default main actor isolation with TCA.

Also for now I think a discussion is best for this rather than an issue, and if anyone explores this more we'd love to hear about what you find.

[Ryota-Kurokawa]

Thanks for answer @mbrandonw !
I agree with you that there are many bugs we should address issues.

At the moment, I think it would be fine to use Reducer<State, Action>.

[gmedori]

As another data point, when I turn on default main actor isolation in my project using the @Reducer macro, I get an error saying simply "Circular reference" on my type. Specifically, the error points to the line in the macro expansion that declares the conformance to the Reducer protocol: