Conformance of 'Feature.Path.State' to protocol 'CaseReducerState' crosses into main actor-isolated code and can cause data races

[ddanilyuk]

Description

While creating a new project with TCA using Xcode 26 beta 7, Swift 6 mode, and SWIFT_DEFAULT_ACTOR_ISOLATION=MainActor.
I encounter a compiler error when applying the @Reducer macro to an enum.

I’m not sure whether this is a TCA issue or a compiler/Xcode, but the macro appears to fail in this configuration.

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

No response

Actual behavior

No response

Reproducing project

@Reducer
struct Feature {
    @ObservableState
    struct State: Equatable { }
}

@Reducer
enum Path {
    case child(Feature)
}

extension Path.State: Equatable { }

The Composable Architecture version information

1.22.2

Destination operating system

iOS 26

Xcode version information

Xcode 26 beta 7

Swift Compiler version information

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

[stephencelis]

@ddanilyuk The minimal case you provide compiles fine for me with main actor isolation:

Can you share a zipped up project that fails to compile?

[ddanilyuk]

@stephencelis
TestPath.zip

[stephencelis]

I keep forgetting Xcode still starts new projects in Swift 5 mode! Thanks, will look into this soon!

[stephencelis]

@ddanilyuk Might take us awhile to unravel this. While main actor isolation is intended to simplify concurrency, it complicates library design/usage. If possible, can you work with nonisolated mode for now?

[ddanilyuk]

@stephencelis
It's be ideal to start new project with everything enabled, but it's okay.

Also, i’ve hit another Swift 6 + @mainactor issue: using ReducerOf<Feature> fails to compile, while Reducer<State, Action> works. (Screenshot attached.)

I also noticed that autocomplete for TCA features has been failing more often, even on small ones.

Xcode 26 will be released in two weeks, so it would be great to see a fix for compile issues before its release.

[mbrandonw]

Also, i’ve hit another Swift 6 + @mainactor issue: using ReducerOf<Feature> fails to compile, while Reducer<State, Action> works. (Screenshot attached.)

This seems pretty clearly to be a Swift bug. Changing to some Reducer<State, Action> fixes it. I'm not sure there is much we can do about Swift bugs. Please do report this issue though.

I also noticed that autocomplete for TCA features has been failing more often, even on small ones.

This too just seems like an unfortunate Swift/Xcode bug. I recommend filing issues/feedbacks.

Xcode 26 will be released in two weeks, so it would be great to see a fix for compile issues before its release.

We can only do so much about the Swift compiler bugs, and the default @MainActor isolation unfortunately pushes a lot of complexity to library maintainers. Most 3rd party libraries are not going to work with default @MainActor isolation from the beginning. We are going to do our best to support it.

[stephencelis]

We are looking into other workarounds here, but the main issue appears to be the nesting of Path in another reducer. Declaring Path reducers at the top-level appear to avoid the isolation errors. In general it seems that nested types are more difficult to apply macros to.

A workaround would be for our macro to look for explicit @MainActor on the Path reducer so that it can use this info in the expansion. This would allow you to continue to nest your Path type with the caveat that you must explicitly annotate it with the @MainActor.

[stephencelis]

2.0 will fully support default main actor isolation and address this issue.