Swift 6 MainActor isolation issue?

[EddieLukeAtmey]

Hi, I'm writing like this in code swift 5.9, it's working:

UIStackView().then($0.addSubview).then {
    $0.axis = .vertical
    $0.addArrangedSubviews(...)
}

However, when I upgrade to swift 6.0 or 6.2, the compiler is complaining:

Converting function value of type '@mainactor (UIStackView) throws -> Void' to '(UIStackView) throws -> Void' loses global actor 'MainActor'

I tried config in build setting's Default actor isolation = MainActor but it's not working. i suspect the cause is in the spm defination:

// swift-tools-version:5.0

maybe this could help fix it? https://www.avanderlee.com/concurrency/default-actor-isolation-in-swift-6-2/

// swift-tools-version: 6.2
...
.target(
    name: "DefaultActorIsolationPackage",
    swiftSettings: [
        /// You can add a new `defaultIsolation` Swift setting to any of your SPM targets.
        .defaultIsolation(MainActor.self)
    ]
)

I'm not totally sure about this, as i'm not an expert on those Actors thingy.

[devxoul]

Thanks for reporting this! The root cause is that the library was compiled with swift-tools-version:5.0 (Swift 5 language mode), where non-escaping closures don't inherit the caller's actor isolation.

In Swift 6 language mode, non-escaping closure parameters automatically inherit the caller's actor isolation — so bumping swift-tools-version to 6.0 fixes this without any API changes. No overloads or @preconcurrency workarounds needed.

PR: #107