Swift 6.2: undesired "Capture of non-sendable type 'T.Type'" warnings

[groue]

Description

With Xcode 26.0 beta (17A5241e), I see a lot of warnings:

• Capture of non-sendable type 'Aggregate.Type' in an isolated closure
• Capture of non-sendable type 'Downstream.Type' in an isolated closure
• Capture of 'recordType' with non-sendable type 'any EncodableRecord.Type' in a '@sendable' closure
• Capture of non-sendable type 'Self.Type' in an isolated closure

Those warnings make no sense: meta-types are Sendable.

EDIT: I initially missed the existence of SE-0470, which introduces non-sendable meta-types. In this issue, the IsolatedConformances and StrictSendableMetatypes experimental features are not enabled. Package.swift mentions swift-tools-version:6.0 or 6.1.

Reproduction

protocol MyProtocol1 { init() }
struct MyStruct<T> {
    var x: @Sendable () -> T.Type
}
func f<T: MyProtocol1>(_ type: T.Type) {
    // ⚠️ Capture of non-sendable type 'T.Type' in an isolated closure
    MyStruct { T.self }
}

protocol MyProtocol2 {
    associatedtype Value
}
func f<T: MyProtocol2>(
    _ type: T.Type,
    // ⚠️ Capture of non-sendable type 'T.Type' in an isolated closure
    closure: @escaping @Sendable (T.Value.Type) -> Void
) {
    Task {
        closure(T.Value.self)
    }
}

Expected behavior

No warning

Environment

Version 26.0 beta (17A5241e)
Apple Swift version 6.2 (swiftlang-6.2.0.9.909 clang-1700.3.9.907)

Additional information

No response

[groue]

WTF, this is a "feature".

https://github.com/swiftlang/swift-evolution/blob/main/proposals/0470-isolated-conformances.md:

Specifically, if a type parameter T does not conform to either Sendable or to a new protocol, SendableMetatype, then its metatype, T.Type, is not considered Sendable and cannot cross isolation boundaries

I don't have the slightest idea of how to remove the warning, though.

[dfed]

I don't think this is a feature unless you've turned on InferIsolatedConformances.

This seems like a bug in the implementation of that feature, though.

[groue]

I don't think this is a feature unless you've turned on InferIsolatedConformances.

I did not enable IsolatedConformances nor StrictSendableMetatypes.

This seems like a bug in the implementation of that feature, though.

OK. Let's wait for this issue to be triaged.

Thanks for your hindsights!

[groue]

See also https://forums.swift.org/t/capture-of-non-sendable-type-concurrency-warning-from-asyncsequence-asynciterator/80379

[luispadron]

+1, were seeing this as well and not immediately obvious how these got turned on.

This was passing for us in Xcode 16.4:

extension AsyncSequence where Self: Sendable, Self.Element: Sendable {
    public func toCurrentValuePublisher(initialValue: Element) -> AnyCurrentValuePublisher<Element, Error> {
        let subject = SendableCurrentValueSubject<Element, Error>(initialValue)

        Task {
            do {
                for try await value in self {
                    await subject.send(value)
                }
                await subject.send(completion: .finished)
            } catch {
                await subject.send(completion: .failure(error))
            }
        }
        return subject.publisher
    }
}

Now it fails with:

Code/iOSAdditions/CombineAdditions/Sources/AsyncSequence+Publisher.swift:14:40: error: capture of non-sendable type 'Self.AsyncIterator.Type' in an isolated closure; this is an error in the Swift 6 language mode
12 |         Task {
13 |             do {
14 |                 for try await value in self {
   |                                        `- error: capture of non-sendable type 'Self.AsyncIterator.Type' in an isolated closure; this is an error in the Swift 6 language mode
15 |                     await subject.send(value)
16 |                 }

Code/iOSAdditions/CombineAdditions/Sources/Publisher+AsyncStream.swift:19:17: error: capture of non-sendable type 'Self.Type' in an isolated closure; this is an error in the Swift 6 language mode
17 |     ) -> AsyncStream<Output> {
18 |         AsyncStream(bufferingPolicy: limit) { continuation in
19 |             let sink = Subscribers.Sink<Output, Never> { _ in
   |                 `- error: capture of non-sendable type 'Self.Type' in an isolated closure; this is an error in the Swift 6 language mode
20 |                 continuation.finish()
21 |             } receiveValue: { value in

[freak4pc]

+1'ing this in a few places, majority of them don't make sense (or actually have Sendable on them)

Specifically cases like

struct MyType<Content: View> { 

}

It will diagnose that Capture of non-sendable type Content.Type in an isolated closure, even though View is @MainActor-isolated, and there shouldn't be anything non-sendable in it? Unless I'm wrong with that assumption,