[concurrency]: data race safety hole with Sendable, global actor isolated subclasses

[jamieQ]

Description

per SE-434, subclassing a non-Sendable, nonisolated class with a global actor isolated subclass should be rejected:

We propose that:
[snip...]

• A global-actor-isolated subclass of a non-isolated, non-Sendable class is allowed, but it must be non-Sendable.

today it is not, which is a data race safety hole.

Reproduction

open class NonSendable {
    var superProp = ""
    func test() {}
}

@MainActor
final class IsolatedSubclass: NonSendable, Sendable { // accepted but should not be
    func trySendableCapture() {
        for _ in 0...100 {
            Task.detached {
                self.superProp += "1"
                print("prop: ", self.superProp)
            }
        }
    }
}

@main
enum App {
    static func main() async {
        let iso = IsolatedSubclass()
        iso.trySendableCapture()
        try? await Task.sleep(for: .seconds(2))
    }
}

Expected behavior

the Sendable, global actor isolated subclass should be rejected by the compiler.

Environment

Swift version 6.2 (swift-6.2-RELEASE)
Target: x86_64-unknown-linux-gnu

Additional information

came up when thinking about this: https://forums.swift.org/t/pitch-sendable-conformance-for-suppressing-sendable-inference/83288/40
godbolt: https://godbolt.org/z/sqz94q3ea

[jamieQ]

FYI @simanerush @mattmassicotte @hborla