GAIT instance is modified simultaneously from different isolations

[rayx]

Description

I did experiments to verify my understanding that a GAIT instance might be accessed from either current isolation or global actor but not both simultaneously. I tried a lot of setup and compiler has caught data races in all of them except this one:

@MainActor
struct S {
    var value = 0

    mutating func modify() {
        value += 1
    }
}

actor A {
    @concurrent func transferToGlobalExecutor(_ s: inout S) async {
        s.value += 1
    }

    func test() async {
        var s = S()
        Task {
            await transferToGlobalExecutor(&s)
        }
        await s.modify()
    }
}

await A().test()

The code compiles, but IIUC s is modified simultaneously by transferToGlobalExecutor() and s.modify(), which runs in different isolations.

Reproduction

See above

Expected behavior

The code shouldn't compile.

Environment

x86-64 swiftc nightly on godbolt

Additional information

No response

[rayx]

It turns out to be a setup issue. It appears that @concurrent in transferToGlobalExecutor() declaration has no effect and the function still runs in actor executor. I filed #85727 to track it.