Strict Concurrency: False Positives?

[ole (Ole Begemann)]

@tera

These checks are not very reliable yet. For example:

import Foundation

@MainActor func mainThreadCall() {}

func foo() {
    DispatchQueue.main.async(execute: mainThreadCall) // ✅ (expected)
    let queue = DispatchQueue.main
    queue.async(execute: mainThreadCall)
//                     🔶╰─ warning: converting function value of type '@MainActor () -> ()' to '@convention(block) () -> Void' loses global actor 'MainActor'; this is an error in the Swift 6 language mode
}

func baz() {
    struct DispatchQueue {
        static let main = Foundation.DispatchQueue.global()
    }
    DispatchQueue.main.async(execute: mainThreadCall) // ✅ but should be a warning
}

For reference, I blogged about why this works the way it does: How the Swift compiler knows that DispatchQueue.main implies @MainActor

tl;dr: it's a hardcoded syntax check in the compiler that looks for DispatchQueue.main.async (and some related APIs), in exactly that spelling.

This was also discussed in this earlier thread: How does swift annotate that DispatchQueue.main.async runs on @MainActor but DispatchQueue.background.async doesn't