Description
The main function is not MainActor if it is provided by an extension function by conforming to a protocol.

Steps to reproduce

import Foundation @main struct Test: P { } protocol P {} extension P { static func main() async { print(Thread.isMainThread) // prints false } }

import Foundation @main struct Test: P { static func main() async { print(Thread.isMainThread) // prints true } } protocol P {} extension P { }

Expected behavior
If I understand SE-0323 correctly, both code should print true, since main function should be implicitly MainActor.

Environment
swift-driver version: 1.75.2 Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)
Target: arm64-apple-macosx13.0
Xcode 14.3
Build version 14E222b

Additional context

I encountered this problem while using Swift Argument Parser. The original code is something like this:

@main struct Command: AsyncParsableCommand { @MainActor func run() async throws { // App is a SwiftUI App App.main() } }

The code works in an old version of Xcode (~13.x), but failed with Fatal error: calling into SwiftUI on a non-main thread is not supported when upgrading to 14.x.