"Approachable concurrency" miscompilation involving function conversions

[jamieQ]

Description

the combination of NonisolatedNonsendingByDefault, default MainActor isolation, and the swift 5 language mode seems like it can lead to function conversion miscompilation in which the implicit nonisolated(nonsending) isolation parameter is incorrectly passed as an explicit parameter.

Reproduction

/*
built with:
xcrun swiftc \
-enable-upcoming-feature NonisolatedNonsendingByDefault \
-default-isolation MainActor \
-swift-version 5 \
-parse-as-library
*/

struct S {
    var action: (AnyObject) async -> Void

	init(action: @escaping (AnyObject) -> Void) {
		self.action = action
	}
}

@main
enum App {
    static func main() async {
        let s = S { print($0) }
        await s.action(42 as AnyObject) // prints: 'Swift.MainActor'
    }
}

changing the param from AnyObject to other types typically results in runtime crashes.

Expected behavior

the code should correctly perform the necessary function conversions without mixing up the parameters.

Environment

Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

Additional information

issue was originally reported and discussed here: https://forums.swift.org/t/exc-bad-access-with-escaping-closure-and-task/83989
godbolt: https://swift.godbolt.org/z/8cdd8zszo