Swift Concurrency inappropriately flags main-actor bound code in non-Sendable closure as not running on main actor

[dfed]

Description

@MainActor-bound code that declares a function inside a non-escaping, non-Sendable anonymous closure fails to compile if the innermost function interacts with the @MainActor unless the innermost function is declared @MainActor.

According to #72382 (comment), a non-sendable closure can never leave the isolation domain it's formed in, and therefore if a non-sendable closure is declared in a @MainActor-bound context, the non-sendable closure should also be @MainActor-bound.

This issue makes utilizing anonymous closures in code-gen in code where strict concurrency code is turned on problematic.

Reproduction

@MainActor
func doSomethingOnMainActor() {}

@MainActor
func testCallingDoSomethingOnMainActor() {
    func doSomething() {
        doSomethingOnMainActor() // Compiles just fine, as expected.
    }
    _ = {
        doSomethingOnMainActor() // Compiles just fine, as expected.
    }()
    func declareDoSomethingInNestedFunction() {
        func doSomething() {
            doSomethingOnMainActor() // Compiles just fine, as expected.
        }
    }
    _ = {
        func doSomething() {
            // The following line should compile, because we are in a method within an anonymous non-escaping, non-sendable closure that is `@MainActor`-bound, which is semantically similar to the `declareDoSomethingInNestedFunction()` function declaration above.
            doSomethingOnMainActor() // Error: Call to main actor-isolated global function 'doSomethingOnMainActor()' in a synchronous nonisolated context
        }
    }()
}

Expected behavior

The code compiles

Environment

Reproduces on both:

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

\swift-driver version: 1.110 Apple Swift version 6.0 (swiftlang-6.0.0.4.52 clang-1600.0.21.1.3)
Target: arm64-apple-macosx14.0

Additional information

No response