The compiler fails to resolve concurrency isolation check

[tapsns]

Description

For a actor with an async method that returns a sending result, the compiler complains:

Pattern that the region based isolation checker does not understand how to check. Please file a bug

Reproduction

import Cocoa

final class Foo {
  func evaluate(_ image: sending NSImage) async {
    // Do something with the image
  }
}

actor Bar {
  private let provider = ImageProvider()

  public func run() async {
    let image = await provider.makeImage()
    let foo = Foo()
    
    // The compiler complains about the following call.
    await foo.evaluate(image)
  }
}

actor ImageProvider {
  func makeImage() async -> sending NSImage {
    return NSImage()
  }
}

Expected behavior

This code should compile without error. This is simple code that demonstrates the bug. In my real code, I am using an actor to isolate access to a resource.

Note that makeImage() simply creates the image and returns it to caller, so there is no use of it outside of the caller's isolation region.

This is clearly a compiler error given that the compiler is literally saying, "Please file a bug."

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.4 clang-1600.0.30)
Target: arm64-apple-macosx15.0

Additional information

Marking makeImage() as nonisolated avoids the compiler error, but I don't think it would be a general workaround due to the implications of that marker.