Pattern that the region based isolation checker does not understand how to check

[dehesa]

Description

I get the message

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

when compiling the code printed below. It is 100% reproducible.

Reproduction

func withTaskProgression<Success: Sendable, Failure: Error>(
  operation: () async throws(Failure) -> sending Success,
  progression: @Sendable (TaskProgress) -> Void,
  isolation: isolated (any Actor)? = #isolation
) async throws(Failure) -> Success {
  try await withoutActuallyEscaping(progression) { (progression) async throws(Failure) -> Success in
    let progress = UnsafeTaskProgress(handler: progression)

    let result: Success
    do {
      result = try await Task.$progress.withValue(progress) {
        try await operation()
      }
    } catch let error as Failure {
      progress.failed(with: error)
      throw error
    } catch { fatalError() } // TODO: Remove when concrete thrown errors are better supported

    progress.finished()
    return result
  }
}

extension Task where Success == Never, Failure == Never {
  @TaskLocal static var progress: UnsafeTaskProgress?
}

final class UnsafeTaskProgress: Sendable {
  private let handler: Handler

  init(handler: @escaping Handler) {
    self.handler = handler
  }

  typealias Handler = @Sendable (TaskProgress) -> Void
  func failed(with error: some Swift.Error) {}
  func finished() {}
}

struct TaskProgress: Sendable {}

Expected behavior

I expect to be able to return Success without issues. In any case, the messages recommended to file a bug and that is what I am doing.

Environment

swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
Target: arm64-apple-macosx15.0

Additional information

I am running macOS 15.0 (Sequoia Beta 24A5331b) with Xcode 16.0 Beta 6 (16A5230g).

A bit outside the scope of the bug, but I am trying to create a Swift Concurrency progression report (as talked about in the Swift Forums).