Runtime crash when passing and executing non-Sendable closure to a class from swiftLanguageMode v5 enabled package

[akbashev]

Description

• Simple main.swift (this is important) app with swift-tools-version: 6.0 package
• Having a old target without any Sendable mark yet with .swiftLanguageMode(.v5) enabled
• Running at top level in main.swift file

App will run but crash when trying to execute this closure with an error Task 1: EXC_BREAKPOINT (code=1, subcode=0x1004cbbc4)

Reproduction

Package.swift

// swift-tools-version: 6.0
// The swift-tools-version declares the minimum version of Swift required to build this package.
import PackageDescription

let package = Package(
  name: "App",
  platforms: [.macOS(.v15)],
  targets: [
    .target(
      name: "System",
      swiftSettings: [.swiftLanguageMode(.v5)]
    ),
    .executableTarget(
      name: "App",
      dependencies: [ "System" ]
    ),
  ]
)

System/System.file

public class System {
  
  public struct Settings { public var name: String = "" }
  
  public let settings: Settings

  public convenience init(configureWith configure: (inout Settings) -> () = { _ in () }) async {
    var settings = Settings()
    configure(&settings) // runtime crash here with `Task 1: EXC_BREAKPOINT (code=1, subcode=0x1004cbbc4)`
    self.init(settings: settings)
  }
  
  public init(settings: Settings) {
    self.settings = settings
  }
  
  public func run() async throws {
    try await Task.sleep(for: .seconds(3))
  }
}

App/main.swift

import System

let system = await System {
  $0.name = "Demo"
}

print(system.settings.name)
try await system.run()
print("Done")

Stack dump

thread #1, queue = 'com.apple.main-thread'
    frame #0: 0x000000018f8b1c34 libsystem_kernel.dylib`mach_msg2_trap + 8
    frame #1: 0x000000018f8c4028 libsystem_kernel.dylib`mach_msg2_internal + 76
    frame #2: 0x000000018f8ba98c libsystem_kernel.dylib`mach_msg_overwrite + 484
    frame #3: 0x000000018f8b1fb4 libsystem_kernel.dylib`mach_msg + 24
    frame #4: 0x000000018f993c80 CoreFoundation`__CFRunLoopServiceMachPort + 160
    frame #5: 0x000000018f9925d8 CoreFoundation`__CFRunLoopRun + 1188
    frame #6: 0x000000018fa50898 CoreFoundation`_CFRunLoopRunSpecificWithOptions + 532
    frame #7: 0x000000018f9e9a44 CoreFoundation`CFRunLoopRun + 64
    frame #8: 0x0000000287fd7ee8 libswift_Concurrency.dylib`CFMainExecutor.run() + 48
    frame #9: 0x0000000287fd7ba0 libswift_Concurrency.dylib`protocol witness for RunLoopExecutor.run() in conformance DispatchMainExecutor + 48
    frame #10: 0x0000000287fd7f94 libswift_Concurrency.dylib`swift_task_asyncMainDrainQueueImpl + 108
    frame #11: 0x0000000287ff8170 libswift_Concurrency.dylib`swift_task_asyncMainDrainQueue + 92
    frame #12: 0x0000000100000c34 App`main at main.swift:0
    frame #13: 0x000000018f535d54 dyld`start + 7184
* thread #3, name = 'Task 1', queue = 'com.apple.root.default-qos.cooperative', stop reason = EXC_BREAKPOINT (code=1, subcode=0x1004cbbc4)
    frame #0: 0x00000001004cbbc4 libdispatch.dylib`_dispatch_assert_queue_fail + 120
    frame #1: 0x0000000100506d30 libdispatch.dylib`dispatch_assert_queue$V2.cold.1 + 116
    frame #2: 0x00000001004cbb48 libdispatch.dylib`dispatch_assert_queue + 108
    frame #3: 0x0000000287f900a4 libswift_Concurrency.dylib`_swift_task_checkIsolatedSwift + 48
    frame #4: 0x0000000287ff0df4 libswift_Concurrency.dylib`swift_task_isCurrentExecutorWithFlagsImpl(swift::SerialExecutorRef, swift::swift_task_is_current_executor_flag) + 356
    frame #5: 0x0000000100001344 App`closure #1($0=(name = "")) -> () in App at main.swift:0
  * frame #6: 0x00000001000016e0 App`System.__allocating_init(configure=0x00000001000012f0 App`closure #1 (inout System.System.Settings) -> () in App at main.swift:3) at System.swift:9:5
    frame #7: 0x0000000100000cec App`async_MainTQ0_ at main.swift:3:20

Expected behavior

If you change main.swift to App.swift file, like:

import System

@main
struct App {
  
  static func main() async throws {
    let system = await System { // Sending value of non-Sendable type '(inout System.Settings) -> ()' risks causing data races
      $0.name = "Demo"
    }
    
    print(system.settings.name)
    try await system.run()
    print("Done")
  }
}

Compiler will fire an error Sending value of non-Sendable type '(inout System.Settings) -> ()' risks causing data races.
Guess should be same when compiled at top level in main.swift.

Environment

macOS 26, Xcode 26.0.1, Swift 6.2 (6.2.0.19.9)

Additional information

I know this an edge case which can be fixed by marking closure as sending or Sendable, just sometimes you don't have this option with 3rd party frameworks.