Compiler fails to catch data race

[rayx]

Description

Using a workaround @jamieQ found (see https://forums.swift.org/t/taskgroup-and-sending-closures/77700/2), I'm able to successfully compile code which has data race.

Reproduction

class NonSendable {}

@MainActor func transferToMainActor(_ x: NonSendable) async { }

func nonIsolatedCallee(_ x: NonSendable) async { }

func nonIsolatedCaller(_ x: NonSendable) async {
  await nonIsolatedCallee(x) // this is ok (as expected)

  let box = { return x } 
  await transferToMainActor(box()) // this should fail but it compiles

  await nonIsolatedCallee(x) // x is accessed concurrently in both MainActor and global executor.
}

The code is based on an example in "Task Isolated Regions" section in SE-0414. It has data race but it compiles.

Expected behavior

It shouldn't compile

Environment

% swift --version

Apple Swift version 6.0.1 (swift-6.0.1-RELEASE)
Target: arm64-apple-macosx14.0

Additional information

No response

[gottesmm]

I am preparing a fix for this