Returned sending value can not be passed into sending parameter in another function

[muukii]

Description

No response

Reproduction

 class _NonSendable {
    init() {
    }
  }

 class Class_1 {
            
    nonisolated func run_sending<T>(_ closure: () -> sending T) -> sending T {  
      let t = closure()
      return t
    }
    
    nonisolated func sub<T>(_ value: sending T) {
      print(value)
    }
    
    nonisolated func main() {
            
      let r = run_sending {
        _NonSendable()
      }
      // ❌ why is this error?
      sub(r)
            
      do {
        let r = _NonSendable()
       // this is fine
        sub(r)
      }
                      
    }
    
  }

Expected behavior

Should not the returned value as sending be able to be passed into a sending parameter in another function?

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

Additional information

No response

[muukii]

this is also related
#77199

[jamieQ]

this example appears to compile on a recent nightly toolchain (compiler explorer example). a potential workaround is to ensure the functions that return sending values do not implicitly take additional parameters that may affect the inferred isolation regions (e.g. the implicit self of instance methods). this can be done in this case by either making the method static or a free function, e.g.

class _NonSendable {}

private func run_sending<T>(
    _ closure: () -> sending T
) -> sending T {
    let t = closure()
    return t
}

class Class_1 {
    nonisolated func sub<T>(_ value: sending T) {
        print(value)
    }

    nonisolated func main() {
        let r = run_sending {
            _NonSendable()
        }
        sub(r)
    }
}