False Sendable data race warnings when passing functions as parameters

[MahdiBM]

Description

There are false Sendable warnings when passing functions as parameters.

Steps to reproduce

struct Something: Sendable {
    @Sendable func doNothing() { }
}
/// ^~~~ The warning is the same, with or without any of the two `Sendable`s.

func doSomething(
    _ block: @Sendable (Something) -> (() -> Void)
    /// ~~~~~^ `@Sendable` required to trigger the warning.
) {
    fatalError()
}

doSomething(Something.doNothing)
/// ^~~~~ Warning: Converting non-sendable function value to '@Sendable (Something) -> (() -> Void)' may introduce data races

The warning goes away if you open the closure instead of directly passing the function:

doSomething { $0.doNothing }
/// ^~~~~ No warnings

Expected behavior
No false warnings.

Environment
Linux CI with swift 5.8 on Ubuntu jammy shows these warnings, as well as my own local environment with Xcode 15 beta 2 and Swift 5.9, so it's fair to say this is not tied to a platform or a Swift version.

~ % swiftc -version
swift-driver version: 1.82.2 Apple Swift version 5.9 (swiftlang-5.9.0.114.10 clang-1500.0.29.1)
Target: arm64-apple-macosx14.0
~ % xcodebuild -version
Xcode 15.0
Build version 15A5161b
~ % uname -a
Darwin Macbook.local 23.0.0 Darwin Kernel Version 23.0.0: Tue Jun 13 21:16:44 PDT 2023; root:xnu-10002.0.116.505.3~3/RELEASE_ARM64_T6000 arm64

Additional Context

• The issue seems to be similar to false Sendable warning if SE-299 (Extending Static Member Lookup in Generic Contexts) is used #64388.
• We noticed the issue when working on making swift-openapi-runtime compatible with -strict-concurrency=complete: Adopt Sendable throughout API to support -strict-concurrency=complete apple/swift-openapi-runtime#22 (comment)
  • To be clear minimal strict-concurrency is enough for this warning to show up.

[hborla]

This is fixed under -enable-experimental-feature InferSendableFromCaptures on main

[MahdiBM]

@hborla can confirm, thanks!
New:

~ docker run --rm -it swiftlang/swift:nightly-main-jammy swift -enable-experimental-feature InferSendableFromCaptures -e "struct Something: Sendable {                                                  
         @Sendable func doNothing() { }
     }
     /// ^~~~ The warning is the same, with or without any of the two `Sendable`s.

     func doSomething(
         _ block: @Sendable (Something) -> (() -> Void)
         /// ~~~~~^ `@Sendable` required to trigger the warning.
     ) {
         print(\"Doing some!\")
     }

     doSomething(Something.doNothing)"
Doing some!

Old:

~ docker run --rm -it swiftlang/swift:nightly-main-jammy swift -e "struct Something: Sendable {                                                                                                             
         @Sendable func doNothing() { }
     }
     /// ^~~~ The warning is the same, with or without any of the two `Sendable`s.

     func doSomething(
         _ block: @Sendable (Something) -> (() -> Void)
         /// ~~~~~^ `@Sendable` required to trigger the warning.
     ) {
         print(\"Doing some!\")
     }

     doSomething(Something.doNothing)"
-e:13:23: warning: converting non-sendable function value to '@Sendable (Something) -> (() -> Void)' may introduce data races
doSomething(Something.doNothing)
                      ^
Doing some!

[MahdiBM]

@hborla It appears there has been a regression re-introducing this warning, based on the latest Docker images:

New, emits warnings:

~ docker run --rm -it swiftlang/swift:nightly-main-jammy@sha256:39867f18bb76cf9a02d4e7e9dbbaf9674154675063c1b1d7928daa63d2658ea2 swift -enable-experimental-feature InferSendableFromCaptures -e "struct Something: Sendable {
              @Sendable func doNothing() { }
          }
          /// ^~~~ The warning is the same, with or without any of the two `Sendable`s.

          func doSomething(
              _ block: @Sendable (Something) -> (() -> Void)
              /// ~~~~~^ `@Sendable` required to trigger the warning.
          ) {
              print(\"Doing some!\")
          }

          doSomething(Something.doNothing)"
-e:13:28: warning: converting non-sendable function value to '@Sendable (Something) -> (() -> Void)' may introduce data races
     doSomething(Something.doNothing)
                           ^
Doing some!

Old, no warnings:

~ docker run --rm -it swiftlang/swift:nightly-main-jammy@sha256:c46884670e329f4f50e6b640be66bdcc16dcb75829878e0e1d22ebc659f3fd3a swift -enable-experimental-feature InferSendableFromCaptures -e "struct Something: Sendable {
              @Sendable func doNothing() { }
          }
          /// ^~~~ The warning is the same, with or without any of the two `Sendable`s.

          func doSomething(
              _ block: @Sendable (Something) -> (() -> Void)
              /// ~~~~~^ `@Sendable` required to trigger the warning.
          ) {
              print(\"Doing some!\")
          }

          doSomething(Something.doNothing)"
Doing some!

Notice the hash difference of the nightly-main images.

[xedin]

@MahdiBM We have promoted the feature to "upcoming" since it has been accepted by the Swift Evolution which means that the flag now is -enable-upcoming-feature instead of -enable-experimental-feature. I think we should warn about this to avoid the confusion...

[MahdiBM]

@xedin -enable-upcoming-feature does work, thanks!