False positive data race error when passing a non-sendable closure in a generic function returning a `sending` result

[Jnosh]

Description

The error message seems to indicate that the compiler thinks that the body closure is returned and crosses isolation domains but the confusion seems to be caused by sending the generic return type.

Reproduction

// Compile in Swift 6 mode

func run<Success>(
    body: () -> Void
) -> sending Success {
    // Returning task-isolated 'body' as a 'sending' result risks causing data races
    _run(body: body)
}

func _run<Success>(
    body: () -> Void
) -> sending Success {
    fatalError()
}

Expected behavior

The snippet should compile without a data race error (although it might warn or error that sending the return value does not have any effect in this case).

Some workarounds/notes:

• The error only appears in a generic context and not with concrete types (sendable or non-sendable).
• Constraining Success to be Sendable "fixes" the error and the compiler no longer complains about the closure.
• So does making the body closure sending or @Sendable - the compiler does seem to think that the closure is returned across isolation domains and is satisfied if it is sendable

Environment

Xcode 16 beta 6

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

No response

[Jnosh]

Fixed in more recent releases.