Error about non-Sendable type when using Swift 5 mode with @preconcurrency API

[michaeljtsai]

Description

After updating to Swift 6, I am getting Swift Concurrency warnings, even with the Swift 5 language mode, for code that used to compile cleanly. For example, the example below produces this error:

warning: capture of 'buffer' with non-sendable type 'UnsafeMutableBufferPointer<NSNumber>' in a `@Sendable` closure; this is an error in the Swift 6 language mode
        buffer[index] = NSNumber(value: index)
        ^
Swift.UnsafeMutableBufferPointer:1:23: note: generic struct 'UnsafeMutableBufferPointer' does not conform to the 'Sendable' protocol
@frozen public struct UnsafeMutableBufferPointer<Element> : Copyable where Element : ~Copyable {

Reproduction

I can reproduce this issue with a CLI project created in Xcode 16, configured to use Swift 5 language mode.

let count = 10
let array = Array(unsafeUninitializedCapacity: count) { (buffer, initializedCount) in
    DispatchQueue.concurrentPerform(iterations: count) { index in
        buffer[index] = NSNumber(value: index)
    }
    initializedCount = count
}

Expected behavior

I expected this to compile without any warnings or errors using the Swift 5 mode of the Swift 6 compiler, as it did with Swift 5 and Xcode 15. DispatchQueue.concurrentPerform() is annotated with @preconcurrency, which I thought was supposed to suppress these sorts of warnings.

Environment

Version 16.1 beta 2 (16B5014f)
swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.11 clang-1600.0.26.2)
Target: x86_64-apple-macosx15.0

Additional information

The problem does not occur if I write it like this:

let count = 10
let array = Array(unsafeUninitializedCapacity: count) { (buffer, initializedCount) in
    let closure = { (index: Int) in
        buffer[index] = NSNumber(value: index)    
    }
    DispatchQueue.concurrentPerform(iterations: count, execute: closure)
    initializedCount = count
}

I am also seeing similar issues calling DispatchQueue.main.sync() as well as these APIs:

@MainActor func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, fileNameForType fileType: String) -> String
nonisolated func filePromiseProvider(_ filePromiseProvider: NSFilePromiseProvider, writePromiseTo url: URL) async throws

and can provide more info on that if you think these are separate issues.

[hborla]

I merged a fix for this to both main and release/6.0.