Linux build failure -- error: cannot inherit from class 'NSLock' (compiled with Swift 6.0) because it has overridable members that could not be loaded in Swift 5.10

[clackary]

Short description of the issue:

RxSwift fails to build on Ubuntu 22.04 against 2024-07-22 nightly Swift toolchains (and newer).

Expected outcome:

Top of RxSwift main (also reproduced with 6.5.0) builds successfully against 2024-07-15 toolchain, but starting failing against newer toolchains, as of 2024-07-22 and later.

What actually happens:

/project/RxSwift/Sources/RxSwift/AtomicInt.swift:11:13: error: cannot inherit from class 'NSLock' (compiled with Swift 6.0) because it has overridable members that could not be loaded in Swift 5.10
 9 | import Foundation
10 | 
11 | final class AtomicInt: NSLock {
   |             `- error: cannot inherit from class 'NSLock' (compiled with Swift 6.0) because it has overridable members that could not be loaded in Swift 5.10
12 |     fileprivate var value: Int32
13 |     public init(_ value: Int32 = 0) {

Self contained code example that reproduces the issue:

Build errors are thrown out of Sources/RxSwift/AtomicInt.swift, but it also reproduces with a simple inherit from NSLock.

import Foundation

class FooLock: NSLock {}

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

6491a16

Platform/Environment

• iOS
• macOS
• tvOS
• watchOS
• playgrounds
• Linux, Ubuntu 22.04

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• easy, 100% repro
• sometimes, 10%-100%
• hard, 2% - 10%
• extremely hard, %0 - 2%

Xcode version:

N/A

⚠️ Fields below are optional for general issues or in case those questions aren't related to your issue, but filling them out will increase the chances of getting your issue resolved. ⚠️

Installation method:

• CocoaPods
• Carthage
• Git submodules

I have multiple versions of Xcode installed:
(so we can know if this is a potential cause of your issue)

• yes (which ones)
• no

Level of RxSwift knowledge:
(this is so we can understand your level of knowledge
and formulate the response in an appropriate manner)

• just starting
• I have a small code base
• I have a significant code base

[clackary]

I figured there's a chance this is a regression out of swift-corelibs-foundation, so I filed a similar issue over there -- apple/swift-corelibs-foundation#5076

If that doesn't go anywhere, this could in theory be resolved within RxSwift by refactoring away from AtomicInt: NSLock inheritance.

[clackary]

Looks like this same failure is showing up in the swift package index testing: https://swiftpackageindex.com/builds/7B923645-D50A-4667-B129-33056F5C7599