Here's a workaround around that bug i created

[IgorMuzyka]

Added package Trait ContemporaryMacOS which acts as marker for propagating @available(macOS 15, *) allowing to evade bumping package version to macOS 15 and instead supplying this Trait when depending on Subprocess package and hiding things which cascade the Atomic & AsyncSequence<Result, Failure> requirement for macOS 15.

@iCharlesHu let me know what you think about this.

[IgorMuzyka]

The bug in question.

Now if i specify package like this:
.package(url: "https://github.com/IgorMuzyka/Subprocess", branch: "charles/mutex-availability", traits: ["SubprocessFoundation", "ContemporaryMacOS"]),

while having my package platforms be .macOS(.v15) it builds without errors.

[IgorMuzyka]

Essentially when you add this trait it just enables the following in places which cascade from those 2cases/3places i pointed out in the issue/pull-request in swiftlang repo:

#if ContemporaryMacOS
@available(macOS 15, *)
#endif

PS: this solution was done under the influence of one liter of 🍺.

[iCharlesHu]

Thanks for the PR! Unfortunately I don't think this approach will work because when ContemporaryMacOS and SubprocessSpan are both on you essentially get conflicting availabilities like

@available(macOS 15.0, *)
@available(macOS 14.0, *)
struct Execution { ... }

I updated swiftlang/swift-subprocess#16 it should now work correctly on ancient macOSs now.

[IgorMuzyka]

My problem wasn't ancient macOS though, i have a package stating macOS(.v15) and it depends on Subprocess which states macOS(.v13).
And it was giving me this error about availability regardless that i clearly target higher.

Will check later if your changes also fix this for me.