Fix native macOS build by excluding SwiftUI WebAuthenticationSession

[radmakr]

WebAuthenticationSession+Utility.swift doesn't compile when building for
native macOS, breaking swift build on a Mac and any SwiftPM consumer
that resolves OAuthenticator for the macOS host.

It builds no problem in Xcode but swift build fails.

[mattmassicotte]

Hey, thanks for reporting this. Do you understand why this is the case? I'm trying to figure it out and I do not understand what exactly is going wrong here.

[mattmassicotte]

I've done some research here, and the problem turns out to be non-trivial. But this does need to be available to macOS targets and this is (apparently) how it is done:

#if canImport(_AuthenticationServices_SwiftUI)
import AuthenticationServices
import SwiftUI
import _AuthenticationServices_SwiftUI

// ...

Something about how the AuthenticationServices framework is integrated with SwiftUI is weird and/or exposing a bug.

[mattmassicotte]

also, what version of Xcode (swift toolchain) did you notice this problem with?

[radmakr]

Do you understand why this is the case?

I don't understand the compiler or build system enough to understand why.

what version of Xcode (swift toolchain)

Xcode Version 26.5 (17F42) but it builds fine in Xcode. It fails to build in the terminal using swift build
I am using swiftly and am currently set to Swift 6.3.0

Since I don't currently need macOS support I am happy to just use my fork's branch. Feel free to close out this PR if you think it is the wrong approach.

This bug really broke my brain. I think it is the first time I had code that could compile with Xcode but not swift build 🤣

[mattmassicotte]

I pushed up what I think is a more robust workaround, but I'm 75% sure this should should just have been working as it was before. Something's up but my patience for figuring it out has expired...

[radmakr]

Thanks @mattmassicotte
The changes you made resolved the issue as far as I can tell.