Swift 5.9 build failure with versioned Package.swift

[sbooth]

I have a package that I believe should build back to Swift 5.3 but SPI is showing a build failure with Swift 5.9. I recently added support for package traits (new in Swift 6.1) and as a result renamed the old package manifest to Package@swift-5.3.swift to attempt to maintain backwards compatibility.

One of the build failures in question is here, summarized:

error: 'spi-builder-workspace': package 'spi-builder-workspace' is using Swift tools version 6.1.0 but the installed version is 5.9.0

The 5.10 and 6.0 builds work correctly- which leads me to believe the right manifest is being used for those builds- but I'm not sure if I'm using versioned manifests incorrectly (perhaps the new one should be Package@swift-6.1.swift?) or if the problem is SPI-specific or lies elsewhere.

[finestructure]

Yeah, it's odd that this fails on 5.9. My best guess is that it's some weirdness around versioned package manifest resolution in SwiftPM.

FWIW, we're only days away from adding Swift 6.2 and dropping 5.9 - so this problem will go away on its own :)

[heckj]

When Swift picks up those alternate package formats is sometimes confusing. The one you created will only activate for Swift 5.3 - if you want one to override Package.swift for 5.9 specifically, you'd need one that was the "Package@swift-5.9.swift.

Think of them as point fixes for single, specific older versions of the swift compiler. I didn't find the pattern intuitive, but once you know... it's a little easier to judge if you want to wrangle that or not.

[finestructure]

The weird thing is that only 5.9 is failing. I'd have expected all except 6.1 to fail.

[heckj]

Me too

[sbooth]

Think of them as point fixes for single, specific older versions of the swift compiler.

I was under the impression (based on blog posts) that Swift's build system would pick the one that is closest to the current toolchain but I haven't seen any official documentation one way or the other. It's confusing to me why 5.9 fails but 5.10 doesn't. Time to download old Xcode versions and experiment I suppose.