Xcode build errors, "Unable to find module dependency: 'NIOCore'"

[snej]

Describe the bug
Added IkigaJSON as a dependency in my Xcode [26.2] project using the GUI. Xcode also added swift-atomics, swift-collections, swift-nio, swift-system dependencies.
When building, even if I don't import IkigaJSON anywhere, I now get a number of build errors, shown below. The compiler is unable to find _NIOJSONs dependencies NIOCore and NIOFoundationCompat.

I was able to reproduce this with a new empty Xcode project created for macOS using the Swift Framework template. All I did was open the project, choose Add Package Dependencies, add IkigaJSON, and build.

The errors begin:

/Users/snej/Library/Developer/Xcode/DerivedData/jsontest-extltjcxnrptnahjlbejrfidqqzw/SourcePackages/checkouts/IkigaJSON/Sources/_NIOJSON/ByteBufferSupport.swift:1:8: error: Unable to find module dependency: 'NIOCore'
import NIOCore
       ^ (in target '_NIOJSON' from project 'IkigaJSON')
/Users/snej/Library/Developer/Xcode/DerivedData/jsontest-extltjcxnrptnahjlbejrfidqqzw/SourcePackages/checkouts/IkigaJSON/Sources/_NIOJSON/ByteBufferSupport.swift:1:8: note: A dependency of main module '_NIOJSON'
import NIOCore
       ^ (in target '_NIOJSON' from project 'IkigaJSON')

A full build log is in the attached zip file.

Sample to Reproduce

Download attached Xcode project. Just open jsontest.xcodeproj and choose Build.
jsontest.zip

Expected behavior
Expected it to build without errors.

Environment (please complete the following information):

• OS: macOS 15.7.4, Xcode 26.2
• Compilation Mode: Debug
• IkigaJSON Version: 2.4.1

Additional context
Add any other context about the problem here.

[snej]

If I add swift-nio to a new project, it builds fine.
If I then add IkigaJSON, build fails just as above.

Looking at the log of the successful build, the initial "Prepare Build" stage includes "Compiling Clang module" steps for _AtomicsShims, CNIOLinux, CNIODarwin, CNIOOpenBSD, CNIOWindows, CNIOPosix, CNIOAtomics.

But in the failed build, the errors start with Unable to find module dependency: '_AtomicsShims' (in target '_NIOJSON' from project 'IkigaJSON') ... which is weird since _AtomicsShims was already built.

Admittedly I'm pretty new to Swift development, but this seems quite strange, like a possible build-system bug.

[Joannis]

Interesting, I'll investigate.

[Joannis]

Thank you for submitting a reproducer!

[snej]

NP! You don't even need that attached project; you can just create your own new project as I described in the 2nd paragraph.

BTW I noticed that the SwiftNIO project released a new version a few days ago; maybe that triggered this?

[Joannis]

The problem is a mix of Swift toolchain version, selected traits and host OS. Some combinations are just poorly handled on my end.