Swift 6.2 linking fails depending on the number of parallel jobs

[ivanovdmitri]

Describe the bug

This bug shows up on both macOS (15.7.1, swift 6.2) and Linux (swift:noble docker image, that runs Ubuntu 24.04.3 LTS, swift-6.2-RELEASE) Linking appears to depend on the number of jobs used

To Reproduce

$ git clone https://github.com/pointfreeco/swift-case-paths
$ git checkout 1.7.2
$ swift test --jobs 2 # fails with the message below but works if the number of jobs is 4 or more

Expected behavior
Expect the unit tests to build (and pass)

Screenshots
error: link command failed with exit code 1 (use -v to see invocation)
/project/.build/aarch64-unknown-linux-gnu/debug/CasePathsMacros-tool.build/CasePathableMacro.swift.o:CasePathableMacro.swift.o:$s15CasePathsMacros12SelfRewriterCMf:(.data+0x1358): error: undefined reference to '$s11SwiftSyntax0B8RewriterC14visitationFunc33_F0C5D882E9301122F145EADC4573BFC8LL3forAA0B0VAHcAH_tF'
/project/.build/aarch64-unknown-linux-gnu/debug/CasePathsMacros-tool.build/CasePathableMacro.swift.o:CasePathableMacro.swift.o:$s15CasePathsMacros15ElementRewriterCMf:(.data+0x2670): error: undefined reference to '$s11SwiftSyntax0B8RewriterC14visitationFunc33_F0C5D882E9301122F145EADC4573BFC8LL3forAA0B0VAHcAH_tF'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[240/315] Linking CasePathsMacros-tool
error: fatalError

Environment

• swift-case-paths 1.7.2
• Xcode 26.0.1
• Swift 6.2
• OS: macOS 15.7.1

Additional context
There is also a tests compilation issue that is readily fixed by updating both Package.swift and Package@swift-6.0.swift with:

  package.targets.append(
    .testTarget(
      name: "CasePathsMacrosTests",
      dependencies: [
        "CasePathsMacros",
        .product(
          name: "MacroTesting",
          package: "swift-macro-testing"
        ),
        .product(name: "SwiftSyntaxMacros", package: "swift-syntax"), // fix compilation issue
      ]
    )
  )

[stephencelis]

@ivanovdmitri I cannot reproduce this on Xcode 26.1 or 26.2 beta 1 (on macOS 26). Is it still an issue for you with the 6.2.1 release? If so, can you provide a project that reproduces the issues along with instructions to reproduce?

This all leads me to believe this is an Apple bug and not a bug with CasePaths, though we will accept workarounds to the issue if anyone has any.

[ivanovdmitri]

@stephencelis This bug consistently shows up when you build your repo in the terminal (either on macOS or on Linux):

$ git clone https://github.com/pointfreeco/swift-case-paths
$ git checkout 1.7.2
$ rm -rf .build # important: be sure to build from a clean state
$ swift test --jobs 2 # fails with the linker error message shown in the original post;  usually works with a larger number of jobs

one workaround is:

$ rm -rf Packages/swift-syntax # if it already exists
$ rm -rf .build # build from a clean state
$ swift package edit swift-syntax # put swift-syntax in editing mode, which changes the behavior of the linker
$ swift test --jobs 2 # links correctly

So yes, it seems like this could be an issue within swift-syntax: the linker sometimes fails to find the internal symbol visitationFunc used by the SyntaxRewriter base class, which is used in your CasePathableMacro target.

[stephencelis]

@ivanovdmitri I've followed your steps using Xcode 26.2's swift from the command line and things build, run, and exit just fine for me:

Test Suite 'All tests' passed at 2025-11-12 13:42:36.315.
	 Executed 121 tests, with 0 failures (0 unexpected) in 1.301 (1.309) seconds
􀟈  Test run started.
􀄵  Testing Library Version: 1400
􀄵  Target Platform: arm64e-apple-macos14.0
􀟈  Test caseSet() started.
􁁛  Test caseSet() passed after 0.001 seconds.
􁁛  Test run with 1 test in 0 suites passed after 0.001 seconds.

Maybe it was fixed in the version of Swift 6.2 that comes with the beta? Regardless, if it's still an issue for you it sounds like the issue should probably be filed on the SPM/SwiftSyntax repos? I'm just not sure there's any action for us to take.

[ivanovdmitri]

@stephencelis Fortunately, this bug manifests itself in a similar way on Linux and we can use Swift's official docker image (swift:noble) with the Swift 6.2 release. The following Dockerfile reproduces the effect in a way that doesn't depend on your host machine's settings:

mkdir thebug
cd thebug

make the Dockerfile with the following:

FROM swift:noble
WORKDIR /thebug
RUN git clone --branch 1.7.2 --depth 1 https://github.com/pointfreeco/swift-case-paths .
CMD ["swift", "test", "--jobs", "2"]

run:

docker build -t thebug . && docker run --rm thebug 

result:

error: link command failed with exit code 1 (use -v to see invocation)
/thebug/.build/aarch64-unknown-linux-gnu/debug/CasePathsMacros-tool.build/CasePathableMacro.swift.o:CasePathableMacro.swift.o:$s15CasePathsMacros12SelfRewriterCMf:(.data+0x1358): error: undefined reference to '$s11SwiftSyntax0B8RewriterC14visitationFunc33_F0C5D882E9301122F145EADC4573BFC8LL3forAA0B0VAHcAH_tF'
/thebug/.build/aarch64-unknown-linux-gnu/debug/CasePathsMacros-tool.build/CasePathableMacro.swift.o:CasePathableMacro.swift.o:$s15CasePathsMacros15ElementRewriterCMf:(.data+0x2670): error: undefined reference to '$s11SwiftSyntax0B8RewriterC14visitationFunc33_F0C5D882E9301122F145EADC4573BFC8LL3forAA0B0VAHcAH_tF'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[255/327] Linking CasePathsMacros-tool
error: fatalError

Update: should we open this issue in swift-syntax repo, using your code (and the Dockerfile) as an example where SyntaxRewriter fails to link? I am not using swift-syntax's SyntaxRewriter in any other dependencies besides this repository.

[stephencelis]

@ivanovdmitri Yup, and you can then link to it here. I don't think tracking things here is helpful when we haven't been able to determine if this is a bug in CasePaths that can be addressed within our package.

[stephencelis]

OK to close this out for now till we determine that there's a resolution that can be made here?

[ivanovdmitri]

@stephencelis : I posted the issue in the swift-syntax repo (swiftlang/swift-syntax#3188), feel free to close the current issue in this repo whenever it's convenient.