Difference in `--project-file-parser=compare`

[philderbeast]

Describe the bug
I encountered a difference in parsing a test project file and am reporting this as requested:

Error: [Cabal-7165]
The legacy and parsec parsers produced different results for the project file. This is unexpected, please report this as a bug.

To Reproduce
In this repository, with the tests of #10933 added (or on that pull request's branch):

$  cd cabal-testsuite/PackageTests/ProjectImport/UniquePathDuplicates
$ rm -rf ./dist-newstyle/
$ cabal build --project-file=yops-0.project --project-file-parser=compare
...
Error: [Cabal-7165]
The legacy and parsec parsers produced different results for the project file.
This is unexpected, please report this as a bug.
...

Expected behavior
No difference in the parser results

System information

• Ubuntu 25.04
• cabal installed and built from 3d032f3, ghc-9.10.2

[philderbeast]

Converting the output to JSON and comparing, it looks like the parsec parser (on the right) has more output:

I asked vscode to format the reported output from each parser as JSON and compared these files, legacy.json and parsec.json.

[mpickering]

Thanks @philderbeast, I'll look into it.

[mpickering]

Thanks for the report @philderbeast, I can't reproduce this with HEAD so it appears to be a problem with your branch.

[mpickering]

In particular, the parsec parser reports that the project depends on this config file (correctly)

E (ProjectConfigPath ("yops-6.config" :| ["yops/yops-5.config","yops-0.project"]))

but the legacy parser doesn't include that path.

[philderbeast]

I can't reproduce this with HEAD so it appears to be a problem with your branch.

I agree, it is not reproducible on master. I checked the branch again. As well as reporting imports of the same file by different paths it tries to filter for unique imports (for an import's canonical file path - not import path) it has seen before. That would explain the difference even if, as I suspect, the filtering is only partially successful.