missing-home-modules reports unreferenced module with GHC 9.4.1

[andreasabel]

With GHC 9.4.1, I am encountering:

Building library for Agda-2.6.3..

<no location info>: error: [-Wmissing-home-modules, -Werror=missing-home-modules]
    These modules are needed for compilation but not listed in your .cabal file's other-modules: 
        Agda.Utils.Ord

The respective module Agda.Utils.Ord exists in the source tree but isn't referenced from anywhere.

To reproduce, unpack error-missing-home-module.zip and type in the root directory:

cabal build -w ghc-9.4.1

This might be a GHC 9.4.1 regression. Upstream issue: https://gitlab.haskell.org/ghc/ghc/-/issues/22017

[ulysses4ever]

Interesting bug, thanks for submitting! I'm wondering if need to account for it on this bug tracker or GHC's one suffices. I mean, can we do something about it on our end?

[andreasabel]

@mpickering found out that the module in question is referenced from a hs-boot file which in turn is not referenced. But apparently, GHC 9.4.1 takes such boot files into account no matter whether they are referenced or not.

I found some hint in the release notes:

GHC is now a bit better at detecting redundant hs-boot files and may report warnings where it previously did not. In such cases the correct solution is generally to remove the hs-boot file in question.

I think the remaining discussion can be carried out on the GHC bug tracker. In the beginning, I was not sure where the problem was, so I first opened the issue here.