A full reproducer with analysis is at https://github.com/andreasabel/bug-cabal-3.12-setup .

I noticed this issue when using ghc-9.10.1 for the first time to build Agda using cabal v1-install. The Agda parser was malfunctioning. Turned out I had a stale Lexer.hs next to my Lexer.x, and this would be used when building with ghc-9.10 but not when building with ghc-9.8. After some hours I traced the problem down to the version of the Cabal library shipped with these GHCs.

In short

Cabal-3.12 passes the include directories to GHC in a different order than Cabal-3.10. Cabal-3.10 puts dist/build before the hs-source-dirs whereas Cabal-3.12 does it the other way round. This leads GHC to pick up a different .hs file:

• In case of Cabal-3.10 the one generated by the build-tool.
• In case of Cabal-3.12 the one sitting in the source tree.

The problem is reproduced in a GitHub workflow run here: https://github.com/andreasabel/bug-cabal-3.12-setup/actions/runs/9291884301 . Passes with GHC 9.8, fails with GHC 9.10, independent of the version of cabal-install.

Full description

It follows a longer description copied from https://github.com/andreasabel/bug-cabal-3.12-setup/blob/61cdad9444a3b13e2b833e1dde36019c4a31f782/README.md

Cabal-3.12 passes include dirs in wrong order to GHC

Conditions:

• GHC 9.10
• cabal v1-install
• custom setup (can be default Setup.hs), so that Cabal-3.12 is used
• building a library (not just an executable)
• using a build-tool: we use happy here to shadow a .hs file by a .y file
• hs-source-dirs is not . but e.g. src

Files:

• fred.cabal with
  • build-type: Custom
  • library with hs-source-dirs: src (in particular not .)
• Setup.hs (standard)
• src/Fred.y: processed by happy
• src/Fred.hs: stale code that should be shadowed by Fred.y always

In this setting cabal v1-install malfunctions if ghc is ghc-9.10.1. (Works with older GHCs.)
It picks up src/Fred.hs instead of dist/build/Fred.hs that is created by happy from src/Fred.y.

Looking at the verbose output cabal v1-install -v3, we notice a difference in the call to ghc when the GHC is 9.10.1.

• GHC 9.10 / Cabal-3.12

    /usr/local/bin/ghc --make -fbuilding-cabal-package \
      ...
      -isrc -idist/build \
      ... \
      Fred
  
    [1 of 2] Compiling Main ( Fred.hs, dist/build/Main.o, ...
  

• GHC 9.8 / Cabal-3.10

    /usr/local/bin/ghc --make -fbuilding-cabal-package \
      ... \
      -idist/build -isrc \
      ... \
      Fred
  
    [1 of 1] Compiling Parser ( dist/build/Parser.hs, ... )
  

When building with GHC 9.8, library Cabal-3.10 is used which places the path dist/build with the generated files correctly before the path . of the source files; but with GHC 9.10, library Cabal-3.12 is used which does it the other way.

Full calls:

• GHC 9.10 / Cabal-3.12

    /usr/local/bin/ghc --make -fbuilding-cabal-package -O -static -dynamic-too -dynosuf dyn_o -dynhisuf dyn_hi \
      -outputdir dist/build -odir dist/build -hidir dist/build \
      -hiedir dist/build/extra-compilation-artifacts/hie \
      -stubdir dist/build -i \
      -i. -idist/build \
      -idist/build/autogen -idist/build/global-autogen \
      -Idist/build/autogen -Idist/build/global-autogen -Idist/build \
      -I/usr/local/opt/icu4c/include -I/usr/local/opt/libxml2/include/libxml2 \
      -optP-include -optPdist/build/autogen/cabal_macros.h \
      -this-unit-id stale-0-Ly26ql3fYpuLnJ9xynYGbf \
      -hide-all-packages -Wmissing-home-modules -package-db dist/package.conf.inplace \
      -package-id array-0.5.7.0-2002 -package-id base-4.20.0.0-8a80 \
      -XHaskell2010 \
      Fred
  
    [1 of 2] Compiling Main ( Fred.hs, dist/build/Main.o, ...
  

• GHC 9.8 / Cabal-3.10

    /usr/local/bin/ghc --make -fbuilding-cabal-package -O -static -dynamic-too -dynosuf dyn_o -dynhisuf dyn_hi \
      -outputdir dist/build -odir dist/build -hidir dist/build \
      -stubdir dist/build -i \
      -idist/build -i. \
      -idist/build/autogen -idist/build/global-autogen \
      -Idist/build/autogen -Idist/build/global-autogen -Idist/build \
      -I/usr/local/opt/icu4c/include -I/usr/local/opt/libxml2/include/libxml2 \
      -optP-include -optPdist/build/autogen/cabal_macros.h \
      -this-unit-id stale-0-F795SeGwWezHPbPW8onWYj \
      -hide-all-packages -Wmissing-home-modules -package-db dist/package.conf.inplace \
      -package-id array-0.5.6.0-28ee -package-id base-4.19.1.0-654f \
      -XHaskell2010 \
      Fred
  
    [1 of 1] Compiling Parser ( dist/build/Parser.hs, ... )