I tried that, but the repository for haskell-opencv contains two packages at the same time and the patch assumes subdirectories opencv and opencv-extra which are uploaded separately to hackage and are therefore (I assume) not contained in the source of opencv for nixpkgs.

On hackage the --with-ld=c++ is still in the Setup.hs file.

if you want to wait, until that gets updated on hackage and the patch is unnecessary, I can change my pull request to just strip the configure flags cabal2nix generates.

If you want to update cabal2nix to not generate those configure flags in this case any more and wait
for that, I can close the pull request.

So, what shall it be?

I'm not sure I follow. Are you saying that the configure flags https://github.com/NixOS/cabal2nix/blob/master/src/Distribution/Nixpkgs/Haskell/FromCabal/PostProcess.hs#L354-L355 are unnecessary today, i.e. we could just remove them and there'd be no harm done?

If I understood matters correctly: No.

At the moment opencv doesn't build in master.

The fix are two parts:

1. Remove --with-ld=c++ from Setup.hs
2. Remove the configure flags cabal2nix generates.

Applying 1. doesn't change the cabal file, so I assumed, cabal2nix would still generate the configure flags, which we need to remove, so 2. is still necessary.

The part 1. will eventually be fixed, if a new version of opencv gets uploaded to hackage event.
The part 2. could be fixed in two ways, either (a) through that part of my pull request, or (b) through a change in cabal2nix (whatever that could be, should cabal2nix look at Setup.hs?)

So if we wait for the change of Setup.hs to hit hackage and hence nixpkgs, we don't need 1. in the pull request.

And if the maintainer of cabal2nix (I assume that is you?) decides to "fix" it, we wouldn't need 2. from my pull request.

Hopefully that is clearer now.

OK, now I understand what is going on. Thank you!

I modified cabal2nix in NixOS/cabal2nix@1b0c0c3 and merged (a slightly modified version of) this PR into haskell-updates as 7fee1034ef7. These changes allow me to build opencv and will be merged into master in a minute. I could not build opencv-extra, however, because ghc crashed trying to compile this code. The result may depend on the exact version of the compiler that's being used, though. I'm inclined to just ignore this issue.

On a side note, it might be a good idea to open a ticket with upstream to make sure that they are aware that their test suite does not compile.

Thank you! I wasn't aware that there is package specific knowledge in cabal2nix already.