conditional packages based on GHC version mismatch between cabal2nix and stack

[domenkozar]

If a cabal file has:

        if impl (ghc < 8.4)
          build-depends: ghc-syb-utils

Then stack using GHC 8.4.x won't consider it as dependency while cabal2nix will. This results into missing "ghc-syb-utils" attribute since package wasn't generated due to stack build plan.

[domenkozar]

This may have been fixed with #115