I was hoping that I can use source-repository-package to point to “optional” dependencies, i.e. dependencies that are used when needed, but not built when not. I want to use this in my cabal.project:

source-repository-package
      type:     git
      location: https://github.com/ghcjs/ghcjs-base
      tag: 01014ade3f8f5ae677df192d7c2a208bd795b96c

because ghcjs-base is not on Hackage yet. The problem is that this package is only a dependency of jsaddle when using ghcjs, but not when using ghc. Furthemore, ghcjs-base has dependencies that cannot be fulfilled with ghc.

So with this in cabal.project, cabal thinks it has to build ghcjs-base, even with using ghc, and fails.