custom-setup solving problem, is Cabal version shared across all Setup.hs?

[phadej]

Happens only with GHC 7.8.4 and older.

With the code in ulidtko/cabal-doctest@742c2d0
Only if I add a cabal-doctests >=1.0.1 bound to example.cabal's setup-depends, I'll get a solver error https://travis-ci.org/phadej/cabal-doctest/jobs/198593113

This is very unfortunate, as I don't see why there can't be valid plan (Cabal for ComponentSetup should interfere with ghc / doctest etc part of the --enable-tests plan)

example:setup.Cabal-1.2.1, example:setup.Cabal-1.1.6 (constraint from minimum
version of Cabal used by Setup.hs requires >=1.20)
rejecting: example:setup.Cabal-1.24.1.0 (dependencies not linked: cannot make
example:setup.Cabal canonical member of
{*Cabal-1.18.1.5/installed-647...,example:setup.Cabal-1.18.1.5/installed-647...};
conflict set: Cabal, example:setup.Cabal, example:setup.cabal-doctest)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: example:setup.Cabal (6609),
example:setup.cabal-doctest (6567), Cabal (3617), doctest (3451),
example-1:test (3447), example (2798), example:setup.base (1432),

[phadej]

cc @grayjay

[phadej]

I have no idea where from the Cabal >= 1.20 bound comes from. Feel free to ask me on IRC for more details.

[ezyang]

new-build requires 1.20 or later because 1.18 setup does not support --dependency, so it's not possible for us to request specific packages. 80de7ff and #4051.

We should make "minimum
version of Cabal used by Setup.hs requires >=1.20" clearer by referencing an actual ticket number; if you grep you'll see where it comes from.

[grayjay]

The problem is that new-build adds all packages from cabal.project to the list of targets for the dependency solver, so they all are required to have consistent dependencies. In this example, the solver would ideally choose the installed Cabal for example's doctests test suite and a newer Cabal for example's setup script. The test suite needs the installed Cabal, because it depends on doctest, which depends on the installed ghc package, and ghc depends on Cabal. However, the two Cabal dependencies are forced to be the same instance because the setup script also depends on cabal-doctest and cabal-doctest is a top-level build target.

One solution is to enable --independent-goals for new-build. Then some packages in a project can be setup dependencies for others and be allowed to have different versions for their dependencies.

/cc @dcoutts

[phadej]

@grayjay thanks for the explanation!

[ezyang]

Note that the --independent-goals flag is currently disabled.

[grayjay]

I think the issue with building cabal-doctest was resolved with the --independent-goals flag. I opened a separate issue for enabling the flag by default: #6460