No ability to use --allow-newer when configuring with Setup.hs

[snoyberg]

A number of different systems (Nix, the Stackage curation tool, Stack) use runghc Setup.hs configure for configuring their packages. There are various reasons for this; since this bug report is about Stackage curation, the reason we have is to avoid having to manually match the Cabal library and cabal-install versions.

@bgamari asked me to run the latest GHC 8 release candidate through the Stackage gauntlet. Unfortunately, due to the lack of --allow-newer, it's quite difficult to do a proper test of GHC 8, which includes a new version of many core libraries. My ultimate workaround is quite ugly: rewriting the .cabal file with all version bounds stripped out.

For the record, all of this GHC release candidate testing is in the repo https://github.com/fpco/ghc-rc-stackage, which is just a Dockerfile, shell script, and some docs.

[23Skidoo]

Can you use runghc Setup.hs configure --exact-configuration? The reason we don't have --allow-newer in Cabal is because Cabal's built-in solver is fairly primitive. Perhaps once the solver is separated from cabal-install (see e.g. #2768), we can make Cabal depend on it and remove its built-in solver, then you'll be able to use all advanced solver features via the runghc Setup.hs configure interface.

/cc @dcoutts

[23Skidoo]

Having looked at the code, I think I might be actually able to do something.

[23Skidoo]

@snoyberg See #3165.

[hvr]

@23Skidoo I still oppose have Cabal depend on the solver library. See also discussion at #2768 (comment). IMHO Cabal should be as lightweight as possible as that's the one component we can't easily update.