Inconsistent behavior between custom and simple build types

[bos]

(Imported from Trac #716, reported by @j3h on 2010-07-23)

If a Custom build-type is used, Setup.hs may be built with a different version of Cabal than the one that cabal-install was built with. This can cause hard-to-find (seemingly random) build failures.

Perhaps cabal-install should warn when it runs Setup.hs with a different version of Cabal than the one that it was built with. Alternatively, it could fail if the version of Cabal that cabal-install was built with is not available when compiling the Setup script.

[bos]

(Imported comment by @dcoutts on 2010-07-23)

I guess we could warn. In general we do need to be able to build using a different Cabal version, even for build-type Simple.

The current behaviour is to use a Cabal lib version that is the same major version as the version that cabal-install was built with, and otherwise to use whatever is the latest available version that meets the version constraints specified in the .cabal file.

[ttuegel]

It should be easy to add a warning, and we've seen an uptick in issues related to this, so I'm changing the milestone.

[ezyang]

@ttuegel is your reconfigure branch going to fix this? If not, are you planning to fix it? (O/w I'm going to unmilestone this.)

[ttuegel]

I can't fix it per se because it's a collection of known and unknown issues, but we can throw out a warning when we fall back to an external setup using an older Cabal library. Actually, that's something I can do in my SetupWrapper refactor...