cabal-install support window for Cabal library version Custom Setup is built with

[ezyang]

Two parts:

1. What should the window be? GHC/Cabal has generally had the property that you need a Setup script built with at least the version of Cabal bundled with GHC to expect things to work. So that would be a reasonable lower bound; Cabal 1.14.0.0 shipped with GHC 7.4.1 so that's probably our window for now.
2. Whatever our support window is, we need to make sure we run cabal-install tests for the sensitive bits on ALL versions of Cabal library. Profiling with Custom setup < 1.24 doesn't work #3790 is a perfect example of a tricky situation that needs tests. So we need tests for 1.14, 1.16, 1.18, 1.20, 1.22 and 1.24.

Seem reasonable?

[23Skidoo]

I think we should decide on a policy for which GHCs we are supporting (for -w, not for being built with), and that'll tell us which Cabals we need to support.

[hvr]

@23Skidoo it should also be taken into account that the supported range for -w will most likely also decide the range of GHCs supported on matrix.hackage.haskell.org as I'm not planning to incur the added complexity of supporting/using more than one cabal-install version.

[23Skidoo]

@hvr And you want to support GHC versions at least back to 7.0, right? This means supporting Cabal 1.10 custom setups.

[hvr]

Tbh, I've considered 7.0 to be expendable with new-build. However, I'd want to support back till GHC 7.4.2 on matrix.h.h.o (which happens to be the first GHC to have proper Generics & SafeHaskell support)

[ezyang]

...which suggests that we want 1.14 as our bound for now? ;)

[ezyang]

For new-build, we have discovered that 1.20 is the first version which supports --dependency, which we need in order to put dependencies on the store accurately. So that's the current bound for new-build.

[hvr]

@ezyang that effectively means that new-build may run into problems with LTS-0/1/2 snapshots?

[ezyang]

@hvr Absolutely. Indeed, even with #4058 an early LTS will probably run into trouble because it will try to build the Custom scripts with an old version of Cabal.