Warn when upgrading base

[snoyberg]

I've seen users get confusing error messages multiple times when trying to upgrade base, often from trying to install a package like ghc-mod. We should give a warning in such situations, and recommend changing GHC versions instead.

[SkyWriter]

@snoyberg , it may sound silly, but I don't suppose you could drop a few lines on how to produce an erroneous case? I'm willing to attack this, but seem to be stuck understanding the actual issue. Thanks!

[snoyberg]

Great question. I'm not sure how the original people suffering from this triggered the bug, but here's an example of steps that could be taken:

1. Create a stack.yaml specifying lts-11.4, e.g.

   resolver: lts-11.4
   packages: []

2. Try to build ghc-mod, e.g. stack build ghc-mod

3. You'll get an error message recommending, among other things, that you upgrade your base version, e.g.:

   - Cabal-1.24.2.0
   - base-4.9.1.0
   - cabal-helper-0.7.3.0
   - extra-1.5.3
   - haskell-src-exts-1.19.1
   - hlint-2.0.15
   - monad-journal-0.7.2
   - optparse-applicative-0.13.2.0

4. If you try to build anything, you'll keep getting error messages about necessary extra deps.

Ideally, two things should instead happen:

1. Stack should never recommend adding base to extra-deps
2. If someone put base in extra-deps, it should give a very loud warning about the fact that you almost certainly didn't mean that.

[SkyWriter]

@snoyberg alright, much clearer now. Especially with the 3907 reference. Thank you! I'll try to get it fixed.

[SkyWriter]

I have submitted my take on implementing the changes. Could someone possibly take a look at the code, and tell me if it's the right direction at all?

[snoyberg]

The PR looks great! I've added a few comments over there, thanks for the contribution.

[snoyberg]

Fixed by #3993, thanks!