Gracefully handling old versions of base with new versions of GHC where base is reinstallable

[TeofilC]

Describe the bug

As of #10982, we now support reinstalling more packages like base when using GHC-9.14. This leads to issues since historical versions of base were released without dependency bounds.

Reproduction instructions

This means that if a package doesn't support GHC-9.14's base, then it will instead pick a very old version of base and fail with:

Error:
    Dependency on unbuildable library from ghc-bignum
    In the stanza 'library'
    In the package 'base-4.19.2.0'

See also: https://gitlab.haskell.org/ghc/ghc/-/issues/26541

Expected behavior

It should fail with a better message.

Proposed solution

One possibility to fix this is to revise old versions of base with something like:

if impl(ghc >= 9.14):
         build-depends: ghc-prim<0

This approach carries some risk. There are a lot of versions of base so the risk of making a mistake is high. We also want to make sure that we don't accidentally make some of these unusable with old versions of cabal.

Perhaps we should implement the fix in cabal. For instance we could avoid picking a version of base which is using the -this-unit-id hack.

We were wondering if you all had an opinion about what the best approach to be. I'm happy to implement something if we come up with a plan.

[geekosaur]

I had proposed that this be partially handled by ghc (a list of non-reinstallable libraries, and optionally a list of dependencies such that cabal can lock dependencies of ghc only when it's in the build plan), but this requires ghcup or etc. retrofitting such into older ghc versions. Otherwise, I don't really see a good way to deal with this.

[TeofilC]

I'm not sure how that would help here, since we do want to reinstall base and treat it like other libraries. It's just that like other libraries it can have missing bounds on Hackage, which make cabal pick bad plans.

Note that this is a new GHC trying to build an old base rather than the an old GHC trying to build a new base. We successfully avoid that combination with the way #10982 is implemented.

[Bodigrim]

One possibility to fix this is to revise old versions of base with something like...

This does not seem to be a valid revision, Hackage Server says "Cannot change the 'if' condition expressions". See https://github.com/haskell/hackage-server/blob/350fd8aac61c76481fcccfeee499ab6675171320/src/Distribution/Server/Util/CabalRevisions.hs#L434-L452

[TeofilC]

I thought about this a bit more and I think rts < 1.0.3 should work. GHC-9.14 will ship with rts-1.0.3 and is the first version of GHC to do so. The rts package is also not published to Hackage so there's no chance that cabal will try to pick a different version from there. It's not a problem if we start publishing it in the future so long as we don't publish old versions.