testuite: doctest: move to ghcup-recommended ghc

[ulysses4ever]

Another stub at #8504: choosing to upgrade existing doctest rather than anything else mentioned in the ticket. It looks like, our issue with upgrading GHC 8.10 -> 9.2 was due to the default -XGHC2021 adopted in 9.2, and nomeata suggested an easy fix (thanks!) to explicitly request -XHaskell2010.

I'm sad to declare defeat at trying to switch to the recommended way of using doctest these days (namely, via cabal repl) due to a failure described in #8504 (comment) It's not only sad because we're using a not (officially) supported way of communication with doctest, but also because we have to employ cabal-env, which has an unclear maintenance status.

I also took a liberty to use ghcup set ghc recommended (which is currently 9.2.5) instead of a specific version or version matrix for a couple of reasons:

• the other "quick job" "Meta checks" have been using it since 475222d
• it makes sense for explicitly secondary job (referring to the name "Quick jobs") to use just one GHC instead of e.g. the whole support window;
• trying single GHC saves time on CI;
• using recommended instead of a specific version will make sure that we're moving forward even if slowly instead of being stuck on one version until someone remembers to upgrade.

---

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• Patches conform to the coding conventions.
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[ulysses4ever]

Also sadly, this doesn't address #8147 because it's still quite tricky to run doctest locally because of the cabal-env business. #8703 i.e. switching to the official way of running doctest via cabal repl may be the right way of solving it, but as noted above and in #8703, this doesn't quite work yet (works for Cabal-syntax but doesn't work for Cabal because the latter's doctests depend on QuickCheck).

[andreasabel]

This is optimistic, but let's try it.

[Mikolaj]

Thank you.

[Mikolaj]

@ulysses4ever: shall we backport immediately or wait until in case it bombs out on master soon?

[ulysses4ever]

@Mikolaj I doubt it will blow anything but as for backporting, I don't feel strongly either way because it's not a user-visible change.

[Mikolaj]

If this, potentially, improves CI and/or its stability, it's worth porting, because the 3.10 branch is going to stay there for a while.

[ulysses4ever]

@mergify backport 3.10

[mergify]

backport 3.10

✅ Backports have been created

• #8731 testuite: doctest: move to ghcup-recommended ghc (backport #8710) has been created for branch 3.10

[ulysses4ever]

@Mikolaj you're welcome to approve #8731!

[Mikolaj]

@ulysses4ever: thank you! CI got cancelled there, so I'm rerunning and keeping watch.