Add exe ghc-supported-languages

[philderbeast]

Fix for #11633.

Note

Should we move this module to cabal-dev-scripts?

Manual QA Notes

Test with various GHC versions.

$ which ghc-9.12.2 | xargs cabal run ghc-supported-extensions

$ echo $status
0

$ which ghc-9.14.1 | xargs cabal run ghc-supported-extensions
The following extensions are known to GHC but are not in the 
extension registry in Language.Haskell.Extension.
  ExplicitLevelImports
  NoExplicitLevelImports
  ImplicitStagePersistence
  NoImplicitStagePersistence
All extensions should be registered, even experimental extensions.

$ echo $status
123

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[philderbeast]

Should we move this module to cabal-dev-scripts?

No, because it needs the development version of KnownExtension:

cabal/Cabal-syntax/src/Language/Haskell/Extension.hs

Lines 125 to 130 in ec8052b

	-- | Known Haskell language extensions, including deprecated and undocumented
	-- ones.
	--
	-- Check <https://downloads.haskell.org/~ghc/9.2.3/docs/html/users_guide/exts/table.html “Overview of all language extensions” in GHC User’s Guide>
	-- for more information.
	data KnownExtension

[philderbeast]

I added a check for language extensions to the preflight section of the release checklist.

[geekosaur]

FWIW that was already there, indirectly, via checking for new options. But nobody ran that for 3.16, because @wz1000 told us there weren't any. Sadly, I'm pretty sure Zubin was thinking -f, not -X, so the new extensions got missed as a result.

[philderbeast]

FWIW that was already there

I think I found what you mean, each GHC release also has its own cabal issue with a checklist, such as #9729.

[geekosaur]

Actually, it looks like someone reverted it at some point. It included a shell recipe for comparing ghc --show-options between two versions, which I'd used locally for a couple of releases but then someone else had to handle a release because I was juggling too many doctor appointments at the time.

(And now it's out of my shell history, sadly.)

[geekosaur]

Okay, re-added it just after yours. The original version also checked -X options, but as you've covered that I left it out.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-03-28 16:04 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking master (87b87bc), #11636 and #11634 together #11671
• ✅ Merged — 2026-03-28 17:56 UTC · at 82823b456b4d1a2710904d0923c0a856dfa77005

This pull request spent 1 hour 52 minutes 40 seconds in the queue, including 1 hour 42 minutes 4 seconds running CI.

Required conditions to merge

• #approved-reviews-by >= 2 [🛡 GitHub branch protection]
  • Add exe ghc-supported-languages #11634
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • Add exe ghc-supported-languages #11634
• #review-threads-unresolved = 0 [🛡 GitHub branch protection]
  • Add exe ghc-supported-languages #11634
• any of [🛡 GitHub branch protection]:
  • check-success = Doctest Cabal
  • check-neutral = Doctest Cabal
  • check-skipped = Doctest Cabal
• any of [🛡 GitHub branch protection]:
  • check-success = Meta checks
  • check-neutral = Meta checks
  • check-skipped = Meta checks
• any of [🛡 GitHub branch protection]:
  • check-success = docs/readthedocs.org:cabal
  • check-neutral = docs/readthedocs.org:cabal
  • check-skipped = docs/readthedocs.org:cabal
• any of [🛡 GitHub branch protection]:
  • check-success = Validate post job
  • check-neutral = Validate post job
  • check-skipped = Validate post job
• any of [🛡 GitHub branch protection]:
  • check-success = fourmolu
  • check-neutral = fourmolu
  • check-skipped = fourmolu
• any of [🛡 GitHub branch protection]:
  • check-success = hlint
  • check-neutral = hlint
  • check-skipped = hlint
• any of [🛡 GitHub branch protection]:
  • check-success = Bootstrap post job
  • check-neutral = Bootstrap post job
  • check-skipped = Bootstrap post job
• any of [🛡 GitHub branch protection]:
  • check-success = whitespace
  • check-neutral = whitespace
  • check-skipped = whitespace
• any of [🛡 GitHub branch protection]:
  • check-success = Check sdist post job
  • check-neutral = Check sdist post job
  • check-skipped = Check sdist post job
• any of [🛡 GitHub branch protection]:
  • check-success = Changelogs
  • check-neutral = Changelogs
  • check-skipped = Changelogs