Internal error even though the correct fix is applied to "not in scope" for `showAst` usage

[Aster89]

I haven't found much in existing issues.

Your environment

Which OS do you use? Up-to-date ArchLinux
Which version of GHC do you use and how did you install it? 9.14.1 from ghcup
How is your project built (alternative: link to the project)? https://github.com/Aster89/haskell-language-server/tree/internal-error-repro-steps

Which LSP client (editor/plugin) do you use?
Terminal Vim+YCM
Which version of HLS do you use and how did you install it? 2.13.0.0 from ghcup, not the local one I linked above.
Have you configured HLS in any way (especially: a hie.yaml file)? No

Steps to reproduce

Ask HLS to fix the "not in scope" error in using showAst at plugins/hls-case-split-plugin/src/Ide/Plugin/CaseSplit.hs:101.

One fundamental detail for the error to occur is that, as you can see, showAst is not in scope. If you fix that (by adding the appropriate import), then the code will still be erroneous because I'm passing undefined to showAst, but HLS correctly offers no FixIt.

Expected behaviour

No internal error.

Actual behaviour

Internal error.

However, one can enter 1/2 and press Enter to get either of these two imports at the top:

import Language.Haskell.GHC.ExactPrint.Utils
import Language.Haskell.GHC.ExactPrint.Utils (showAst)

which is correct.

Debug information

2026-05-26T11:00:04.508498Z | Error | ghcide-code-actions-bindings: Internal Error: Exception in plugin PluginId "ghcide-code-actions-bindings" while processing SMethod_TextDocumentCodeAction: GHC.Internal.Data.Data.dataTypeConstrs is not supported for ModuleName, as it is not an algebraic data type.

[Aster89]

Some further details were discussed on Element.

What I think is more relevant, from @fendor, is copied below:

Yes, hovering seems to be sufficient

VSCode requests a code action on hover, which causes the internal error

reproducible on GHC 9.10 and 9.14 for me

showAst is also iirelevant, error is triggered on any unknown identifier

Also, not trivially triggerable in other modules, the CaseSplit module seems to be necessary 🤔

I suspect the addArgumentAction might be to blame. It should be the only code action that triggers for this particular diagnostic and performs some data analysis on the sources

Yes, that seems likely to me, replacing showAst with EP.showAst doesn't trigger the error for me, which is something we explicilty guard against in addArgumentAction

[Aster89]

Also, not trivially triggerable in other modules, the CaseSplit module seems to be necessary 🤔

@fendor, given we're talking of using (1) HLS executable on (2) a clone of the HLS codebase, can you please clarify what you mean? Does triggering the bug for you require that CaseSplit module must be in (1), in (2) or both? Currently I suffer this when using the upstream HLS in the repo where I'm working on the plugin, so for me the answer is surely not (1) nor "both".

[fendor]

I meant that simply using showAst in other modules within HLS doesn't seem to exhibit the same behaviour. That's all!

[Aster89]

@fendor, I've spent a few minutes to craft better minimal reproduction steps:

• in Aster89@1ad0363 you'll see no new plugin, just a few more explicit import lists on an existing plugin, together with a TODO comment telling you how to trigger the error;
  • I hope this cuts the plugin I'm working on out of the equation;
• in Aster89@62b1339 there's a new, dummy plugin with almost nothing in it, and TODO comments telling you how to trigger/not trigger the error;
  • the change to the .cabal file is not exacly minimal, but I don't think that's particularly important,
  • because the code of the plugin itself shows you two instances of very similar code where the error isn't and is triggered.

It looks like what triggers the error is the presence of LSP. on line 21 as opposed to line 16. How that can cause the error is right now out of my reach. But I'm happy to follow advice on further investigation, if that can help.

For now, at least, I've understood how to workaround the issue.