error message when dune lang version is too new in dune-package is unclear

[Alizter]

When a user downgrades their version of dune, and tries to use an installed dune package. They are met with the following situation:

$ dune build
File ".../dune-package", line 1, characters 11-15:
1 | (lang dune 3.23)
               ^^^^
Error: Version 3.23 of the dune language is not supported.
Supported versions of this extension in version 3.23 of the dune language:
- 1.0 to 1.12
- 2.0 to 2.9
- 3.0 to 3.21
-> ...

This is really unhelpful.

Instead we should say something like:

Error: The installed `dune` package was built and installed with an
unsupported version of dune. The current dune version is ....
Hint: Upgrade your version of dune.

[Sudha247]

I have been confused by this error message before - thanks for opening an issue.

When a user downgrades their version of dune, and tries to use an installed dune package

Good point, but doesn't the same error message show up when a user is trying to build a project with a newer lang dune, with an older Dune? For example, if my local project has (lang dune 3.23) and I'm trying to build it with Dune 3.21.0, won't the same error message show up? In that case, should the new error message be something more generic than saying "package was installed with"? I will try to propose something here.

[Alizter]

Yes, generally the version of dune should be exactly the same. I've written up a detailed reason why recently here: rocq-prover/rocq#22026 (comment)