Warn when incompatible dune version detected in dune-project

[ulrikstrid]

This is quite hacky but something to get the ball rolling.

Fixes #3700

[ulrikstrid]

I seem to have had the wrong ocamlformat version and then it didn't format. I formatted the code and force pushed now.

[rgrinberg]

LGTM. Can you add a test case for this behavior and make sure it only fires for dune 2.8 and above? We try not to alter dune's behavior for a fixed version.

@kit-ty-kate do you agree with the behavior proposed here?

[kit-ty-kate]

note that currently if (lang dune ...) is set to 2.6 or above, and the constraint is set, dune won't do anything clever and will just add two constraints (i.e. will become "dune" {>= "2.6" & >= "2.6"}). It's not wrong but visually annoying imo, so maybe we could add something for that or simply say "remove the constraint if (dune lang ...) is set to 2.6 or above". Idk. I don't mind too much either way.

[kit-ty-kate]

make sure it only fires for dune 2.8 and above? We try not to alter dune's behavior for a fixed version.

Does this apply even if it's just a warning?

[rgrinberg]

Does this apply even if it's just a warning?

That's what we've done when introducing other warnings. For example, the merlin warning.

[kit-ty-kate]

the thing is that otherwise the warning is rather useless as from (lang dune 2.6) the issue has been fixed anyway by the automatic addition of >= {dune_version}

[rgrinberg]

the thing is that otherwise the warning is rather useless as from (lang dune 2.6) the issue has been fixed anyway by the automatic addition of >= {dune_version}

Even if the user already set a constraint manually?

[kit-ty-kate]

yeah :/

dune/src/dune/opam_create.ml

Line 153 in ea9020b

| Some c -> And [ constraint_; c ] )

[rgrinberg]

This behavior seems a bit weird.

For example, if I add a >= 2.3.0 constraint manually on dune, I generate the following opam file:

+  "dune" {>= "2.7" & >= "2.3.0"}

I wonder if would be better to just have a warning rather than use the existing constraint.

I suppose we could add the warning retroactively, but I think we need to make sure it doesn't fire up in the vendored directory.

[ulrikstrid]

Added tests but they will fail until 2.8 is a valid language version.

[ulrikstrid]

@rgrinberg what can I do to move this PR forward?

[rgrinberg]

Isn't the issue solved by upgrading your dune language version already? Then the up to date constraint is always inserted.