No error is raised when building an empty package not defined in the dune-project file

[kit-ty-kate]

It looks like somewhere between the merge of #4867 and the release of dune 3.0.0, the feature lost all its tests in #5120 and suffers from a regression:

$ cat dune-project
(lang dune 3.0)
$ touch test.opam

$ dune build # With dune >= 3.0.3 no error

$ ~/dune/dune.exe build # With 5c6eb65b149a9254dc2e9bd618df9e198a321705 we get the error
Error: The package test does not have any user defined stanzas attached to it.
If this is intentional, add (allow_empty) to the package definition in the
dune-project file
-> required by _build/default/test.install
-> required by alias all
-> required by alias default

$ echo "(package (name test))" >> dune-project
$ dune build # now dune >= 3.0.3 shows a warning
Error: The package test does not have any user defined stanzas attached to it.
If this is intentional, add (allow_empty) to the package definition in the
dune-project file
-> required by _build/default/test.install
-> required by alias all
-> required by alias default

[emillon]

According to the description in #5120, that's intentional. This warning only happens when the package is defined in dune-project. So I don't think it's a bug or a regression (has there been a released version which triggers the warning on opam files?). We can discuss about extending the behavior, of course.

[kit-ty-kate]

Ah.... Well, it makes the whole feature pretty much useless so I strongly disagree with #5120. Could we revert that?

[emillon]

2 different things:

• I agree that the feature needs some tests
• we can extend the behavior to packages defined through an opam file

For the latter, the "classic" way to proceed would be to add the warning, and make it fatal for lang dune >= 3.5 but I'd like to evaluate if it triggers a lot on opam-repository to avoid adding to much noise.

[emillon]

I'll add tests (first part of the above) but the for second part, I'm not sure how to proceed: if we add a warning for packages that are defined through an opam file, what would be the hint to fix the package? If the solution is to add (allow_empty), this means that the definition of the package needs to be added to dune-project (so, we're not in the situation where the package is defined by an opam file anymore).

[kit-ty-kate]

Something like this maybe:

Error: The package <pkg> does not have any user defined stanzas attached to it.
If this is intentional, add (package (name <pkg>) (allow_empty)) to your dune-project file.

[emillon]

In addition, while writing tests I noticed that the warning case can never happen. I'll fix that next week.

[rgrinberg]

I'm going to remove the 3.6.0 milestone. This issue isn't new and a PR fixing it doesn't seem likely in time for the release. Instead, let's discuss it in a meeting and actually assign this to someone if this is ever to get done.