dune 1.7.0 expects files where they aren't (exhibited by lwt.3.2.1)

[pirbo]

Note that I'm not using cutting edge lwt.4.1.0 but an old lwt.3.2.1 (because my real use case is in the context of cross compilation for windows where we don't manage to get lwt.4.1.0 but the issue is unrelated to cross compilation. If I downgrade lwt to 3.2.1 on any of my machine, the problem arises.)

After that (travis-ci) updated to dune 1.7.0, I started to see some

dune build --only-packages kappa-library,kappa-binaries
File "agents/dune", line 9, characters 8-14:
9 |   (name KaStor)
            ^^^^^^
Error: File unavailable: /home/pboutill/.opam/for_nightly/lib/lwt/log/lwt_log.a
File "agents/dune", line 9, characters 8-14:
9 |   (name KaStor)
            ^^^^^^
Error: File unavailable: /home/pboutill/.opam/for_nightly/lib/lwt/log/lwt_log.cmxa

[...]

and indeed there is no /home/pboutill/.opam/for_nightly/lib/lwt/log/ and /home/pboutill/.opam/for_nightly/lib/lwt/unix/. All the files "dune" looks for are directly in /home/pboutill/.opam/for_nightly/lib/lwt/.

Is this a problem on my side or a general problem of dune?

[rgrinberg]

As far as I understand, the issue is that Lwt 3.2.1 has some hacks where it post processes the META file. Since dune >= 1.7.0 uses its own format for external packages (dune-package) all of those hacks are ignored. So essentially, Lwt 3.2.1 will not work with dune 1.7 and above.

[hcarty]

Lwt 3.2.1 and above will not work with dune 1.7 and above

@rgrinberg Do you mean "Lwt 3.2.1 and below will not work with dune 1.7 and above" here? A quick local test seems to show Lwt 4.1.0 and several dependent packages building with dune 1.7.0.

[pirbo]

I see and indeed newer lwt work fine.
If you think it's worth having a mention of the incompatibility here, reopen the issue. Else let's document the incompatibility somewhere (probably the opam file) and close here...