Drop support for old ocaml versions

[raphael-proust]

This PR removes support for ocaml<=4.07. This is following a suggestion (from @hannesm) and a user poll on discuss.

[raphael-proust]

We'll need to get ocaml-ci to drop tests for dropped ocaml versions. That might need to wait until we merge or at least we agree to merge.

[hannesm]

I guess with recent dune (>= 2.7.0) and bisect_ppx (2.5.0) this could be done differently; namely in the dune files add the stanza (instrumentation (backend bisect_ppx))) (as explained in bisect_ppx README). :)

[hannesm]

I removed some more code, see #949 (which is against the branch here). Please have a look :)

[raphael-proust]

Thanks for the additional removals! I've left one (1) comment which we may possibly ignore and merge anyway. Otherwise looks good to me.

[raphael-proust]

After adding all the constraints, the ocaml-ci dropped build attempts on legacy compilers. (Thanks Tmsc!)

However, the only compiler version where all the packages are now buildable is 4.14+domains. That's because ocaml-ci attempts to build all packages, including lwt_domain.opam. We should split the lwt_domain.opam in its own repository to avoid this. I'll do that in a separate PR.

[dinosaure]

However, the only compiler version where all the packages are now buildable is 4.14+domains. That's because ocaml-ci attempts to build all packages, including lwt_domain.opam. We should split the lwt_domain.opam in its own repository to avoid this. I'll do that in a separate PR.

You probably should make an issue on ocaml-ci I think.

[tmcgilchrist]

@raphael-proust We have a fix for ocam-ci that allows this PR to build.

See https://github.com/tmcgilchrist/lwt/tree/drop-support-for-old-ocaml-versions for a successful build against all supported packages / ocaml versions. Note it skips lwt_domains on unsupported compilers but includes it on ocaml-4.12+domains, and will pickup OCaml 5.0 once that is available.

There are 2 changes I needed to make in LWT:

1. Add the "base-domains" dependency to lwt_domains.opam
2. Tag the lwt_domains tests as belonging to that package drop-support-for-old-ocaml-versions...tmcgilchrist:drop-support-for-old-ocaml-versions

I didn't change the other tests but should be modified to identify which package they belong to.

[raphael-proust]

I've made a merge-request (into this branch) of your branch just so I could use the GH UI to merge. Thanks a lot for that fix.

[raphael-proust]

I've added one last commit to mark other tests with what packages they correspond to. But that breaks the build because there now several main.exe tests for the lwt package. So I rolled it back.

I think that a follow-up PR to modernise the dune files might be good. We can avoid these executable+alias in the test/ and replace by a simple test stanza, we can make the change @hannesm suggests above, we can bump the version requirement for dune (to match the new version requirement for ocaml), etc.

[raphael-proust]

No one has come forward with some reason to not merge this. And the MR does restore a functioning CI. So I'm merging this.

[olafhering]

lwt_ppx.opam has now two constraints for ocaml.

[raphael-proust]

Thanks @olafhering . I've fixed the issue in the release branch and added the same fix suggestion to the release PR.