[Proposal] Add previews of some of the platform packages with OCaml 4.13 compatibility

[kit-ty-kate]

This PR copies some of the platforms packages from https://github.com/kit-ty-kate/opam-alpha-repository/ to make OCaml 4.13 simpler to test for people.

Proposal

This PR adds a preview of the next version of ocamlformat, odoc and mdx namely:

• ocamlformat.0.19.0~4.13preview
• odoc.1.5.3~4.13preview
• mdx.1.11.0~4.13preview

For the moment they point to specific git branches to make them support OCaml 4.13. They all have their corresponding PRs or are already merged in their main branch (see #18791 for details)

The goal is to update the package and the branch as the alpha/beta cycle progresses (there should only be one version of these preview packages at all time, no ~4.13preview2 and whatnot), and once the proper release for them land, we can simply mark it as available: false and forget about the previews.

This version number scheme should allow you to still release alpha/beta/rc versions and let them be taken first above those preview packages. It also allows us to be more explicit about what this version is (a simple preview of this package's next version with OCaml 4.13 support)

In parallel, those packages will also be available in opam-alpha-repository but marked available with the older stable versions of the compiler for more broad testing purpose.

cc @jonludlam for odoc
cc @NathanReb @emillon for mdx
cc @gpetiot @emillon for ocamlformat

What are your (package maintainers) views on this? Are the proposed version numbers the correct one for the next release?

[rgrinberg]

Sounds good to me.

[avsm]

Superb, thanks @kit-ty-kate! In the interests of longer term reproducibility, how about replacing the git branches with explicit sha256 changesets instead? This'll be a little more work for us to maintain, but in return those packages will build into the longer term with just a checkout of opam-repository.

[pitag-ha]

Thanks @kit-ty-kate! For OMP2 that sounds good to me. For ppxlib in the end we'll follow your idea and add the 4.13 support on top of the last release rather than the main branch for now, so the first ppxlib version supporting 4.13 will be

ppxlib.0.22.2

It will still depend on OMP2 and it isn't implemented yet, but we can re-use one of your commits (thanks for separating them!).

[avsm]

Thinking about this more:

how about replacing the git branches with explicit sha256 changesets instead

I think merging just the branches as they are in this PR is fine, and then "freeze" them to sha256 changesets when we release 4.13 and mark the package as unavailable. That'll preserve the history but not make day-to-day development a pain.

[kit-ty-kate]

I've removed OMP from the PR as it was properly released with OCaml 4.13 support yesterday in #18902

[avsm]

@Octachron @jonludlam this appears to be failing on 4.13...

#   The return type of this pattern-matching is ambiguous.
#   Please add a type annotation, as the choice of `s' is not principal.
# File "src/parser_automaton_internal.ml", line 485, characters 44-66:
# 485 |     | Sexp -> if is_not_ignoring state then Sexp (Atom str, stack) else stack
#                                                   ^^^^^^^^^^^^^^^^^^^^^^
# Error: The constructor Sexp expects 0 argument(s),
#        but is applied here to 1 argument(s)
#     ocamlopt src/.parsexp.objs/native/parsexp__Parser_automaton_internal.{cmx,o} (exit 2)
# (cd _build/default && /home/opam/.opam/4.13/bin/ocamlopt.opt -w -40 -g -I src/.parsexp.objs/byte -I src/.parsexp.objs/native -I /home/opam/.opam/4.13/lib/base/caml -I /home/opam/.opam/4.13/lib/sexplib0 -intf-suffix .ml -no-alias-deps -open Parsexp__ -o src/.parsexp.objs/native/parsexp__Parser_automaton_internal.cmx -c -impl src/parser_automaton_internal.ml)
# File "src/parser_automaton_internal.ml", lines 440-454, characters 6-66:
# 440 | ......match state.kind with
# 441 |       | Positions ->
# 442 |         (* Note we store end positions as inclusive in [Positions.t], so we use [delta:0],
# 443 |            while in the [Cst] case we save directly the final ranges, so we use
# 444 |            [delta:1]. *)
# ...
# 451 |           add_pos state ~delta:0;
# 452 |           make_list [] stack)
# 453 |         else stack
# 454 |       | Cst -> make_list_cst (current_pos state ~delta:1) [] stack
# Warning 18 [not-principal]: 
#   The return type of this pattern-matching is ambiguous.
#   Please add a type annotation, as the choice of `s' is not principal.
# File "src/parser_automaton_internal.ml", line 485, characters 44-66:
# 485 |     | Sexp -> if is_not_ignoring state then Sexp (Atom str, stack) else stack
#                                                   ^^^^^^^^^^^^^^^^^^^^^^
# Error: The constructor Sexp expects 0 argument(s),
#        but is applied here to 1 argument(s)

[kit-ty-kate]

This failure is in parsexp not odoc. It's a known issue in the first alpha: ocaml/ocaml#10364
I have a temporary fix in opam-alpha-repository here: https://github.com/kit-ty-kate/parsexp/tree/v0.14, I might have to move it to opam-repository then.

[avsm]

This should work after #18977 is merged

[kit-ty-kate]

Given the rest of the packages were all released and ppxlib.0.23.0 is not necessary anymore I've updated this PR to include ocamlformat, mdx and odoc (1.5 branch, instead of 2.0)

Are the proposed version numbers what they are expected to be when the support for OCaml 4.13 is going to be included?

[gpetiot]

The version number for ocamlformat is correct indeed (0.19.0), I just merged the 4.13 support by the way. The release is expected before July 16th.