Add support for OCaml 4.13

[kit-ty-kate]

Ports ocaml-ppx/ocaml-migrate-parsetree#114

[jberdine]

I don't know if it's useful, but I pinned this PR in a 4.13-alpha1 switch and the build fails:

dune build --auto-promote @install
      ocamlc traverse/.ppxlib_traverse.objs/byte/ppxlib_traverse.{cmi,cmo,cmt} (exit 2)
(cd _build/default && /Users/jjb/.opam/sledge_test/bin/ocamlc.opt -w @[email protected]@30..39@[email protected]@[email protected] -strict-sequence -strict-formats -short-paths -keep-locs -safe-string -g -bin-annot -I traverse/.ppxlib_traverse.objs/byte -I /Users/jjb/.opam/sledge_test/lib/stdlib-shims -I ast/.ppxlib_ast.objs/byte -I src/.ppxlib.objs/byte -I stdppx/.stdppx.objs/byte -I traverse_builtins/.ppxlib_traverse_builtins.objs/byte -no-alias-deps -opaque -o traverse/.ppxlib_traverse.objs/byte/ppxlib_traverse.cmo -c -impl traverse/ppxlib_traverse.pp.ml)
>> Fatal error: OCaml and preprocessor have incompatible versions
Fatal error: exception Misc.Fatal_error
make: *** [Makefile:5: default] Error 1

This is after pinning omp to #114.

[kit-ty-kate]

@jberdine thanks a lot, the magic number got bumped the day before yesterday and I hadn't seen it. It should be fixed now.

[jberdine]

Thanks, that worked and let me get somewhat further. There are still a number of ppx_* packages that don't build with 4.13 so I haven't done any real testing so far.

[kit-ty-kate]

Feel free to use https://github.com/kit-ty-kate/opam-alpha-repository/, it contains some of the fixes you might need.

[pitag-ha]

Thanks @kit-ty-kate! Would you mind adding a changelog entry to point out that this commit makes ppxlib compatible with 4.13? And could you also sign the commit?

[pitag-ha]

There are still a number of ppx_* packages that don't build

@jberdine, are you referring to the packages listed here? Or are there more?

[pitag-ha]

Would you mind adding a changelog entry

Never mind this anymore, since the support will already be added in 0.22.2 and we have a changelog entry for it there. I've added the no changelog label.

And could you also sign the commit?

That would still be great, though!

[pitag-ha]

Just in case someone comes across this PR: this PR is currently not very active, since we've also backported it to 0.22 and released it there. Concretely, ppxlib.0.22.2 is 4.13 compatible.

[pitag-ha]

@kit-ty-kate, could you sign the commits?

[kit-ty-kate]

@pitag-ha Done

[pitag-ha]

Thanks @kit-ty-kate!