Migrate Ptyp_open from 502 to 501

[patricoferris]

A bug fix to support migration syntax like type t = M.(t) to the 501 AST via, our now, standard approach of using extensions. This really only impacts those who have not upgraded ppxlib to 0.36.0.

So, to help people, perhaps we backport this to a 0.35.1 release?

[NathanReb]

I'm not sure this would improve the situation much, especially since as you point out, our AST will not go back to 5.1. Now the error would be a Uninterpreted Extension .... The extension name would hint at ppxlib which is good but I still feel like it wouldn't beat a good error message there.
I think the quality of the uninterpreted extension error is balanced by the improved compatibility that we gain from allowing new features to be used alongside ppx-es but that won't be the case here since it would only happen when using older compilers.

How about simply improving the error message and backport it to 0.35 as you suggest?

[patricoferris]

ppxlib.0.35.0 is installable with OCaml 5.2 and 5.3 so it is possible (as was the case here: https://mastodon.social/@jonmsterling@mathstodon.xyz/116085163199163273) that you could use type t = M.(t) and expect to be preserved?

[NathanReb]

Yeah I guess that's right, I didn't really expect to backport this kind of support but given how hard it can be to upgrade from 0.35 to 0.36, I'd say it's fine indeed!