Hm, if this is a problem, we need to sort it out (before 1.2.1). I think having comparison on compiler versions behave differently from comparison on package versions is a pretty bad idea.

• I have no objection on allowing + back in compiler versions, but then we need to be clear on the distinction between name and version: the first is the label of this specific compiler, the second the general OCaml version that it provides. Follows that a package specifying ocaml-version = "4.02.1" won't be installable on a compiler with version: "4.02.1+dev72" ¹.
• @bactrian (https://github.com/bactrian/opam-sync-github-prs/blob/master/generate.ml) correctly generates the version: field to match the OCaml version without suffix ; I am not clear how this specific issue surfaced from OCaml's tests, wasn't it a bug in opam-repo ?
• Previous OPAM versions simply truncated the .comp's version: field on file load, so not using + in there gives 100% compatibility with those.

¹ Well, unless we change the global meaning of the version comparison = to mean "the shortest of the two is a prefix of the longer one". Which would be in this case equivalent to ocaml-version >= "4.02.1" & ocaml-version < "4.02.2" (not considering ~ versions). While this is a bit verbose, this change is a bad idea IMHO because the semantics are too complicated. Adding some fancy syntax like ocaml-version = "4.02.1*" or ocaml-version =~ "4.02.1" for such cases could be an idea, but doesn't seem worth the added complexity either.

Could you provide a reference on the version comparison opam implements for packages ? I'm sure you provided doc about this at a certain point but I can't find it anymore.

It's in the FAQ

I think having comparison on compiler versions behave differently from comparison on package versions is a pretty bad idea.

Especially if compiler as packages become a reality.

Ah! I finally located the real source of this error (thanks to the "allowed" failure at https://travis-ci.org/ocaml/opam/builds/50899181): OPAM itself generates the version: field for the system compiler from running ocamlc -version, leading to this error when you have a development OCaml preinstalled. In that case, the name in compiler is just system and gives no information about the precise version.

Thanks a lot @planar for noticing this and pushing me in the right direction.

This is quite messy, I'll try to find a sane way to solve this:

• version: should, consistently, be the patch-version of OCaml (without suffix)
• We have the preinstalled boolean variable so having system as the compiler name is not needed, besides not really making sense. There may be checks for "system" lurking around though, we'll need to be careful.
• Just truncating version: would be consistent with the old behaviour (if rather itchy, and losing the detailed version information)

Just filed a PR to fix this, at least on the surface since we're already in beta stage: the version reported by ocamlc -version will simply be truncated in the generated system compiler definition. I'm not completely happy, since the complete compiler version isn't stored there.

Note that this means that a change in suffix of the system OCaml version won't trigger a reinstall. I believe that was already the case (well, we can suppose people installing dev versions of OCaml know that it happened and can do a switch reinstall manually I guess)

I would like, as mentionned above and for consistency, to have the system compiler be defined with its complete version in the name rather than just "system" ; but that would be quite a change, and raises some concerns (the name is used as a key throughout, and you'd need a way to specify wether you want the system switch or the source switch with the same version when creating a new switch). This is not a problem after installation since compilers are referred to by their aliases.

There is also some special magic that allows you to write tests like ocaml-version = "system" (rather than compiler = "system", or just preinstalled), that I would be very happy removing.

Follows that a package specifying ocaml-version = "4.02.1" won't be installable on a compiler with version: "4.02.1+dev72"

Note I'll be very sad if you do that, because I won't be able to use an unmodified OPAM to test development versions of the compiler. Unless you provide a switch to truncate the compiler version.

@planar: yes, that's precisely why I wanted to avoid it. You should be fine now, same behaviour as before except that when you write a .comp by hand, you should explicitely not put +xxx in the version, rather than having silently ignored (almost the same as your proposed patch, actually, but with fixed system-compiler version change detection).

You won't yet be able to easily access the complete 4.02.1+dev72 string from OPAM though.