v0.3 regression on OCaml < 4.07

[sim642]

I have some nightly CI builds that suddenly started failing (with no code changes!) and the only difference is seq 0.3 being installed instead of 0.2.2, so this seems to be the culprit. The regression is a runtime crash:

Fatal error: exception File "src/select_version.ml", line 11, characters 11-17: Assertion failed

which looks like the case here:

seq/src/select_version.ml

Line 11 in 7b95791

| _ -> assert false

I'm confused by what Select_version is still for, it doesn't seem to be used for any build time version selection anymore, but it's still there, so I guess it gets implicitly linked into the library and unintentionally runs some code every time?

The regression didn't show in opam-repository CI because the latest released version of my package (goblint-cil) doesn't yet have a transitive dependency on seq, but the development version does (via yojson 2.0.0).
Also, I guess my use of -linkall is what reveals the issue (by running Select_version). Default linking would exclude the unused module.

[sim642]

Coincidentally, a very similar issue just appeared on a completely unrelated project as well: namely, dune's mdx support also uses -linkall and if seq 0.3 is installed, mdx crashes with the same exception.

[sim642]

The regression also now shows in the latest opam-health-check diff: http://check.ocamllabs.io/diff/1658498706-41f2345be04c54b77fe87c9100aef20218bcac77..1658671506-f1eef1a5b413443a841a24a1c7f655ef64772692. May of the now failing are with the same exception.

The same can be seen by grepping the build logs for select_version.ml on that run: http://check.ocamllabs.io/run/1658671506-f1eef1a5b413443a841a24a1c7f655ef64772692?comp=4.02&comp=4.03&comp=4.04&comp=4.05&comp=4.06&comp=4.07&comp=4.08&comp=4.09&comp=4.10&comp=4.11&comp=4.12&comp=4.13&comp=4.14&comp=4.14%2Balpha-repo&comp=5.0&comp=5.0%2Balpha-repo&available=4.02&available=4.03&available=4.04&available=4.05&available=4.06&available=4.07&available=4.08&available=4.09&available=4.10&available=4.11&available=4.12&available=4.13&available=4.14&available=4.14%2Balpha-repo&available=5.0&available=5.0%2Balpha-repo&maintainers=&logsearch=select_version.ml&logsearch_comp=4.06.

Surprisingly fewer than I expected based on my personal experience with this crash.