Actually, I wonder: right now we do julia_compat = "~$(julia_version.major).$(julia_version.minor)" but perhaps that should be changed to julia_compat = "^$(julia_version.major).$(julia_version.minor)". Rationale: this way, until there is libjulia_jll 1.6, one can use the cxxwrap built for Julia 1.5 in Julia 1.6 (and more generally: that for 1.x in 1.(x+1)), and with some luck, it'll work; in the worst case, it doesn't work, but then one isn't worse off than before.
Oh, and since we increment the JLL version along with the Julia version, even if there letter is an actual Julia 1.6 version, it'll have a higher version and thus Pkg will prefer it over the Julia 1.5 version.

Hmm, lots of build failures:

[22:03:22] /opt/x86_64-linux-gnu/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../x86_64-linux-gnu/bin/ld: warning: libunwind.so.8, needed by /workspace/destdir/lib/libjulia.so, not found (try using -rpath or -rpath-link)
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_ULx86_64_step'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_Ux86_64_getcontext'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_U_dyn_register'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_ULx86_64_init_local'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_ULx86_64_local_addr_space'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_ULx86_64_get_reg'
[22:03:22] /workspace/destdir/lib/libjulia.so: undefined reference to `_ULx86_64_get_proc_info_by_ip'
[22:03:22] collect2: error: ld returned 1 exit status
[22:03:22] make[2]: *** [test/CMakeFiles/test_cxxwrap.dir/build.make:106: test/test_cxxwrap] Error 1

Wonder if that indicates a potential issue with libjulia_jll 1.5... sigh

Damn, this is turning out to be quite the rabbit hole :/

These all seem to be from libunwind.

From my perspective, it's been a rabbit hole for several weeks (months?) now already, so I'll call this par for the course :-).

Anyway, yeah, this probably needs to be addressed in libjulia_jll 1.5: I notice that it lacks a dependency on LibUnwind_jll... I am pretty sure we had pretty good reasons to do that, but unfortunately we did not document those reasons via comments sigh.

I'll try to figure it out next week, but now I need to catch some sleep. Perhaps @vchuravy has an idea in the meantime...

Since version 1.5 julia uses a shared libunwind on linux + freebsd instead of a static library which explains why this happens for these architectures and not for 1.3 and 1.4. (I am not sure about macos ...)

The build script seems to fall back to it's own downloader for libunwind since neither USE_SYSTEM_LIBUNWIND nor a depencency is specified (maybe because it was easier this way to choose the correct version instead of replicating the logic in the build script again). See ad1f5f9

But that shared library is never installed into $destdir. So either building with libunwind_jll / libosxunwind_jll needs to be fixed or we could just copy that file into $libdir.

So if I add BuildDependency("LibUnwind_jll") this builds fine! But I'd prefer to have this fixed in libjulia_jll; the PR for that was merged, now waiting for JuliaRegistries/General#25308 to be merged.

In the meantime, I also modified this PR here to set the julia_compat as I described above (to which nobody reacted, which I hope means people agree, but perhaps they also just didn't read it? @barche ?)

I've tested the macOS build of the artifacts here on my computer; it seems to work well (e.g. Singular.jl test suite passes with it).

So from my POV this is ready for merging @barche @giordano

It's 18 hours too late, but this is indeed perfect, thanks @fingolfin !