With Julia 1.5.0-beta1 on Ubuntu 18.04.4 I’m getting:
(@v1.5) pkg> build Cxx
Building Cxx → `~/.julia/packages/Cxx/1RaOv/deps/build.log`
┌ Error: Error building `Cxx`:
│ ERROR: LoadError: could not load library "libLLVM-9.0.1"
│ libLLVM-9.0.1.so: cannot open shared object file: No such file or directory
Ubuntu 18.04 package manager doesn’t provide libLLVM-9.0.1, libLLVM-9 is the newest available. What is the solution?
I’m not sure if this helps, but did you try to install them by
sudo apt install llvm-9 llvm-9-dev
?
They should be available according to: Ubuntu – Details of package llvm-9-dev in bionic but maybe you still need to wait for: WIP: Fix source build for Julia 1.5 by Gnimuc · Pull Request #466 · JuliaInterop/Cxx.jl · GitHub
I have libLLVM-9.so installed, but Cxx requires libLLVM-9.0.1.so.
Should I just abandon Ubuntu and install Manjaro, which has llvm toolchain 9.0.1 available? 
BTW, I have the same problem with Julia 1.4.2, which requires libLLVM-8.0.1.so for Cxx package. Unobtanium for Ubuntu 18.04.4.
Or just upgrade to Ubuntu 20.04 LTS, which has LLVM up to version 10?
I noticed that. Unfortunately, I’m tied up to ROS Melodic, which is not available for Ubuntu 20.04. Constraint satisfaction failed! 
I am not a user of ROS, but if I understand correctly you can just compile whatever version you need from sources:
https://wiki.ros.org/melodic/Installation/Source
so you may get Melodic working on 20.04. YMMV.
Theoretically yes, but it is huge collection of modules and I’ve heard many horror stories about building ROS from source gone wrong. I also have other dependencies. I’m trying to find an easy way before I do something more dramatic. I’m tempted to give Manjaro a try. Ubuntu restrictions on available package versions are pretty annoying.
I’m having the same problem with Julia docker image:
paul@desktop:~$ docker run -it --rm julia
(@v1.4) pkg> add Cxx
.....
Building Cxx → `~/.julia/packages/Cxx/1RaOv/deps/build.log`
┌ Error: Error building `Cxx`:
│ ERROR: LoadError: could not load library "libLLVM-8.0.1"
Any suggestions for this environment?
I believe there is something wrong in Cxx.jl, you shouldn’t install libLLVM, it comes bundled with Julia. @Gnimuc
Ok, Cxx.jl only works for Julia up to v1.3: https://github.com/JuliaInterop/Cxx.jl/issues/464
Out of curiosity I tried julia:1.0 docker image and no cigar either:
paul@desktop:~$ docker run -it --rm julia:1.0
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.0.5 (2019-09-09)
_/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |
(v1.0) pkg> add Cxx
Cloning default registries into /root/.julia/registries
Cloning registry General from "https://github.com/JuliaRegistries/General.git"
Resolving package versions...
ERROR: Unsatisfiable requirements detected for package Cxx [a0b5b9ef]:
Cxx [a0b5b9ef] log:
├─possible versions are: [0.3.0-0.3.4, 0.4.0] or uninstalled
├─restricted to versions * by an explicit requirement, leaving only versions [0.3.0-0.3.4, 0.4.0]
└─restricted by julia compatibility requirements to versions: uninstalled — no versions left
There was never any version of Cxx that supported Julia 1.0. You would have better chances with 1.1-1.3.
I was looking for version 1.3, but official Julia docker images give me only 1.0.5 or 1.4.2 options and nothing in between.
I just did that. The same problem on Ubuntu 20.04.1, unfortunately. Installing llvm-9 and llvm-9-dev does not help. What kind of Linux is actually compatible with Cxx? Is the difference between LLVM 9.0 and 9.0.1 so ground-breaking to justify incompatibility with the most popular Linux distribution?