llvm 12.0.1

Error log

The package failed to build. The error log from opam follows.

Opam plugin "depext" may require upgrading/reinstalling. Reinstall the plugin on the current switch? [Y/n] y
The following actions will be performed:
  - recompile opam-depext 1.2.3

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> retrieved opam-depext.1.2.3  (cached)
-> removed   opam-depext.1.2.3
-> installed opam-depext.1.2.3
Done.

<><> opam-depext.1.2.3 installed successfully <><><><><><><><><><><><><><><><><>
=> opam-depext is unnecessary when used with opam >= 2.1. Please use opam install directly instead
# Run eval $(opam env) to update the current shell environment

<><> Carrying on to "opam depext -viy labrys.0.1 uutf.1.0.3 topkg.1.0.7 stdlib-shims.0.3.0 seq.base re.1.12.0 pprint.20211129 ocamlfind.1.9.6 ocamlbuild.0.15.0 num.1.5-1 msgpack.1.3.0 menhirSdk.20211128 menhirLib.20211128 menhir.20211128 llvm.12.0.1 integers.0.7.0 dune-configurator.3.16.0 dune.3.16.0 ctypes.0.23.0 csexp.1.5.2 containers.2.8.1 conf-python-2-7.1.2 conf-llvm.12.0.1 conf-cmake.1 conf-bash.1 cmdliner.1.3.0 bigarray-compat.1.1.0 base-bytes.base" 

Since version 2.1, opam now handles external dependencies alongside OCaml ones, and the `depext` plugin interface is provided for backwards compatibility only. Consider using your usual `opam install` command to install both OCaml and system dependencies, or `opam install <pkg> --depext-only` if you want to only install external dependencies.
# Detecting depexts using vars: arch=x86_64, os=linux, os-distribution=debian, os-family=debian
[ERROR] No solution for labrys.0.1 & uutf.1.0.3 & topkg.1.0.7 & stdlib-shims.0.3.0 & seq.base & re.1.12.0 & pprint.20211129 & ocamlfind.1.9.6 & ocamlbuild.0.15.0 & num.1.5-1 & msgpack.1.3.0 & menhirSdk.20211128 & menhirLib.20211128 & menhir.20211128 & llvm.12.0.1 & integers.0.7.0 & dune-configurator.3.16.0 & dune.3.16.0 & ctypes.0.23.0 & csexp.1.5.2 & containers.2.8.1 & conf-python-2-7.1.2 & conf-llvm.12.0.1 & conf-cmake.1 & conf-bash.1 & cmdliner.1.3.0 & bigarray-compat.1.1.0 & base-bytes.base:   * Missing dependency:
            - conf-llvm = 12.0.1
            depends on the unavailable system package 'llvm-12-dev'. Use `--no-depexts' to attempt installation anyway, or it is possible that a depext package name in the opam file is incorrect.


Command failed: opam --cli=2.1 list --readonly --external '--resolve=labrys.0.1,uutf.1.0.3,topkg.1.0.7,stdlib-shims.0.3.0,seq.base,re.1.12.0,pprint.20211129,ocamlfind.1.9.6,ocamlbuild.0.15.0,num.1.5-1,msgpack.1.3.0,menhirSdk.20211128,menhirLib.20211128,menhir.20211128,llvm.12.0.1,integers.0.7.0,dune-configurator.3.16.0,dune.3.16.0,ctypes.0.23.0,csexp.1.5.2,containers.2.8.1,conf-python-2-7.1.2,conf-llvm.12.0.1,conf-cmake.1,conf-bash.1,cmdliner.1.3.0,bigarray-compat.1.1.0,base-bytes.base' --color=never returned 20