error during installing clang8 on Ubuntu 20.4

[miketimofeev]

Discussed in #4927

^{Originally posted by iscanGH January 21, 2022}
I need to install clang-8 to be compatible with another package (Carla).
First, I removed clang-11 and its dependencies (I think), and
sudo apt-get update

Then I failed at
sudo apt-get install clang-8
The following packages have unmet dependencies: clang-8 : Depends: libclang-common-8-dev (= 1:8.0.1+svn369350-1~exp1~20200114191400.80) but 1:8.0.1-9 is to be installed E: Unable to correct problems, you have held broken packages.
I installed aptitude and also run several combinations of resolving issues, but failed. For example when I run
sudo aptitude install clang-8
I get
`The following NEW packages will be installed:
gcc-7-base{a} lib32gcc1{a} libasan4{a} libcilkrts5{a} libgcc-7-dev{a}
libobjc-7-dev{a} libstdc++-7-dev{a} libubsan0{a}
The following packages will be REMOVED:
libobjc-8-dev{u}
The following packages will be upgraded:
clang-8 libclang-common-8-dev libclang1-8 libllvm8{b}
4 packages upgraded, 8 newly installed, 1 to remove and 3 not to upgrade.
Need to get 36.2 MB of archives. After unpacking 35.8 MB will be used.
The following packages have unmet dependencies:
libllvm8 : Depends: libffi6 (>= 3.0.4) which is a virtual package and is not provided by any available package

llvm-8-dev : Depends: libllvm8 (= 1:8.0.1-9) but 1:8.0.1+svn369350-1exp120200114191400.80 is to be installed
The following actions will resolve these dependencies:

 Remove the following packages:                       

1. clang-8 [1:8.0.1-9 (focal, now)]                   
   

   Keep the following packages at their current version:

2. libclang-common-8-dev [1:8.0.1-9 (focal, now)]     
   

3. libllvm8 [1:8.0.1-9 (focal, now)] 
   

Accept this solution? [Y/n/q/?]
`

[miketimofeev]

Hi @iscanGH!
We will take a look at what can be done here.

[miketimofeev]

@iscanGH Could you try the following, it works for me

    steps:
      - name: Install clang8
        run: |
          sudo apt-get update
          sudo apt install -y --allow-downgrades clang-8

The issue is most likely the same as described here #4589 (comment)