@Cleophee Could you please copy-paste all of the output you receive when you try to install RStudio with gdebi?
I think it makes sense given your description. The flag --installed limits the results to only installed packages. Since RStudio failed to install, it isn't listed when you specify --installed. It appears that libclang-dev isn't installed either. This is disappointing because I was hoping it would list some other software that you had installed that was conflicting with RStudio.
Here are some additional commands to try to understand why you can't install libclang:
# List any installed libclang packages
apt list --installed 'libclang*'
# See the update information
apt-cache policy libclang-dev
apt-cache policy libclang-8-dev
Could you please copy-paste the output you receive?
Here is what I get when I run the above in the Docker container:
$ apt list --installed 'libclang*'
Listing... Done
libclang-8-dev/disco,now 1:8-3 amd64 [installed,automatic]
libclang-common-8-dev/disco,now 1:8-3 amd64 [installed,automatic]
libclang-dev/disco,now 1:8.0-48~exp1ubuntu1 amd64 [installed,automatic]
libclang1-8/disco,now 1:8-3 amd64 [installed,automatic]
$ apt-cache policy libclang-dev
libclang-dev:
Installed: 1:8.0-48~exp1ubuntu1
Candidate: 1:8.0-48~exp1ubuntu1
Version table:
*** 1:8.0-48~exp1ubuntu1 500
500 http://archive.ubuntu.com/ubuntu disco/universe amd64 Packages
100 /var/lib/dpkg/status
$ apt-cache policy libclang-8-dev
libclang-8-dev:
Installed: 1:8-3
Candidate: 1:8-3
Version table:
*** 1:8-3 500
500 http://archive.ubuntu.com/ubuntu disco/universe amd64 Packages
100 /var/lib/dpkg/status