A user has reported, and I have confirmed, that r-cran-gert from c2d4u is uninstallable on bionic because it is compiled against a version of libgit2 that is not included with c2d4u: # apt-get install r-cran-gert The following packages have unmet dependencies: r-cran-gert : Depends: libgit2-27 (>= 0.26.0) but it is not installable E: Unable to correct problems, you have held broken packages.
From the c2d4u buildlog I can see that it is compiling r-cran-gert
against the libgit2 backports from ppa:cran/libgit2. So I think the easiest solution is to put a copy of that libgit2 into c2d4u. The same seems to hold for r-cran-v8. The problem does not exist for ubuntu:focal because here the stock versions of libgit2 and libv8 are sufficient and we do not provide ppa backports.