14.04 to 16.04 LTS gcc-5-base downgrade
Solution 1:
In addition to what the OP describes, my do-release-upgrade was interrupted by this issue. Josh.F's comment did the trick for me:
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get -f install
That fixed gcc-5 for me and then I still needed to sudo apt-get upgrade
Solution 2:
The comment by @josh-f fixed this for me:
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install -y r-base-dev