MarsSeed [1] filed a deletion request for ros-melodic-roscpp-git [2]: Bogus, abandoned VCS package from 2019, which downloads the code for a different ROS distro (Noetic, not Melodic).
There's still an abandoned, unmaintained, but at least relevant stable package which uses the correct source: AUR/ros-melodic-roscpp [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/ros-melodic-roscpp-git/
