Comment 25 for bug 1754872

There is no correct behavior here.

xubuntu-desktop depends on lightdm, lightdm-gtk-greeter; so apt will install lightdm which Recommends unity-greeter | lightdm-greeter | lightdm-kde-greeter. Hence apt installs unity-greeter. Then it installs lightdm-gtk-greeter.

Telling apt to install xubuntu-desktop and lightdm-gtk-greeter means you made the choice to use that, and thus don't need unity-greeter.

Just not installing unity-greeter is (a) not possible in a greedy algorithm and (b) leads to wrong results in other cases (like installing an sqlite backend for something instead of an mysql one because it has fewer packages). The EDSP solvers and the kalel solver can do that.

That said, for the future solver generation, my goal is to behave similar to now, but more strict. If something depends on A|B, the solver will inject a Recommends: A, unless B is already installed or manually marked for install. This will cause even more packages to be installed than now in some situations, but it's the best compromise we can offer.