Comment 8 for bug 457221

To make things easier here is my comment:

It's actually ubuntu-release-upgrader that handles the distribution upgrade logic now and it does have some logic for dealing with specific packages. This can be seen in DistUpgrade.cfg:

# information about the individual meta-pkgs
[ubuntu-desktop]
KeyDependencies=lightdm, unity, ubuntu-artwork, ubuntu-sounds
# those pkgs will be marked remove right after the distUpgrade in the cache
PostUpgradeRemove=xscreensaver, gnome-cups-manager, powermanagement-interface, deskbar-applet, nautilus-cd-burner
ForcedObsoletes=desktop-effects, cups-pdf, gnome-app-install, policykit-gnome, gnome-mount

and I believe it was put in ForcedObsoletes when cups-pdf became unseeded.

update-manager (1:0.93.4) intrepid; urgency=low

  * DistUpgrade/removal_blacklist.cfg:
    - add openssh-blacklist-extra and openssh-extra
  * UpdateManager/UpdateManager.py:
    - do not crash if lsb_release can not be run (LP: #255319)
  * DistUpgrade/DistUpgradeApport.py:
    - do not generate apport reports against a package if
      the error indicates that its a full disk, this is a bug
      in update-manager than (failed to do the full disk checking)
  * DistUpgrade/DistUpgrade.cfg:
    - force obsoletion of cups-pdf on ubuntu-desktop, kubuntu-desktop
      and xubuntu-desktop
  * DistUpgrade/DistUpgradeController.py:
    - increase KERNEL_INITRD_SIZE a bit

 -- Michael Vogt <email address hidden> Fri, 15 Aug 2008 19:40:46 +0200