I carried out a --force-conflicts install of libraw20.
So now apt is broken but theoretically I could install kstars-bleeding et al manually. Removing libraw20 restores apt functionality to normal.
At any rate, libraw19 and libraw20 do not have any conflicting files, which I was able to confirm with 'dpkg-query -L'.
It comes down to the fact that libraw20's control file has 'Conflicts' and 'Replaces' entries against libraw19, while the latter has no such entries against any prior versions of libraw.
So would there be any harm in just deleting those entries in libraw20's control file to allow installation to proceed?
IIRC, I added Conflicts and Replaces because there were indeed conflicts, maybe in the development files I don't exactly remember. I'd be more than happy to make it install alongside libraw19 if someone figures out a way for them to live happily togeather.
Worked for me. Installed it from a modified deb, apt keeps wanting to update it from the one in the ppa but it won't as long as the conflict is still there.
Yes, the devel files will conflict. The usual way is to split this in two parts, one with only the libs (i.e., libraw19, libraw20 containing libraw.so.XX and libraw_r.so.XX), and one, unversioned, with the devel files (headers, unversioned .so link), libraw-dev. Those will conflict, and there it is fine to replace the earlier version, as one usually wants to compile against the latest version. But normal users probably wouldn't install the devel part anyhow...
Sure, but the dependencies should only be that libraw-dev (version 20) requires libraw20 and replaces libraw-dev (version <=19). libraw20 however should not replace earlier versions, so that you can keep different versions of the library in parallel (which is the idea behind using librawXX instead of just libraw).
As Aurneth suggests, it seems it is only the two lines
Conflicts: libraw19
Replaces: libraw19
in the control file (section libraw20) that should just be deleted. But I have no Debian system (and only guess this from similarities to the SPEC files of rpm)