Debian - package is installed already but apt says "depends: x but it is not going to be installed"

Question

I don't have a lot of experience with debian. I am troubleshooting a friend's debian squeeze setup who probably pasted both squeeze and wheezy sources in his sources.list. Since then I have corrected it. Installing gcc-4.3 throws:

gcc-4.3 : Depends: cpp-4.3 (= 4.3.5-4) but it is not going to be installed

# apt-cache policy cpp-4.3
cpp-4.3:
  Installed: 4.3.5-4
  Candidate: 4.3.5-4
  Version table:
 *** 4.3.5-4 0
        500 http://ftp.au.debian.org/debian/ stable/main i386 Packages
        100 /var/lib/dpkg/status

Why doesn't apt find my already installed package?

Answer 1

If you use mixed sources (some mix of stable, unstable, testing, experimental and/or others) you might want to tell APT about which one the desired package should be drawn from - if you try to upgrade a package that is already installed from another source, the dependencies installed from that source might be incompatible with the new version.

So try 'apt-get install gcc-4.3/squeeze cpp-4.3/squeeze' or ''apt-get install -t squeeze gcc-4.3'

If you are desperate, you might want to try to download the package and then 'dpkg -i --force-depends mypackage.deb' - but that is a last resort and it will likely break stuff : if APT tells you there is a dependency problem, it usually knows better than me...