Thanks to hard work of Adam, Julien, Jonathan, Matthias, Scott, Simon and many others, the GCC-5/libstdc++ transition has progressed to a state, where we are ready to migrate the bulk of it to testing.
It should be a mostly smooth ride. However, there will a few packages that are going to be uninstallable in testing for a few days and some packages will be temporarily removed from testing. If APT is unable to provide you with an upgrade for all of your packages, please try again in a few days. We apologise for the inconvenience.
Currently, we expect about 36 binary packages to become temporarily uninstallable on amd64 and 34 on i386. This involves Britney accepting at least 4800 “change items” on testing (some of these are removals). Many thanks to Julien for providing a proposed set of hints and Adam extending them.
Update: We now got a list of the packages being removed and a list of packages becoming uninstallable. It will be available on debian-devel within 20 minutes from now.
nthykier 
Hi,
thanks for the great work, unfortunately on (at least my) multiarch setup (amd64+i386) testing is uninstallable due to bug 797463 (ffmpeg binnmu breakage). At least that is where I got stuck while solving dependencies with aptitude…