Then how can you get into the situation that the shipped binary packages can't be built with the shipped toolchain or shipped dependencies I think that Matthew

Then how can you get into the situation that the shipped binary packages can't be built with the shipped toolchain or shipped dependencies I think that Matthew might be suggesting that not all packages are rebuilt with every release? A binary package might be compiled with GCC-4, then the distribution is upgraded to GCC-5, dropping GCC-4. If for some reason the package is not compatible with the new compiler, you would then be in a situation where the binary cannot be recreated from source without outside tools. Do you know if Canonical recompiles everything on each update to the toolchain?