Using the changes in staging / master, we should be able to start isolating the remaining problems breaking adoption of gcc5. I already maintain my own branch which has defaulted to gcc5 for a while so a lot of major kinks have already been worked out. Please test the changes in staging.post-15.06 if you would like to help the effort of getting nixpkgs ready for gcc5. The migration should be easiest if we mainline transition patches into master / staging where appropriate, as they have been entirely backward compatible with the old compiler. Merging the patches into mainline should reduce the effort of keeping a separate branch, as the branch is just the switch to gcc5.