01d9831f4b
What I did was to add two versions of the same package into the same collection, i.e. ghc-7.0.4 and ghc-7.2.1. I was surprised to see the build succeed, because there are file collisions between these two. So I (wrongly) assumed that the collection function was to blame, but in fact this is a "feature" of nix-env. Apparently, file collision detection doesn't take place when the user installs two versions of the same package simultaneously. File collisions are detected only between different packages! For example, the command nix-env -i ghc-7.0.4 ghc-7.2.1-wrapper is going to fail, but nix-env -i ghc-7.0.4-wrapper ghc-7.2.1-wrapper is going to succeed. Maybe I just didn't read the documentation thoroughly enough, but this behavior sure is unexpected to me. svn path=/nixpkgs/trunk/; revision=28638