For instance, once I'm done splitting std.datetime (which I've slowly been working on), it'll take advantage of the new `package.d` feature, which does not exist in 2.063, so it'll be impossible to compile Phobos with an older version of dmd. In general, it's just bad advise to tell people to use the current dmd when working on druntime or Phobos. You need to be using git master. Sometimes, it works to use the latest release, and it's great when it does, but you cannot rely on it.