What we'd like to do is to test whether some very mildly breaking changes to <system_error> are so mild as to affect no users, and on that basis propose WG21 to make those mildly breaking changes to <system_error> in the next C++ standard. And to test this, we'd like to modify Boost.System with those mildly breaking changes, ship a release and see how many users complain. If it is zero, we have evidence for WG21 that this is not a consequential change.
The usual way is to announce the upcoming change in release nnn and then make the change in release nnn + 1. What is wrong with using this method in your case, if all parties are in agreement to do it ?
It goes without saying that this forthcoming change would be announced one release ahead as per Boost custom. So, next release would have it #ifdef'd off by default, following release #ifdef's it on by default. A macro would allow users to select which implementation. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/