I have a question how building Boost should behave when the user provides wrong configuration information. E.g., suppose you have this in user-config.jam: using python : 3.20 : /nowhere ; or alternatively, in 1.42 a user runs Boost.Build as follows: ./bjam -sZLIB_INCLUDES=/still-nowhere ... What should happen? - A hard error. After all, the user wanted that specific configuration. And warnings are easy to miss. - A warning. Autodetection will still run and possibly find a random version - A hard error, but also run autodetection and let user know what was found. For example: Could not find Python version 3.20 in /nowhere. There is a valid installation of 2.5 in /usr. Remove the 'using python' statement to use it. Comments? Thanks, Volodya