I know I can try this out but I will ask here anyway just to make sure my test is not being incorrectly done. Can mpl::if_/mpl::eval_if be used to pick the type for which a default template parameter is set based on some metaprogramming boolean test of a previous template parameter type ? I know that C++ allows a template parameter default to be set based on a previous template parameter, but I don't know if mpl::if_/mpl::eval_if can be used in this context.