The original proposal for std::bases was rejected, in part because it arrived too late for a then immediately upcoming new standard. Seems like that specific proposal has not resurfaced again.
However, there is a new and more general proposal P0194 Static reflection that includes ways to not only interrogate base classes, but also has a much wider target.
Right now, that proposal contains some types that seems to do something like what the std::bases proposal suggested:
template <Class T> struct get_public_base_classes;
template <Class T> struct get_accessible_base_classes;
template <Class T> struct get_base_classes;
However (again), in Herb Sutter's latest report from the ISO C++ meeting he projects these kinds of features not to appear in C++20, but perhaps in a future C++23 standard.
