The legacy adaptable function API was removed from C++20 and added to the list of zombie names, so most of those identifiers are indexed as zombies. However, 'result_type' was not added to the zombie list as it remains used in 'std::function', the seed sequence requirements, the uniform random bit generator requirements, the random number distribution requirements, and most of the standard library URBs and distributions. None of these uses are referenced by the index of library names.