Man pages matching name std::__detail::_equality_base in section 3cxx:

std::__detail::_Equality_base.3cxx.en	debian 10 / libstdc++-7-doc
std::__detail::_Equality_base.3cxx.en	debian 10 / libstdc++-8-doc
std::__detail::_Equality_base.3cxx.en	debian 11 / libstdc++-9-doc