<xstd/ints/ext/boost.hpp>

#include <boost/int128.hpp>
#include <boost/hash2/hash_append.hpp>

namespace xstd {
  template<> struct make_unsigned<boost::int128::int128>;
  template<> struct make_signed<boost::int128::uint128>;

  template<class Hash, class Flavor, integer T>
    constexpr auto hash_append_int(Hash& h, const Flavor& f, T v) -> void;
}

Header	Declares
<xstd/ints/ext/boost/int128.hpp>	make_signed, make_unsigned
<xstd/ints/ext/boost/hash2.hpp>	hash_append_int

The umbrella exports <xstd/ints/ext/boost/int128.hpp> and <xstd/ints/ext/boost/hash2.hpp>, and each exports the Boost header it adapts, so including one is enough to name Boost's types and flavors. No header of the library includes it: an adapted library is asked for by name. Through it, Boost.Int128's pair is paired and hashes as xstd's own 128-bit pair does.