How to static assert to disallow “mixed endianness” in a non-templated member function

I suggest asserting that it’s either big or little endian:

#include <bit>
#include <compare>

struct pawned_pw {
    std::strong_ordering operator<=>(const pawned_pw& rhs) const {
        static_assert(std::endian::native == std::endian::big ||
                          std::endian::native == std::endian::little,
                      "mixed-endianess architectures are not supported");

        if constexpr (std::endian::native == std::endian::little) {
            return ...;
        } else {
            // big
            return ...;
        }
    }
};