tion-only concept defining the behavior of an object capable of reliably/in-order sending ...">Native_handle_sender</a> (and other such doc-only classes) can become an actual concept formally implemented by class(es) that, today, implement it via the "honor system." Currently it is a class <code>#ifdef</code>-ed out from actual compilation but still participating in doc generation. Note that Doxygen (current version as of this writing: 1.9.3) claims to support doc generation from formal C++20 concepts. </p>