Portably detect __VA_OPT__ support? : <p>In C++20, the preprocessor supports <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0306r4.html" rel="noreferrer"><code>__VA_OPT__</code></a> as a way to optionally expand tokens in a variadic macro if the number of arguments is greater than zero. (This obviates the need for the <code>##__VA_ARGS__</code> GCC extension, which is a non-portable and ugly hack.)</p>

<p>Clang SVN has implemented this feature, but they haven't added a feature test macro for it. Can any clever preprocessor hacker figure out a way to detect the presence or absence of <code>__VA_OPT__</code> support without causing a hard error or a portability warning?</p>
