Hmmm … not really I searched around briefly through the papers and their dependencies, and couldn’t find a single example of how they are supposed to actually be used.
So I can’t tell if your example is invalid or if (more likely) gcc (or its C++ Standard Library) doesn’t support this yet.
I’ve been waiting for constexpr vectors and constexpr strings for a long time. Hope I come across a nice way to use those in my codes soon, now that C++20 is done!