Making a flake for quick generic setup of C++ projects (that is to say, adding C++ to this collection) has been somewhere below page 50 of my TODO list for quite some time. I try to avoid C++ like the plague, and for the situations where I’m obliged to use C++, I have ad-hoc nix solutions which work for me, so the motivation to get a clean, shareable generic solution is somewhat lacking. I’ll have a look if I can get this into a usable state if I find a bit of spare time.
On the specific topic of clang not finding standard headers, I recall I had (and solved) this problem somewhere … here. This solution is cmake-specific, but the whole topic does discuss the problem in a slightly broader context. Maybe you’ll find something relevant.