I've always found SerenityOS to have quite nice C++ source code. The project runs on very recent versions of C++ (to the point where the standard compilation sc

I've always found SerenityOS to have quite nice C++ source code. The project runs on very recent versions of C++ (to the point where the standard compilation script for Ubuntu will compile a modern compiler first) and the OS intentionally doesn't stick to POSIX, allowing some very nice API improvements that only work in a C++-first world. Its main author moved on to Ladybird, though, so I haven't really browsed the code recently. I'm not sure if SerenityOS uses concepts and other such recent additions.