As a Qt user, I'd like warning-free Qt headers even if I compile in C++20 mode

As a qt developer, whenever I try to compile the project in C+20 mode, I first need to go and fix some compile errors, and there are lots of warnings, too. Of course, I'm done with that at some point, but when I then start adding C+20-only API to Qt, I would hope it gets non-zero test coverage on the CI, too.