Here are a few small modifications I needed to make to get Qt 5.15.17 to build with MSVC when configured to use C++23 (i.e., configure.bat -c++std c++2b/cl.exe /std:c++latest).
Qt 5 refers to C++20 as c++2a and C++23 as c++2b (holdovers from before they were published).
If you don't care about the details, you can just apply the attached patches. Note that there may be other issues I haven't encountered, since I try to disable all Qt functionality I'm not actively using.