When compiling vs. Qt headers >= 6.10.0, using `-Werror -pedantic`, and in C++20 mode, one may get a lot warnings such as the following:

 
In file included from /Users/calin/Qt512/6.10.0/macos/lib/QtCore.framework/Headers/QHash:1:
In file included from /Users/calin/Qt512/6.10.0/macos/lib/QtCore.framework/Headers/qhash.h:12:
/Users/calin/Qt512/6.10.0/macos/lib/QtCore.framework/Headers/qlist.h:451:11: warning: use of the 'assume' attribute is a C++23 extension [-Wc++23-attribute-extensions]

This is due to changes such as in this commit, new to 6.10.0: https://github.com/qt/qtbase/commit/84467d70a831eea78f288f76fff91c5def01f14f

To reproduce, add the flags using e.g. qmake as so to any project compiled against Qt 6.10.0: 

qmake -o foo foo.pro 'QMAKE_CXXFLAGS+=-Wextra -pedantic' CONFIG+=c++20

Some comments:

• The committer wrongly asserts that [[assume]] is a C+20 attribute. It is not. It is a C+23 attribute. Reference: https://en.cppreference.com/w/cpp/language/attributes/assume
• clang in C++20 mode will return true to `__has_cpp_attribute(assume)`, sadly.
• But it still warns about the attribute being used if in C++20 mode.

I am not sure how you guys can resolve this but this is very annoying to users because compiling with `-Wextra -pedantic` is not uncommon.

This is new in the 6.10.x series. In 6.9.x and before everything compiles without warnings.

Please fix.