The function creates an object library, that compiles the cpp file generated from the rcc file without passing the -DQT_NAMESPACE define.

Most likely it should link against Qt6::Core or Qt5::Core so that the define is pulled in. The define is set for Qt6::Core, but I'm not sure about Qt5::Core. So this might need fixing in Qt 5.15.x too.

Afaik we don't have a qmake test for resources_big, but there's one commented out CMake test in tests/auto/cmake/test_add_big_resource.

Note that we don't currently provide a way to query if a Qt build is namespaced from CMake (we do for qmake). We should probably add a public property or variable for that.