#warning preprocessor macro is used in qtdeprecationdefinitions.h, but this macro is not supported by MSVC.

As the result, if MSVC is used and QT_DISABLE_DEPRECATED_BEFORE is set to a value smaller than a certain version, the build process will be stopped with C1021 error instead of being continued with a warning message as follows :

C:\Qt\include\QtCore/qtdeprecationdefinitions.h(16): fatal error C1021: invalid preprocessor command 'warning' 

Under MSVC, #pragma should be used instead of #warning.