This issue was previously filed as QTBUG-50083, but dismissed as not in the source. That was incorrect.

The problematic code lives in corelib/global/qglobal.cpp, function QSysInfo::windowsVersion(). Various web guidance suggests that the message can be detected and suppressed by the user installing their own message handler via qInstallMsgHandler(). Regrettably however, code that triggers this warning is part of QT's WinMain(), so the error arrives before the user can do anything about it.