I tried debugging or running a Qt5-based test application (https://cgit.kde.org/knewstuff.git/tree/tests/khotnewstuff_upload.cpp) that uses qcDebug with QtCreator (4.7.0). The associated logging category was according to http://doc.qt.io/qt-5/qloggingcategory.html#details activated either with QLoggingCategory::setFilterRules("*.debug=true) or by specifying the environment variable QT_LOGGING_RULES.

Unfortunately, QtCreator does not display debug output (stderr) from the application. The debug output is only visible if the application was started from a console or if "run in terminal" was checked in the configuration of the Qt Creator project.

Qt4 applications do not have this problem.