SignOn/authpluginif.h:42:7: error: using typedef-name ‘using QStringList = class QList’ after ‘class’

I'm getting the following error when trying to compile it with Qt 6.9.1:

g++ -c -pipe -O2 -fno-exceptions -Wall -Wextra -D_REENTRANT -fPIC -DDEBUG_ENABLED -DNO_SIGNON_USER -DQT_NO_EXCEPTIONS -DQT_NO_DEBUG -DQT_CORE_LIB -I. -I.. -I/usr/qt6/include -I/usr/qt6/include/QtCore -I. -I/usr/qt6/mkspecs/linux-g++ -o moc_authpluginif.o moc_authpluginif.cpp
In file included from moc_authpluginif.cpp:9:
SignOn/authpluginif.h:42:7: error: using typedef-name ‘using QStringList = class QList<QString>’ after ‘class’
   42 | class QStringList;
      |       ^~~~~~~~~~~
In file included from /usr/qt6/include/QtCore/qtypeinfo.h:9,
                 from /usr/qt6/include/QtCore/qglobal.h:45,
                 from /usr/qt6/include/QtCore/qnamespace.h:12,
                 from /usr/qt6/include/QtCore/qobjectdefs.h:12,
                 from /usr/qt6/include/QtCore/qobject.h:10,
                 from SignOn/authpluginif.h:31:
/usr/qt6/include/QtCore/qcontainerfwd.h:40:7: note: ‘using QStringList = class QList<QString>’ has a previous declaration here
   40 | using QStringList = QList<QString>;
      |       ^~~~~~~~~~~
make[3]: *** [Makefile.signon-plugins:992: moc_authpluginif.o] Error 1
make[3]: Leaving directory '/usr/src/git/signond/lib/plugins'
make[2]: *** [Makefile:45: sub-signon-plugins-pro-make_first-ordered] Error 2
make[2]: Leaving directory '/usr/src/git/signond/lib/plugins'
make[1]: *** [Makefile:99: sub-plugins-make_first] Error 2
make[1]: Leaving directory '/usr/src/git/signond/lib'
make: *** [Makefile:50: sub-lib-make_first] Error 2

Any hints?