gcc (7.4.1 here) says:
ktcpsocket.h:171:22: warning: attribute ignored in declaration of ‘class KTcpSocket’ [-Wattributes]
class KIOCORE_EXPORT KTcpSocket: public QIODevice
^~~~~~~~~~
ktcpsocket.h:171:22: note: attribute for ‘class KTcpSocket’ must follow the ‘class’ keyword