Commit 4ce110c50c7c2511626e79ad2d2270537c5ac279 exported QAbstractSocketPrivate so that it could be used in QtCoap. The class itself is derived from QAbstractSocketEngineReceiver, which was previously declared in the same file as QAbstractSocketEngine. As a result, the now-exported QAbstractSocketPrivate class included the definition of the non-exported QAbstractSocketEngine. This caused linking errors when using the QAbstractSocketPrivate in QtCoap: qcoapqudpconnection.cpp.obj : error LNK2001: unresolved external symbol "public: static struct QMetaObject const QAbstractSocketEngine::staticMetaObject" (?staticMetaObject@QAbstractSocketEngine@@2UQMetaObject@@B) To fix it, move QAbstractSocketEngineReceiver into its own header, and only include this new header in QAbstractSocketPrivate. This also requires to adjust a couple of places that were relying on the previous includes. Task-number: QTBUG-139697 Task-number: QTBUG-80704 Change-Id: Icb01399fe0d8941f7b91583cb3687419cb15cc02 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io> Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>