The compiler reports the error "field has incomplete type 'Attica::Content' whenever it compiles entryinternal.cpp. I tried three versions of gcc (4.9.3, 5.3.0, and 6.1.0) and clang 3.9 and got the same error.

Reproducible: Always

Steps to Reproduce:
1. build kde-frameworks/knewstuff-9999
2.
3.

Actual Results:  
[ 36%] Building CXX object src/CMakeFiles/KF5NewStuff.dir/core/entryinternal.cpp.o
cd /var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999_build/src && /usr/bin/clang++  -DKCOREADDONS_LIB -DKDE_DEFAULT_DEBUG_AREA=550 -DKF5NewStuff_EXPORTS -DQT_CORE_LIB -DQT_DBUS_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_CAST_FROM_BYTEARRAY -DQT_NO_CAST_TO_ASCII -DQT_NO_DEBUG -DQT_NO_SIGNALS_SLOTS_KEYWORDS -DQT_NO_URL_CAST_FROM_STRING -DQT_USE_FAST_OPERATOR_PLUS -DQT_USE_QSTRINGBUILDER -DQT_WIDGETS_LIB -DQT_XML_LIB -DTRANSLATION_DOMAIN=\"knewstuff5\" -D_GNU_SOURCE -D_LARGEFILE64_SOURCE -I/var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999_build/src -I/var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src -isystem /usr/include/KF5/Attica -isystem /usr/include/KF5 -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtCore -isystem /usr/lib64/qt5/mkspecs/linux-g++ -isystem /usr/include/qt5/QtNetwork -isystem /usr/include/KF5/KXmlGui -isystem /usr/include/qt5/QtDBus -isystem /usr/include/qt5/QtXml -isystem /usr/include/qt5/QtWidgets -isystem /usr/include/qt5/QtGui -isystem /usr/include/KF5/KConfigCore -isystem /usr/include/KF5/KConfigWidgets -isystem /usr/include/KF5/KCodecs -isystem /usr/include/KF5/KWidgetsAddons -isystem /usr/include/KF5/KConfigGui -isystem /usr/include/KF5/KAuth -isystem /usr/include/KF5/KCoreAddons -isystem /usr/include/KF5/KService -isystem /usr/include/KF5/KIOCore -isystem /usr/include/KF5/KIOWidgets -isystem /usr/include/KF5/KJobWidgets -isystem /usr/include/KF5/KCompletion -isystem /usr/include/KF5/KArchive -isystem /usr/include/KF5/KI18n -isystem /usr/include/KF5/KItemViews -isystem /usr/include/KF5/KIconThemes -isystem /usr/include/KF5/KTextWidgets -isystem /usr/include/KF5/SonnetUi   -DQT_NO_DEBUG -DNDEBUG -march=sandybridge -g -Ofast -pipe -fstack-protector-strong  -std=c++0x -fno-exceptions -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -pedantic -Wno-gnu-zero-variadic-macro-arguments -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -fPIC -std=gnu++11 -o CMakeFiles/KF5NewStuff.dir/core/entryinternal.cpp.o -c /var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src/core/entryinternal.cpp
In file included from /var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src/core/engine.cpp:22:
In file included from /var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src/core/engine_p.h:27:
In file included from /usr/include/qt5/QtCore/QSharedPointer:1:
In file included from /usr/include/qt5/QtCore/qsharedpointer.h:45:
In file included from /usr/include/qt5/QtCore/qshareddata.h:46:
/usr/include/qt5/QtCore/qhash.h:152:7: error: field has incomplete type 'Attica::Content'
    T value;
      ^
/usr/include/qt5/QtCore/qhash.h:533:19: note: in instantiation of template class 'QHashNode<QString, Attica::Content>' requested here
    concrete(node)->~Node();
                  ^
/usr/include/qt5/QtCore/qhash.h:573:20: note: in instantiation of member function 'QHash<QString, Attica::Content>::deleteNode2' requested here
    x->free_helper(deleteNode2);
                   ^
/usr/include/qt5/QtCore/qhash.h:252:37: note: in instantiation of member function 'QHash<QString, Attica::Content>::freeData' requested here
    ~QHash() { if (!d->ref.deref()) freeData(d); }
                                    ^
/var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src/attica/atticaprovider_p.h:47:7: note: in instantiation of member function
      'QHash<QString, Attica::Content>::~QHash' requested here
class AtticaProvider: public Provider
      ^
/usr/include/qt5/QtCore/qsharedpointer_impl.h:217:29: note: in instantiation of member function 'QtSharedPointer::CustomDeleter<KNS3::AtticaProvider,
      QtSharedPointer::NormalDeleter>::execute' requested here
            realself->extra.execute();
                            ^
/usr/include/qt5/QtCore/qsharedpointer_impl.h:532:65: note: in instantiation of member function
      'QtSharedPointer::ExternalRefCountWithCustomDeleter<KNS3::AtticaProvider, QtSharedPointer::NormalDeleter>::deleter' requested here
        typename Private::DestroyerFn actualDeleter = &Private::deleter;
                                                                ^
/usr/include/qt5/QtCore/qsharedpointer_impl.h:322:7: note: in instantiation of function template specialization
      'QSharedPointer<KNS3::Provider>::internalConstruct<KNS3::AtticaProvider, QtSharedPointer::NormalDeleter>' requested here
    { internalConstruct(ptr, QtSharedPointer::NormalDeleter()); }
      ^
/var/tmp/portage/kde-frameworks/knewstuff-9999/work/knewstuff-9999/src/core/engine.cpp:193:24: note: in instantiation of function template specialization
      'QSharedPointer<KNS3::Provider>::QSharedPointer<KNS3::AtticaProvider>' requested here
            provider = QSharedPointer<KNS3::Provider> (new AtticaProvider(m_categories));
                       ^
/usr/include/KF5/Attica/attica/provider.h:62:7: note: forward declaration of 'Attica::Content'
class Content;
      ^
1 error generated.