Fails to build: Invalid use of incomplete type 'class QVariant' in QtWebEngineCore::URLRequestNotification::notify()

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-Wall -Wextra -D_REENTRANT -Wno-unused-parameter -Wno-unused-variable -Wno-deprecated-declarations -c /home/florian/code/qt/5.14/qtwebengine/src/core/net/url_request_notification.cpp -o obj/QtWebEngineCore/url_request_notification.o
../net/url_request_notification.cpp: In member function ‘void QtWebEngineCore::URLRequestNotification::notify()’:
../net/url_request_notification.cpp:111:52: error: invalid use of incomplete type ‘class QVariant’
  111 |             if (!interceptor->property("deprecated").toBool())
      |                                                    ^
In file included from ../../../../qtbase/include/QtCore/qmetatype.h:1,
                 from ../../../../qtbase/include/QtCore/../../src/corelib/kernel/qobject.h:54,
                 from ../../../../qtbase/include/QtCore/qobject.h:1,
                 from ../../../../qtbase/include/QtCore/../../src/corelib/tools/qsharedpointer_impl.h:64,
                 from ../../../../qtbase/include/QtCore/qsharedpointer_impl.h:1,
                 from ../../../../qtbase/include/QtCore/../../src/corelib/tools/qsharedpointer.h:48,
                 from ../../../../qtbase/include/QtCore/qsharedpointer.h:1,
                 from ../../../../qtbase/include/QtCore/../../src/corelib/kernel/qpointer.h:43,
                 from ../../../../qtbase/include/QtCore/qpointer.h:1,
                 from ../../../../qtbase/include/QtCore/QPointer:1,
                 from ../net/url_request_notification.h:46,
                 from ../net/url_request_notification.cpp:40:
../../../../qtbase/include/QtCore/../../src/corelib/kernel/qmetatype.h:134:21: note: forward declaration of ‘class QVariant’
  134 |     F(QVariant, 41, QVariant) \
      |                     ^~~~~~~~
../../../../qtbase/include/QtCore/../../src/corelib/kernel/qmetatype.h:1981:11: note: in definition of macro ‘QT_FORWARD_DECLARE_STATIC_TYPES_ITER’
 1981 |     class Name;
      |           ^~~~
../../../../qtbase/include/QtCore/../../src/corelib/kernel/qmetatype.h:1983:1: note: in expansion of macro ‘QT_FOR_EACH_STATIC_CORE_CLASS’
 1983 | QT_FOR_EACH_STATIC_CORE_CLASS(QT_FORWARD_DECLARE_STATIC_TYPES_ITER)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~