qt6apptestqt_debug_metatypes.json: illegal value

EDIT

My CMake file looks like this:

cmake_minimum_required(VERSION 3.16)project(testqt VERSION 0.1 LANGUAGES CXX)set(CMAKE_AUTOMOC ON)set(CMAKE_CXX_STANDARD_REQUIRED ON)find_package(Qt6 6.4 REQUIRED COMPONENTS Quick)qt_standard_project_setup()qt_add_executable(apptestqt    main.cpp test.cpp test.hpp)qt_add_qml_module(apptestqt    URI testqt    VERSION 1.0    QML_FILES Main.qml)# Qt for iOS sets MACOSX_BUNDLE_GUI_IDENTIFIER automatically since Qt 6.1.# If you are developing for iOS or macOS you should consider setting an# explicit, fixed bundle identifier manually though.set_target_properties(apptestqt PROPERTIES#    MACOSX_BUNDLE_GUI_IDENTIFIER com.example.apptestqt    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}    MACOSX_BUNDLE TRUE    WIN32_EXECUTABLE TRUE)target_link_libraries(apptestqt    PRIVATE Qt6::Quick)include(GNUInstallDirs)install(TARGETS apptestqt    BUNDLE DESTINATION .    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})

ORIGINAL QUESTION

I have simple testing code as follow:

test.hpp

#pragma once#include <QQmlEngine>class test_gadget {    Q_GADGET    QML_ELEMENT    Q_PROPERTY(int test MEMBER test)public:    int test;};class test_object : public QObject {    Q_OBJECT    QML_ELEMENTpublic:    test_object(const test_gadget&);private:    test_gadget gadget;};

test.cpp

#include <test.hpp>test_object::test_object(const test_gadget& other) {    this->gadget = other;}

main.cpp

#include <QQmlContext>#include <test.hpp>int main(int argc, char *argv[]){    QGuiApplication app(argc, argv);    QQmlApplicationEngine engine;    const QUrl url(u"qrc:/testqt/Main.qml"_qs);    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,&app, []() { QCoreApplication::exit(-1); },        Qt::QueuedConnection);    test_gadget gadget;    test_object test_obj(gadget);    engine.rootContext()->setContextProperty("test_obj", &test_obj);    engine.load(url);    return app.exec();}

However when I try to compile, there were errors:

[  5%] Built target apptestqt_qmlimportscan[ 10%] Built target apptestqt_tooling[ 15%] Automatic MOC and UIC for target apptestqt[ 20%] Built target apptestqt_autogen[ 25%] Running AUTOMOC file extraction for target apptestqt[ 25%] Built target apptestqt_automoc_json_extraction[ 30%] Automatic QML type registration for target apptestqtError 5 while parsing C:/Users/USER/source/repos/build-testqt-Desktop_Qt_6_6_0_MinGW_64_bit-Debug/meta_types/qt6apptestqt_debug_metatypes.json: illegal valuemingw32-make.exe[2]: *** [CMakeFiles\apptestqt.dir\build.make:88: apptestqt_qmltyperegistrations.cpp] Error 1mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:97: CMakeFiles/apptestqt.dir/all] Error 2mingw32-make.exe: *** [Makefile:135: all] Error 213:22:22: The process "C:\Qt\Tools\CMake_64\bin\cmake.exe" exited with code 2.Error while building/deploying project testqt (kit: Desktop Qt 6.6.0 MinGW 64-bit)When executing step "Build"

Note:build-testqt-Desktop_Qt_6_6_0_MinGW_64_bit-Debug\meta_types\qt6apptestqt_debug_metatypes.json is empty

How should I fix this?

Qt version: 6.6.0