qt6_wrap_cpp doesn't work for C++ source files

// myobject2.h
#ifndef MYOBJECT2_H
#define MYOBJECT2_H

void doSomethingWithMyObject2();

#endif

// myobject2.cpp
#include "myobject2.h"
#include <QObject>

class MyObject2 : public QObject {
    Q_OBJECT
public:
    MyObject2() = default;
};

void doSomethingWithMyObject2()
{
    MyObject2 obj;
    obj.setObjectName("whatever");
}

#include "myobject2.moc"

# CMakeLists.txt
cmake_minimum_required(VERSION 3.16)
project(qt-playground)
set(CMAKE_AUTOMOC ON)
find_package(Qt6 COMPONENTS Widgets REQUIRED)
add_executable(qt-playground
    main.cpp
    myobject2.cpp
)
qt_wrap_cpp(genMocs myobject2.cpp TARGET qt-playground)
message("XXX ${genMocs}")
target_sources(qt-playground PRIVATE ${genMocs})