QML_ELEMENT does not work as expected with precompiled_headers

To declare a QML element from C++, one can use QML_ELEMENT with qt_add_qml_module, and it works nicely... until you add precompiled_headers to the mix.

See attached project, the precompiled_headers contains QQmlEngine, and the TestItem is declared as is:

#ifndef TESTITEM_H
#define TESTITEM_H

#include <QObject>

class TestItem : public QObject{
    Q_OBJECT
    QML_ELEMENT
public:
    explicit TestItem(QObject *parent = nullptr);
};
#endif // TESTITEM_H 

As you can see, QQmlEngine is not included here, and everything compiles fine thanks to precompiled_headers.

But when running, you got this error:

qrc:/qt/qml/test_bug/Main.qml:11:5: TestItem is not a type 

TestItem is not in the module... adding the include QQmlEngine fixes it.

As long as the code compile, the QML_ELEMENT tag should be handled properly, even without the QQmlEngine inc;ude.