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User mentioned (1): @Some
User mentioned (0): @Marek
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Low reputation (1):

Compilation works now that the inclusion inside the class has been moved outside:

headerWorking.h
#ifndef HEADER_WORKING_H
#define HEADER_WORKING_H

#include <QObject>
#include <qqml.h>
#include "header1.h"

class ClassWorking: public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    Q_ENUM(TestType_t)
    .
    .
    .
};
#endif

headerNotWorking.h
#ifndef HEADER_NOT_WORKING_H
#define HEADER_NOT_WORKING_H

#include <QObject>
#include <qqml.h>
#include "header1.h"

class ClassNotWorking: public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    Q_ENUM(TestType_t)
    Q_INVOKABLE void foo(TestType_t testType);
    .
    .
    .
};
#endif

I just recently started working on this project and didn't question why the inclusion happened inside the class.

Thanks @Some programmer dude and @Marek R

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