The moc build step to generate moc_TestRawString.cpp results in the following:
TestRawString.h:0:1: note: No relevant classes found. No output generated.
and indeed, moc_TestRawString.cpp is an empty file.

Once I comment the variable "raw_double_quotes" in TestRawString.h, the project will compile and run correctly. Is it a feature or something?

This phenomenon is easy to reproduce and here are my example codes.

TestRawString.h

#include <QObject>
class TestRawString : public QObject
{
    Q_OBJECT
public:
    explicit TestRawString(QObject *parent = nullptr);
    std::string raw_double_quotes = R"++(")++";
    std::string raw_single_quote = R"++(')++";
    std::string normal_double_quotes = "\"";
};

TestRawString.cpp (generated by Qt)

#include "TestRawString.h"

TestRawString::TestRawString(QObject *parent)
    : QObject{parent}
{}

main.cpp

#include "TestRawString.h"

int main()
{
    TestRawString trs;
    assert(trs.normal_double_quotes == R"++(")++");
    return 0;
}

.pro

QT -= gui

CONFIG += c++11 console
CONFIG -= app_bundle

HEADERS += \
    TestRawString.h

SOURCES += \
        TestRawString.cpp \
        main.cpp

enviroment:

Qt: (online installation)
QMake version 3.1
Using Qt version 6.2.4 in C:/Qt/Qt6.2/6.2.4/mingw_64/lib

compiler: gcc version 11.2.0 (x86_64-posix-seh-rev3, Built by MinGW-W64 project)

os: windows 10 21H2 x86_64