QVersionNumber seems broken on -O1 and higher with g++

If we use QVersionNumber and enable compile optimization the initial usage of idx is not correct.

#include <QVersionNumber>
#include <QDebug>

int main(int argc, char** argv)
{
        const QString pVersion = QStringLiteral("1.12.2");
        int idx = 0;
        const auto& version = QVersionNumber::fromString(pVersion, &idx);
        if(idx == 0)
                qDebug() << "broken";
        else
                qDebug() << "ok";

        qDebug() << "idx:" << idx;
        return 0;
}

g++ with -O0

$ g++ main.cpp -std=c+11 -fPIC `pkg-config --libs --cflags Qt5Core` -lstdc+ -O0
$ ./a.out
ok
idx: 6

g++ with -O2

$ g++ main.cpp -std=c+11 -fPIC `pkg-config --libs --cflags Qt5Core` -lstdc+ -O2
$ ./a.out
broken
idx: 6

clang++ with -O2

$ clang++ main.cpp -std=c+11 -fPIC `pkg-config --libs --cflags Qt5Core` -lstdc+ -O2
$ ./a.out
ok
idx: 6

If we do not initialize idx to 0, it will work.

        int idx;