QFileInfo::isRoot() unexpected behavior

#include <QCoreApplication>

#include <QFileInfo>
#include <QDir>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    auto foo = [](const QString &path) {
        QFileInfo fi{path};
        qDebug() << path << fi.canonicalFilePath();
        qDebug() << fi.isRoot();
        qDebug() << QDir(fi.canonicalFilePath()).isRoot();
    };

    foo("C:\\");
    foo("C:\\test\\..\\");
    foo("C:\\..\\");

    return a.exec();
}

"C:\\" "C:/"
true
true
"C:\\test\\..\\" "C:/"
false
true
"C:\\..\\" "C:/"
false
true