Do standard exercise when triaging bugs (adding title with version):

QWidget mainWin;
mainWin.setWindowTitle(QT_VERSION_STR + 'a' + QString::number(2));

Note line is highlighted and Quick fix on QT_VERSION_STR is "Apply fix: Use array indexing" which results in

mainWin.setWindowTitle(&QT_VERSION_STR [ 'a'] + QString::number(2));

Expected Quick fix would be "Enclose QT_VERSION_STR in QLatin1String()" (as reported before and was under rare circumstances working with the old model).