QLocale::formattedDataSize should be const

class MyDelegate : public QStyledItemDelegate
{
public:
    QString displayText(const QVariant &, const QLocale &) const override;
};

QString MyDelegate::displayText(const QVariant & value, const QLocale & locale) const
{
    return locale.formattedDataSize(value.toDouble()); 
    // error: passing 'const QLocale' as 'this' argument discards qualifiers [-fpermissive]
}