declare int as lambda result type will generate error red line

#include <QCoreApplication>
#include <QString>
#include <QList>
#include <functional>
#include <QDebug>

auto a = [](const QString &data) -> int { return data.toInt() + 1; };
auto b = [](const QString &data) -> int { return data.toInt() + 1; };

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
    qDebug() << b("123");
    qDebug() << a("123");
}