Strange behaviour of std::vector<std::pair<QString,int*> >

(As reported on StackOverflow.)

The following code produces an unexpected output when compiled with g++ main.cpp -O3 -std=c++17 -Wall -fPIC -I/usr/include/qt -I/usr/include/qt/QtCore -lQt5Core (GCC version 8.1.1, Qt version 5.11.1, Arch Linux).

#include <QString>
#include <QDebug>
#include <vector>
#include <tuple>

int main() {
    typedef std::pair<QString,int*> A;
    std::vector<A> v;
    v.emplace_back("111", nullptr);
    v.emplace_back("222", nullptr);
    v.emplace_back("333", nullptr);
    v.emplace_back("444", nullptr);
    v.emplace_back("555", nullptr);
    for (const A &a : v)
        qDebug() << a.first;
    return 0;
}

The output is as follows.

""
""
""
""
"555"

It seems like the vector resize triggers some problem with the move constructor.

Adding the -fno-strict-aliasing compiler option circumvents the problem. (According to the GCC web page, this is a hint that it's a bug in the code, not in GCC.)