warning: iteration 2u invokes undefined behavior [-Waggressive-loop-optimizations]

[blackrabbit]

Here is a piece of code:

QDataStream &operator<<(QDataStream &out, const sq_settings_t &t)
{

for (int i=0; i<4;i++)
{
    out << (quint8)t.io_type[i]
        << t.pd_100k[i]
        << t.probe_diff[i]
        << t.pull_ena[i]
        << t.pull_val[i];
}

out << t.adr_cap_lim
    << t.adr_gen_lim
    << t.max_trig_steps
    << t.post_trig_len
    << t.s_clk
    << t.t_clk
    << t.thresh
    << t.tvcc;

return out;

}

The problem is, the line "<< t.probe_diff[i]" generates a warning, that i don't understand at all.

Can someone explain to me what is this warning, or at least how to remove it?

ps: i know it's warning and it's not an error, but still, i am cleaning up my code and getting rid of all warnings.