or is not valid C++ : why does this code compile?

Here is a very simple C++ application I made with QtCreator :

int main(int argc, char *argv[])
{
    int a = 1;
    int b = 2;

    if (a < 1 or b > 3)
    {
       return 1;
    }
    return 0;
}

To me, this is not valid C++, as the keyword or is not a reserved keyword.

But if I compile and run it, it works fine without any warnings ! The exit code is 0 and if I change b = 4, the exit code is 1 !

I'm not including anything to make sure there is no hidden define.

This is really strange to me. Is this something Qt is defining ? I didn't find anything in the documentation regarding that.