Use-after-free on 64-bit coordinates

[plopresti]

#include <boost/polygon/polygon.hpp>
#include <iterator>
#include <cstdint>

namespace bp = boost::polygon;

typedef int64_t Coord;
typedef bp::point_data<Coord> Vec2;
typedef bp::polygon_data<Coord> Polygon;
typedef bp::polygon_set_data<Coord> PolySet;

int main()
{
    const Vec2 pts[] = {
        Vec2(0,0),
        Vec2(-219215130788,1374389535),
        Vec2(-195163313930,204784040673), // BAD
        //Vec2(-195163313929,204784040673), // OK
    };

    Polygon poly;
    set_points(poly, std::cbegin(pts), std::cend(pts));
    PolySet pset;
    pset.insert(poly);
    std::vector<Polygon> c;
    pset.get_trapezoids(c);
}

Compile with -fsantitize=address to see the use-after-free error, although my glibc complains even without it.

Note that these coordinates are all less than 40 bits.

The suggestions in #36 (use gmp_override.hpp) and #39 (update 64-bit coordinate_traits) do not help.

What does help is changing the last line of evalAtXforYlazy() to return roundl(evalAtXforYret). I do not understand the code well enough to say whether this is a good idea in general.