I am trying to use an rtree as an index to a set of triangles to perform some spatial queries.
The rtree stores std::pair<box, triangles> where box is the boost geometry box.

The problem occurs with the nearest query and, in particular, when two or more elements have equivalent boxes,
 and I look for just one element, the result may be wrong.

Look at the code below; the problem is that the two triangles have equivalent bounding boxes, and since
the query returns just one element, the result is the one I inserted first. Even though the bounding boxes
are the same, the two elements are not, I'd like the query to returns t2 instead.

I think I need to provide an IndexableGetter to my rtree, but according to
https://www.boost.org/doc/libs/1_70_0/libs/geometry/doc/html/geometry/spatial_indexes/creation_and_modification.html
the returned value has to be one of the Indexable concepts (Point, Box, or Segment) and none of them seems to satisfy
my requirements.

Do I need to make my triangle Indexable? I could not find any reference on how to do that and what methods are required
for a type to be indexable; can anyone provide any resources that may be useful?

No, only boxes should be used for indexing.

You have to iterate the query iterator and check a condition until you're satisfied with the result, e.g.:

double distance_to_closest_triangle = MAX;
for(auto it = rtree.qbegin(nearest(pt, rtree.size())) ; it != rtree.qend() ; ++it) {
    if (distance_to_closest_triangle <= distance(pt, it->first))
        break;
    distance_to_closest_triangle = min(distance(pt, triangles[it->second]), distance_to_closest_triangle);
}