This is a good example. Without trampolines, this could look like this (Godbolt: https://godbolt.org/z/nK5fqMxjs).

    int main() {
        struct Point {
            int x, y;
        } points[3] = {
            { 3, 1 },
            { 2, 2 },
            { 5, 7 }
        };

        struct Point target = { 4, 5 };

        long dsq(const struct Point *p) {
            long dx = p->x - target.x, dy = p->y - target.y;
            return dx*dx + dy*dy;
        }

        typedef typeof(dsq) dsq_f;

        int compare(const void *p, const void *q, void *data) {
           wide(dsq_f) *dsq = data;
           long a = CALL(*dsq, (p)), b = CALL(*dsq, (q));
           return (a > b) - (a < b);
        }

        qsort_r(points, 3, sizeof(struct Point), compare, &CLOSURE(dsq_f, dsq));

        for (int i = 0; i < 3; ++i)
            printf("%d,%d\n", points[i].x, points[i].y);
    }

There are slightly different ways how to define the helper macros, I am still experimenting a bit. Here you could avoid the typedef if defined differently. But ideally, there would be native language support that avoids these macros.