For elaborate objects defining comparison, clamp() should return the leftmost object unless it falls outside the limits established by the other two parameters.

Consider:

    struct A {
        int x, y;
        int opCmp(ref const A rhs) const { return (x > rhs.x) - (x < rhs.x); }
    }
    A x, lo, hi;
    x.y = 42;
    assert(x.clamp(lo, hi).y == 42);

Also, clamp() needs to be constrained appropriately.