sorting a mutable array of immutable objects

[Jeff Thompson via Digitalmars-d]

I can create a mutable array of immutable objects, thanks to the 
answers to my other question 
https://forum.dlang.org/post/nf57f5$e6v$1...@digitalmars.com .

Now I want to sort the array, but the sort function doesn't "see" 
the opCmp in class C. Do I need to define a custom compare 
function for each class where I want to use Rebindable? Or can 
Rebindable itself be updated to "pass through" methods like opCmp?

class C {
  this(int x) immutable { this.x = x; }
  override int opCmp(Object o) const { return x - (cast(C)o).x; }
  int x;
}

void main(string[] args)
{
  auto array = new Rebindable!(immutable(C))[2];
  array[0] = new immutable C(20);
  array[1] = new immutable C(10);
  sort(array); // Error: "Invalid predicate passed to sort: "a < 
b""
}