sorting a mutable array of immutable objects

[Jeff Thompson via Digitalmars-d]

On Wednesday, 20 April 2016 at 09:54:13 UTC, Jeff Thompson wrote:
> 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""
> }

Actually, maybe my problem is that the opCmp override should take 
a const Object (for comparing with an immutable object). Is there 
a way to do that?