@nogc with opApply

[Alex]

Hi all,
maybe I misunderstand something but having this:

´´´
import std.experimental.all;

static assert(isIterable!S);

void main()
{
     S s;
     s.each!(el => el.writeln);
}

struct S
{
     private Nullable!uint member = 0;
     Nullable!uint front() @nogc { return member; }
     //void popFront(){} // not implementable.
     //bool empty(){} // not implementable
     Nullable!uint successor(uint current) @nogc { return 
Nullable!uint.init; }

     /**
     the opApply method grants the correct foreach behavior
     */
     int opApply(scope int delegate(ref uint) /*@nogc*/ 
operations) //@nogc
     {
         int result;

         for(auto leading = front; !leading.isNull; leading = 
successor(leading.get))
         {
             result = operations(leading.get);

             if(result)
             {
                 break;
             }
         }
         return result;
     }
}
´´´

Everything works fine, before I try to use the opApply function 
inside a @nogc function.

If I do, compiler complains, that opApply is not marked as @nogc. 
Ok.
If I try to mark opApply @nogc, I would have to mark operations 
delegate also as @nogc, but I can't do this, as I do not know a 
priori, how it will be used.

Now, as I learned at some point, a possibility would be, to 
templatify the function, as the compiler can then derive, if 
@nogc apply or not.
But if I write
´´´
int opApply()(...){...}
´´´
Then the static assert from above refuses to compile.

So... how to solve this case?