writing iterators without code duplication. inout?

[pompei2]

> int delegate(int delegate(ref int)) doIter() const
>   {
>74      return (int delegate(ref int) dg)
>       {
>         cast(typeof(this))(this).doIter()
>77            (
>78              (ref int i)
>               {
>                 int copy = i; dg(copy);
>               }
>           );
>       }	
>   }

I see what you are doing there. Unfortunately, the innermost 
delegate (the one doing the copy trick) somehow got @system 
attribute, which leads to the following compile errors (I added 
line numbers and reordered your code in the citation above):

constiter.d(78): Error: cannot implicitly convert expression 
(__dgliteral2) of type void delegate(ref int i) @system to int 
delegate(ref int)
constiter.d(77): Error: cast has no effect in expression 
(cast(const(Container))this.doIter()((__error)))
constiter.d(74): Error: cannot implicitly convert expression 
(__dgliteral1) of type void delegate(int delegate(ref int) dg) 
@system to int delegate(int delegate(ref int))

Where does this @system come from? How do I get rid of it?

> Until
> int delegate(ref inout int) opApply() inout;
> and
> int delegate(int delegate(ref inout int)) doIter() inout;
> are made to work. (I actually don't know if there is any 
> obstacles to do this).

Can you point me to the bugreport so I can vote for it? I'm not 
sure if it is #4838 or if they are unrelated.