Struct immutable data and dict

[Timoses]

On Thursday, 6 October 2016 at 02:09:44 UTC, Adam D. Ruppe wrote:
> On Thursday, 6 October 2016 at 01:23:35 UTC, Patric Dexheimer 
> wrote:
>> Why?
>
> Because you'd be overwriting that immutable member. Structs 
> just put structure around their contents, but it doesn't change 
> their nature. That struct is no different than if you wrote 
> `immutable size_t` as the value - and of course, overwriting 
> that; changing that violates that promise that you won't change 
> it.
>
> You could store pointers to those structs though, and overwrite 
> the pointer.

Hm... so is there any way around this?

In my case I have a RandomAccessRange which caches it's results. 
Reduced example:

     struct Res
     {
         immutable int i;
     }

     struct Range
     {
         Res[size_t] c;

         Res fun(size_t i)
         {
             if (auto p = i in c)
                 return *p;
             else
             {
                 c[3] = Res(3);   /// ERROR line 26
                 return c[3];
             }
         }
     }

     unittest
     {
         auto r = Range();
         r.fun(3);
     }

which errors at compilation with
onlineapp.d(26): Error: cannot modify struct this.c[3] Res with 
immutable members

A workaround I thought could be to use an array

     struct Range
     {
         //Res[size_t] c;
         struct Con { Res c; bool valid; }
         Con[] c;

         Res fun(size_t i)
         {
             if (c.length > i && c[i].valid)
                 return c[i].c;
             else
             {
                 if (c.length <= i) c.length = i + 1;
                 auto r = Con(Res(3), true);
                 c[i] = r;   /// same ERROR!
                 return c[i].c;
             }
         }
     }

However, of course this also fails because randomly assigning the 
array elements will overwrite it. So the associative array seems 
like the better idea. However, not being able to INITIALIZE an 
assoc array element disallows its usage.

Is there any solution, trick or workaround??