Why do associative arrays return from opIndexAssign by value?

[TommiT]

On Tuesday, 18 June 2013 at 10:18:23 UTC, Jonathan M Davis wrote:
> On Tuesday, June 18, 2013 11:41:03 TommiT wrote:
>> At least int assignment isn't lvalue. I find this quite
>> surprising:
>> 
>> void edit(ref int) { }
>> 
>> void main()
>> {
>>      int n;
>>      //edit(n = 4); // Error
>>      edit(n += 4);
>> }
>
> Maybe that's by design, but it's not what I would have 
> expected. I don't know
> if that's a bug or not. Certainly, it doesn't match how it 
> works in C++, and I
> don't know why D would be any different in this regard.
>
> - Jonathan M Davis

This must be by design, because associative arrays return lvalue 
from += operator as well:

int[string] values;

foreach (string key; keys)
{
     int* ptr = key in values;

     if (ptr == null)
     {
         // Now it's quite a decent workaround:
         ptr = &(values[key] += 0);
         *ptr = initValueFor(key);
     }
     edit(*ptr);
}