Why do we have transitive const, again?

[Andrei Alexandrescu]

On 09/23/11 20:52, Mehrdad wrote:
> On 9/23/2011 3:56 PM, Jonathan M Davis wrote:
>> The complaints have generally been about the lack of logical const and
>> the inability to cast away const and modify variables _not_ about
>> transitiveness. I'm not say that there have been _no_ complaints about
>> the transitiveness about const, but I can't recall even _one_ issue
>> with it at the moment. It's the lack of logical const that's generally
>> complained about and has shown itself to be a major blocker for const
>> in some cases. - Jonathan M Davis
>
> But the only reason logical const is an issue is indeed the fact that
> const is transitive.
>
> To illustrate, I run into problem almost every other time I use D, and
> to me it's *only* a problem because source is const:
>
> class MyRange : InputRange!char
> {
> char peeked;
> InputRange!char source;
> char front() const
> {
> // How do I lazy-load the value?
> if (peeked == '\0')
> {
> peeked = source.front;
> // ERROR, popFront isn't const...
> source.popFront();
> }
> return peeked;
> }
> }
>
> If this isn't because of transitivity, then what is it? If only source
> wasn't implicitly const (and it wouldn't be in C++, since it'd be a
> const pointer to a non-const object) then this would be a piece of cake.
>
> I run into this problem and similar ones so often that it's driven me
> away from D, as much as I was a fan. If you have a nice solution then
> please let me know.

This strikes me as a pretext. If you frame that obscure matter as a 
showstopper, you'll have no trouble finding another one the moment 
you're given a solution.

Andrei