front stability

[Steven Schveighoffer via Digitalmars-d]

On 6/30/16 10:59 AM, jmh530 wrote:
> On Thursday, 30 June 2016 at 14:17:22 UTC, Steven Schveighoffer wrote:
>>
>> Jack Stouffer has created a PR to formalize this.
>>
>> Please comment if you have objections (the group that argued for
>> current generate behavior was absent from this post that was meant to
>> be a debate). I think this is the right thing to do.
>
> It looks like this is just documentation changes. This really doesn't
> prevent anyone from making a Range that violates front stability. For
> instance, suppose isInputRange is changed so that
>
> auto h = r.front; // can get the front of the range
>
> becomes
>
> auto h = r.front; // can get the front of the range
> auto j = r.front;
> assert(h == j);  //ensure front stability

This doesn't solve it, and it can't be solved -- halting problem. We 
have some expectations that are assumed and some that are mechanically 
tested. This PR clarifies the assumptions.

-Steve