Make istreambuf_iterator::_M_sbuf immutable and add debug checks

[François Dumont]

On 16/11/2017 19:12, Petr Ovtchenkov wrote:
> On Thu, 16 Nov 2017 18:40:08 +0100
> François Dumont <frs.dumont@gmail.com> wrote:
>
>> On 16/11/2017 12:46, Jonathan Wakely wrote:
>>
>>> Let me be more clear: I'm not going to review further patches in this
>>> area while you two are proposing different alternatives, without
>>> commenting on each other's approach.
>>>
>>> If you think your solution is better than François's solution, you
>>> should explain why, not just send a different patch. If François
>>> thinks his solution is better than yours, he should state why, not
>>> just send a different patch.
>>>
>>> I don't have time to infer all that from just your patches, so I'm not
>>> going to bother.
>>>
>>>
>> Proposing to revert my patch doesn't sound to me like a friendly action
>> to start a collaboration.
> I'm already say that this is technical issue: this patch present only in
> trunk yet.
Doesn't explain why you want to revert it.
>   Series is more useful for applying in different branches.
Which branches ? There are mostly maintenance branches. None of your 
patches is fixing a bug so it won't go to a maintenance branch.
> BTW, https://gcc.gnu.org/ml/libstdc++/2017-11/msg00037.html
> was inspired by you https://gcc.gnu.org/ml/libstdc++/2017-10/msg00029.html
Which was already rejected, why submitting it again ?
>
>> So current implementation is
>> just fine to me and I'll let Petr argument for any change.
> Please, clear for me: what is the "current implementation"?
> Is it what we see now in trunk?
Yes. Other proposals were mostly code quality changes. None of them had 
no impact so Jonathan decision to keep current implementation is just fine.