New istreambuf_iterator debug check

[François Dumont]

On 24/01/2018 18:53, Petr Ovtchenkov wrote:
> On Wed, 24 Jan 2018 17:39:59 +0100
> François Dumont <frs.dumont@gmail.com> wrote:
>
>> Hi
>>
>>   Â Â Â  I'd like to propose this new debug check. Comparing with non-eos
>> istreambuf_iterator sounds like an obvious coding mistake.
>>
>>   Â Â Â  I propose it despite the stage 1 as it is just a new debug check,
>> it doesn't impact the lib in normal mode.
>>
>>   Â Â Â  Tested under Linux x86_64, ok to commit ?
>>
>> François
>>
>         bool
>         equal(const istreambuf_iterator& __b) const
> -      { return _M_at_eof() == __b._M_at_eof(); }
> +      {
> +	bool __this_at_eof = _M_at_eof();
> +	bool __b_at_eof = __b._M_at_eof();
> +
> +	__glibcxx_requires_cond(__this_at_eof || __b_at_eof, _M_message(
> +	  "Abnormal comparison to non-end-of-stream istreambuf_iterator"));
> +	return __this_at_eof == __b_at_eof;
> +      }
>
> Looks strange for me. It is legal and possible that istreambuf_iterator
> will be in EOF state.
>
Sure, but consider rather the associated 3_neg.cc showing the debug 
check purpose:

 Â  cistreambuf_iter it1(istrs), it2(istrs);
 Â  it1 == it2; // No sens