Make istreambuf_iterator::_M_sbuf immutable and add debug checks

[Petr Ovtchenkov]

On Mon, 6 Nov 2017 22:19:22 +0100
François Dumont <frs.dumont@gmail.com> wrote:

> Hi
> 
>      Any final decision regarding this patch ?
> 
> François

https://gcc.gnu.org/ml/libstdc++/2017-11/msg00036.html
https://gcc.gnu.org/ml/libstdc++/2017-11/msg00035.html
https://gcc.gnu.org/ml/libstdc++/2017-11/msg00037.html
https://gcc.gnu.org/ml/libstdc++/2017-11/msg00034.html

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   - ptr