[Bug libstdc++/81857] istreambuf_iterator not work as input iterator

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81857

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |rs2740 at gmail dot com

--- Comment #1 from TC <rs2740 at gmail dot com> ---
istreambuf_iterator is specified to call sbumpc() only on increment, not upon
construction; the implementation is as specified.

The copy_n aspect is https://cplusplus.github.io/LWG/issue2471.