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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/64814] std::copy_n advances InputIterator one *less* time than necessary.
Date: Tue, 27 Jan 2015 16:29:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64814

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The current behaviour is decades old, coming from the copy_n in the SGI STL,
and like the standard https://www.sgi.com/tech/stl/copy_n.html says nothing
about postconditions for the input iterator.

Note http://cplusplus.github.io/LWG/lwg-active.html#2242 though.