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

[abominable-snowman at yandex dot ru]

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

--- Comment #3 from Petr Ovtchenkov <abominable-snowman at yandex dot ru> ---
Created attachment 42042
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42042&action=edit
proposed test for issue

https://gcc.gnu.org/ml/libstdc++/2017-08/msg00036.html

May be worth to add (see issue2471) words to copy_n spec:

 "copy_n return result + n (i.e. increment OutputIterator n times) and
  increment InputIterator max(0, n - 1)."

to avoid ambiguity?