https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81964
--- Comment #3 from Petr Ovtchenkov <abominable-snowman at yandex dot ru> ---
Looks "not a bug".
Standard say (with a bit misoriented words):
27.6.1 Class template istream_iterator [istream.iterator]
1 The class template istream_iterator is an input iterator (27.2.3)
that reads (using operator>>) successive elements from the input
stream for which it was constructed.
-> After it is constructed, and every time ++ is used, the iterator
-> reads and stores a value of T.
...
27.6.1.1 istream_iterator constructors and destructor
[istream.iterator.cons]
...
istream_iterator(istream_type& s);
-> 3 Effects: Initializes in_stream with addressof(s). value may be
initialized
-> during construction or the first time it is referenced.
4 Postconditions: in_stream == addressof(s).
Current position is "After it is constructed ... the iterator reads and stores
a value of T.":
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0738r0.html
