9 Iterators library [iterators]

9.7 Iterator adaptors [iterators.predef]

9.7.3 Move iterators and sentinels [iterators.move]

9.7.3.2 move_iterator operations [move.iter.ops]

9.7.3.2.3 move_iterator conversion [move.iter.op.conv]

constexpr I base() const;

Returns: current.