Indexing operator of iterator_range<counting_iterator<...>> produces undefined behavior

首先编译来自 test_counting_iterator_range_indexing.cpp 的复现程序，并检查 boost/range/iterator_range_core.hpp 中 iterator_range_base::operator[] 附近的代码。确认 counting_iterator 的警告和悬空引用行为，然后添加一个回归测试，展示 range[0] 在不产生未定义行为的情况下得到 0。

Example:

#include <boost/iterator/counting_iterator.hpp>
#include <boost/range/iterator_range.hpp>

#include <iostream>
#include <type_traits>

int main(int, char **)
{
    using iterator = boost::counting_iterator<int>;
    using iterator_range = boost::iterator_range<iterator>;

    iterator first(0), last(3);
    iterator_range range(first, last);

    std::cout << "iterator::reference is a reference type: " << std::is_reference<iterator::reference>::value << std::endl;
    std::cout << "iterator_range::reference is a reference type: " << std::is_reference<iterator_range::reference>::value << std::endl;

    std::cout << "first[0] returns a reference type: " << std::is_reference<decltype(first[0])>::value << std::endl;
    std::cout << "range[0] returns a reference type: " << std::is_reference<decltype(range[0])>::value << std::endl;
    
    std::cout << "first[0] == " << first[0] << std::endl;
    std::cout << "range[0] == " << range[0] << std::endl; // <-- segfault
}

Output:

iterator::reference is a reference type: 1
iterator_range::reference is a reference type: 1
first[0] returns a reference type: 0
range[0] returns a reference type: 1
first[0] == 0
Segmentation fault (core dumped)

Expected behavior: range[0] returns 0.

If Boost headers are copied to ./boost and the program is compiled with g++ -I. option, the compiler produces the following warning:

In file included from ./boost/range/iterator_range.hpp:13,
                 from test_counting_iterator_range_indexing.cpp:2:
./boost/range/iterator_range_core.hpp: In instantiation of ‘boost::iterator_range_detail::iterator_range_base<IteratorT, boost::iterators::random_access_traversal_tag>::reference boost::iterator_range_detail::iterator_range_base<IteratorT, boost::iterators::random_access_traversal_tag>::operator[](boost::iterator_range_detail::iterator_range_base<IteratorT, boost::iterators::random_access_traversal_tag>::difference_type) const [with IteratorT = boost::iterators::counting_iterator<int>; boost::iterator_range_detail::iterator_range_base<IteratorT, boost::iterators::random_access_traversal_tag>::reference = const int&; boost::iterator_range_detail::iterator_range_base<IteratorT, boost::iterators::random_access_traversal_tag>::difference_type = long int]’:
test_counting_iterator_range_indexing.cpp:23:43:   required from here
./boost/range/iterator_range_core.hpp:391:32: warning: returning reference to temporary [-Wreturn-local-addr]
         return this->m_Begin[at];
                                ^

The reason for the segmentation fault is that iterator_range::operator[] returns a reference to a stack-allocated object returned by counting_iterator::operator[].