warning: dereferencing pointer does break strict-aliasing rules

I get this warning while compiling the program below with g++-4.4.3:

$ g++ -O2 -Wall test.cc
test.cc: In function ?int main()?:
test:8: warning: dereferencing pointer ?<anonymous>? does break strict-aliasing
rules
/usr/include/c++/4.4/bits/stl_list.h:214: note: initialized from here

I don?t see anything suspicious in the following program, but there
should be a reason for the warning. Can anyone explain this?

/* 1 */ #include <list>
/* 2 */
/* 3 */ struct X
/* 4 */ {
/* 5 */ struct P
/* 6 */ {
/* 7 */ const void* p;
/* 8 */ bool operator==(const P& o) const { return p == o.p; }
/* 9 */ };
/* 10 */
/* 11 */ typedef std::list<P> PL;
/* 12 */
/* 13 */ struct I
/* 14 */ {
/* 15 */ PL::const_iterator itr;
/* 16 */ bool operator==(I o) const { return *itr == *o.itr; }
/* 17 */ bool operator!=(I o) const { return !(*this == o); }
/* 18 */ I& operator++() { ++itr; return *this; }
/* 19 */ };
/* 20 */
/* 21 */ PL list;
/* 22 */
/* 23 */ I begin() const { I i = {list.begin()}; return i; }
/* 24 */ I end() const { I i = {list.end()}; return i; }
/* 25 */ };
/* 26 */
/* 27 */ int main()
/* 28 */ {
/* 29 */ X x;
/* 30 */ for (X::I it = x.begin(); it != x.end(); ++it) { }
/* 31 */ }

Line 214 of /usr/include/c++/4.4/bits/stl_list.h looks like this:

/* 182 */ /**
/* 183 */ * @brief A list::const_iterator.
/* 184 */ *
/* 185 */ * All the functions are op overloads.
/* 186 */ */
/* 187 */ template<typename _Tp>
/* 188 */ struct _List_const_iterator
/* 189 */ {
/* ... */
/* 210 */ // Must downcast from List_node_base to _List_node to get to
/* 211 */ // _M_data.
/* 212 */ reference
/* 213 */ operator*() const
/* 214 */ { return static_cast<_Node*>(_M_node)->_M_data; }
/* ... */
/* 260 */ };

$ g++ --version
g++ (Debian 4.4.2-9) 4.4.3 20100108 (prerelease)
Copyright (C) 2009 Free Software Foundation, Inc.
....

--
Seungbeom Kim

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