This an old patch I had prepared a long time ago, I don't think I ever 
submitted it.

     libstdc++: [_GLIBCXX_DEBUG] Do not consider detached iterators as 
value-initialized

     An attach iterator has its _M_version set to something != 0. This 
value shall be preserved
     when detaching it so that the iterator does not look like a value 
initialized one.

     libstdc++-v3/ChangeLog:

             * include/debug/formatter.h (__singular_value_init): New 
_Iterator_state enum entry.
             (_Parameter<>(const _Safe_iterator<>&, const char*, 
_Is_iterator)): Check if iterator
             parameter is value-initialized.
             (_Parameter<>(const _Safe_local_iterator<>&, const char*, 
_Is_iterator)): Likewise.
             * include/debug/safe_iterator.h 
(_Safe_iterator<>::_M_value_initialized()): New. Adapt
             checks.
             * include/debug/safe_local_iterator.h 
(_Safe_local_iterator<>::_M_value_initialized()): New.
             Adapt checks.
             * src/c++11/debug.cc (_Safe_iterator_base::_M_reset): Do 
not reset _M_version.
             (print_field(PrintContext&, const _Parameter&, const 
char*)): Adapt state_names.
             * testsuite/23_containers/deque/debug/iterator1_neg.cc: New 
test.
             * testsuite/23_containers/deque/debug/iterator2_neg.cc: New 
test.
             * 
testsuite/23_containers/forward_list/debug/iterator1_neg.cc: New test.
             * 
testsuite/23_containers/forward_list/debug/iterator2_neg.cc: New test.

Tested under Linux x86_64 _GLIBCXX_DEBUG mode.

Ok to commit ?

François