Another version of this patch with just a new test case showing what 
wrong code was unnoticed previously by the _GLIBCXX_DEBUG mode.

On 04/08/22 22:56, François Dumont wrote:
> 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