[PATCH] Improvements to valid range checks in debug mode

[François Dumont]

On 18/08/21 5:55 pm, Jonathan Wakely wrote:
> On Wed, 18 Aug 2021 at 15:40, Jonathan Wakely wrote:
>> On Mon, 16 Aug 2021 at 20:26, Jonathan Wakely via Libstdc++
>> <libstdc++@gcc.gnu.org> wrote:
>>> On Fri, 13 Aug 2021 at 18:10, François Dumont via Libstdc++
>>> <libstdc++@gcc.gnu.org> wrote:
>>>> I just come back to this email and it sounds like the right moment to
>>>> commit it.
>>> Ah yes, thanks for the reminder!
>>>
>>> I'll push it tomorrow.
>>>
>>>> On 27/02/20 2:06 pm, Jonathan Wakely wrote:
>>>>> These should wait for stage 1 but I'm posting them now for comment.
>>>>>
>>>>> With the change to __gnu_debug::__valid_range we now get a debug
>>>>> assertion for:
>>>>>
>>>>>    std::string s;
>>>>>    std::min_element(std::string::iterator{}, s.end());
>>>>>
>>>>> where previously it would just crash with undefined behaviour.
>> Actually, that change doesn't work. Some of our container iterators
>> use a value-initialized iterator as the past-the-end value, so the
>> check in the new __valid_range_aux function incorrectly rejects some
>> valid ranges. Maybe I can make it work for bidirectional iterators,
>> which must be attached to a container to be valid.
> The attached patch is good enough for the original motivation, as shown above.
>
> This makes the debug checks work with basic_string iterators, and for
> all other container types we already have safe iterators.

It would be great, especially considering my proposal to activate 
__valid_range check in _GLIBCXX_ASSERTIONS mode.