[PATCH][_GLIBCXX_DEBUG] Enhance __gnu_debug::vector iterator invalidation

François Dumont frs.dumont@gmail.com
Sun May 31 20:17:59 GMT 2026


On 5/28/26 13:46, Jonathan Wakely wrote:
> On Thu, 21 May 2026 at 22:20 +0200, François Dumont wrote:
>> Hi
>>
>> Following your remark Jonathan regarding how iterators are 
>> invalidated in vector I reviewed the std::__debug::vector 
>> implementation.
>
> Can you remind me where I said that? I remember discussing it for
> inplace_vector::assign (2025-11-03 in the thread about
> __debug::inplace_vector).
>
> I don't remember the comment you're referring to, so I don't know what
> I said :-)

In this message:

https://gcc.gnu.org/pipermail/libstdc++/2025-November/064301.html

You said:

Is it too aggressive to invalidate all iterators?

I know the generic Sequence requirements say that assign(i, j)
invalidates all iterators, but I think maybe that's a defect.

As std::vector is also a Sequence I thought you were talking about both 
inplace_vector and vector.

Maybe I over interpreted it but I also think that when you are assigning 
a vector to some new content the iterators do not have to be invalidated.

Reading this email again I think I'll have to consider the problem of 
the end iterator, in a future patch...

>> I'll do something similar on std::__debug::inplace_vector.
>>
>>     libstdc++: [_GLIBCXX_DEBUG] Enhance __gnu_debug::vector iterator 
>> invalidation
>>
>>     Rework the way std::__debug::vector's iterators are being 
>> invalidated. Use a guard
>>     approach which on destructor invalidates iterators based on the 
>> state of the container.
>>
>>     Moreover update invalidation policy in following vector methods:
>>     - operator=(initializer_list)
>>     - assign(Iterator, Iterator)
>>     - assign(size_t, const _Tp&)
>>     - assign(initializer_list)
>>
>>     Current behavior to invalidate all iterators unconditionally is 
>> preserved only if
>>     _GLIBCXX_DEBUG_PEDANTIC is defined. Otherwise if a reallocation 
>> occurs all iterators
>>     are invaliidated else only iterators after the new container size 
>> are invalidated.
>>
>>     libstdc++-v3/ChangeLog:
>>
>>             * include/debug/assertions.h [_GLIBCXX_DEBUG_STD_C]: New 
>> macro.
>>             * include/debug/vector (_Safe_vector<_Tp, _Allocator>): 
>> Update template parameter.
>>             (_Safe_vector::_M_requires_reallocation): Remove.
>>             (_SafeVectorGuard): New.
>>             [_GLIBCXX_DEBUG_PEDANTIC](_SafeVectorPedanticGuard): New.
>>             (_SafeVectorInsertGuard): New.
>>             (vector): Adapt methods to use new guard type.
>>             * 
>> testsuite/23_containers/vector/cons/destructible_debug_neg.cc: Adapt 
>> error line
>>             number.
>>             * 
>> testsuite/23_containers/vector/debug/initializer_list.cc: New test case.
>>             * 
>> testsuite/23_containers/vector/debug/initializer_list_pedantic.cc: 
>> New test case.
>>
>> Partially tested on x86_64 w/o _GLIBCXX_DEBUG mode.
>>
>> Ok to commit once tests completed ?
>>
>> https://forge.sourceware.org/gcc/gcc-TEST/pulls/159
>>
>> François
>>
>>
>
>> diff --git a/libstdc++-v3/include/debug/assertions.h 
>> b/libstdc++-v3/include/debug/assertions.h
>> index c4993dc8596..768a7f05228 100644
>> --- a/libstdc++-v3/include/debug/assertions.h
>> +++ b/libstdc++-v3/include/debug/assertions.h
>> @@ -31,6 +31,12 @@
>>
>> #include <bits/c++config.h>
>>
>> +#ifdef _GLIBCXX_DEBUG
>> +# define _GLIBCXX_DEBUG_STD_C ::std::_GLIBCXX_STD_C
>> +#else
>> +# define _GLIBCXX_DEBUG_STD_C ::std
>> +#endif
>> +
>> #ifndef _GLIBCXX_DEBUG
>> // Verify that [_First, _Last) forms a non-empty iterator range.
>> # define __glibcxx_requires_non_empty_range(_First,_Last)    \
>> diff --git a/libstdc++-v3/include/debug/vector 
>> b/libstdc++-v3/include/debug/vector
>> index 61e5ff78a7a..1e1a971036e 100644
>> --- a/libstdc++-v3/include/debug/vector
>> +++ b/libstdc++-v3/include/debug/vector
>> @@ -51,11 +51,12 @@ namespace __gnu_debug
>>    * detecting code which relies on non-portable implementation 
>> details of
>>    * the libstdc++ reallocation policy.
>>    */
>> -  template<typename _SafeSequence,
>> -       typename _BaseSequence>
>> +  template<class _Tp, typename _Allocator>
>>     class _Safe_vector
>>     {
>> -      typedef typename _BaseSequence::size_type size_type;
>> +      typedef std::__debug::vector<_Tp, _Allocator> _SafeSequence;
>> +      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
>> +      typedef typename _StdVector::size_type size_type;
>>
>>       _GLIBCXX20_CONSTEXPR
>>       const _SafeSequence&
>> @@ -103,10 +104,6 @@ namespace __gnu_debug
>>
>>       size_type _M_guaranteed_capacity;
>>
>> -      bool
>> -      _M_requires_reallocation(size_type __elements) const 
>> _GLIBCXX_NOEXCEPT
>> -      { return __elements > _M_seq().capacity(); }
>> -
>>       _GLIBCXX20_CONSTEXPR
>>       void
>>       _M_update_guaranteed_capacity() _GLIBCXX_NOEXCEPT
>> @@ -115,6 +112,66 @@ namespace __gnu_debug
>>       _M_guaranteed_capacity = _M_seq().size();
>>       }
>>     };
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    class _SafeVectorGuard
>> +    {
>> +      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
>> +      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
>> +      typedef typename _StdVector::size_type size_type;
>> +
>> +      _SafeVector& _M_vect;
>> +      size_type _M_capacity, _M_size;
>> +
>> +    public:
>> +      bool _M_active;
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      _SafeVectorGuard(_SafeVector& __vector);
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      ~_SafeVectorGuard();
>> +    };
>> +
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +  template<class _Tp, typename _Allocator>
>> +    class _SafeVectorPedanticGuard
>> +    {
>> +      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
>> +      _SafeVector& _M_vect;
>> +
>> +    public:
>> +      bool _M_active;
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      _SafeVectorPedanticGuard(_SafeVector& __vector);
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      ~_SafeVectorPedanticGuard();
>> +    };
>> +#endif
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    class _SafeVectorInsertGuard
>> +    {
>> +      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
>> +      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
>> +      typedef typename _StdVector::size_type size_type;
>> +      typedef typename _StdVector::difference_type difference_type;
>> +
>> +      _SafeVector& _M_vect;
>> +      size_type _M_capacity;
>> +      difference_type _M_offset;
>> +
>> +    public:
>> +      bool _M_active;
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      _SafeVectorInsertGuard(_SafeVector& __vector, difference_type 
>> __offset);
>> +
>> +      _GLIBCXX20_CONSTEXPR
>> +      ~_SafeVectorInsertGuard();
>> +    };
>> }
>
> All the guard types above need comments explaining what they do. When
> should you use each one? How are they different from each other?
>
> How do we deviate from the standard when not in pedantic mode?
>
> I'm unclear what they actually do, and how that corresponds to the
> standard.
>
> _SafeVectorGuard invalidates all iterators if the size increased to
> greater than capacity. That's used by resize and emplace_back and
> push_back, right? And if the size was reduced, invalidate iterators to
> the erased elements. This must not be used for reserve, because it
> would not invalidate all iterators on reallocation, because the size
> would not change.
>
> _SafeVectorPedanticGuard invalidates all iterators unconditionally,
> which is what the standard says to do for assign.
>
> _SafeVectorInsertGuard seems to be almost the same as _SafeVectorGuard
> but using an arbitrary position for the "invalidates all after pos"
> instead of only invalidating at the end.

Yes, I wonder if I could use only 1 implementation for both but I think 
it would make this implementation more complicated to get it right.

Ideally we should have a 4th implementation just invalidating all 
iterators in case of reallocation. I preferred to use _SafeVectorGuard 
(_ResizeGuard) in this case, which is fine to use when size if always 
increased like in push_back or emplace_back.

>
> Would it be more clear to have _ResizeGuard, _AssignGuard (which
> changes behaviour for pedantic vs non-pedantic), and _InsertGuard?
>
I've adopted those names.

And doing so I changed the type of guard used in assign_range and 
insert_range.

>>
>> namespace std _GLIBCXX_VISIBILITY(default)
>> @@ -128,14 +185,33 @@ namespace __debug
>>     : public __gnu_debug::_Safe_container<
>>     vector<_Tp, _Allocator>, _Allocator, __gnu_debug::_Safe_sequence>,
>>       public _GLIBCXX_STD_C::vector<_Tp, _Allocator>,
>> -      public __gnu_debug::_Safe_vector<
>> -    vector<_Tp, _Allocator>,
>> -    _GLIBCXX_STD_C::vector<_Tp, _Allocator> >
>> +      public __gnu_debug::_Safe_vector<_Tp, _Allocator>
>>     {
>>       typedef _GLIBCXX_STD_C::vector<_Tp, _Allocator> _Base;
>>       typedef __gnu_debug::_Safe_container<
>>     vector, _Allocator, __gnu_debug::_Safe_sequence> _Safe;
>> -      typedef __gnu_debug::_Safe_vector<vector, _Base>        
>> _Safe_vector;
>> +      typedef __gnu_debug::_Safe_vector<_Tp, _Allocator>    
>> _Safe_vector;
>> +
>> +      typedef __gnu_debug::_SafeVectorGuard<_Tp, _Allocator> _Guard;
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +      typedef __gnu_debug::_SafeVectorPedanticGuard<_Tp, _Allocator>
>> +    _MaybePedanticGuard;
>> +#else
>> +      typedef _Guard _MaybePedanticGuard;
>
> This name seems focused on the detail of how we decide which type to
> use, not what it actually does. And _SafeVectorGuard is also "maybe
> pedantic" because its destructor changes behaviour for pedantic mode.
>
> Would _AssignGuard be a better name? That reflects where it's actually
> used.

Yes, looks better. Only _ResizeGuard looks a little bit surprising in 
push_back/emplace_back/append_Range.

Maybe I should define _ReallocGuard for those methods, even it would 
just be another alias of _SafeVectorGuard.

>
>> +#endif
>> +      typedef __gnu_debug::_SafeVectorInsertGuard<_Tp, _Allocator>
>> +    _InsertGuard;
>> +
>> +      template<typename _Tp2, typename _Alloc2>
>> +    friend class ::__gnu_debug::_SafeVectorGuard;
>> +
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +      template<typename _Tp2, typename _Alloc2>
>> +    friend class ::__gnu_debug::_SafeVectorPedanticGuard;
>> +#endif
>> +
>> +      template<typename _Tp2, typename _Alloc2>
>> +    friend class ::__gnu_debug::_SafeVectorInsertGuard;
>>
>>       typedef typename _Base::iterator _Base_iterator;
>>       typedef typename _Base::const_iterator _Base_const_iterator;
>> @@ -276,12 +352,9 @@ namespace __debug
>>       vector&
>>       operator=(initializer_list<value_type> __l)
>>       {
>> +    _MaybePedanticGuard __guard(*this);
>>     _Base::operator=(__l);
>> -    if (!std::__is_constant_evaluated())
>> -      {
>> -        this->_M_invalidate_all();
>> -        this->_M_update_guaranteed_capacity();
>> -      }
>> +    __guard._M_active = true;
>>     return *this;
>>       }
>> #endif
>> @@ -303,26 +376,23 @@ namespace __debug
>>       typename __gnu_debug::_Distance_traits<_InputIterator>::__type 
>> __dist;
>>       __glibcxx_check_valid_range2(__first, __last, __dist);
>>
>> +      _MaybePedanticGuard __guard(*this);
>>       if (__dist.second >= __gnu_debug::__dp_sign)
>>         _Base::assign(__gnu_debug::__unsafe(__first),
>>               __gnu_debug::__unsafe(__last));
>>       else
>>         _Base::assign(__first, __last);
>>
>> -      this->_M_invalidate_all();
>> -      this->_M_update_guaranteed_capacity();
>> +      __guard._M_active = true;
>>     }
>>
>>       _GLIBCXX20_CONSTEXPR
>>       void
>>       assign(size_type __n, const _Tp& __u)
>>       {
>> +    _MaybePedanticGuard __guard(*this);
>>     _Base::assign(__n, __u);
>> -    if (!std::__is_constant_evaluated())
>> -      {
>> -        this->_M_invalidate_all();
>> -        this->_M_update_guaranteed_capacity();
>> -      }
>> +    __guard._M_active = true;
>>       }
>>
>> #if __cplusplus >= 201103L
>> @@ -330,12 +400,9 @@ namespace __debug
>>       void
>>       assign(initializer_list<value_type> __l)
>>       {
>> +    _MaybePedanticGuard __guard(*this);
>>     _Base::assign(__l);
>> -    if (!std::__is_constant_evaluated())
>> -      {
>> -        this->_M_invalidate_all();
>> -        this->_M_update_guaranteed_capacity();
>> -      }
>> +    __guard._M_active = true;
>>       }
>> #endif
>>
>> @@ -428,13 +495,9 @@ namespace __debug
>>     if (std::__is_constant_evaluated())
>>       return _Base::resize(__sz);
>>
>> -    bool __realloc = this->_M_requires_reallocation(__sz);
>> -    if (__sz < this->size())
>> -      this->_M_invalidate_after_nth(__sz);
>> +    _Guard __guard(*this);
>>     _Base::resize(__sz);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    this->_M_update_guaranteed_capacity();
>> +    __guard._M_active = true;
>>       }
>>
>>       _GLIBCXX20_CONSTEXPR
>> @@ -444,25 +507,17 @@ namespace __debug
>>     if (std::__is_constant_evaluated())
>>       return _Base::resize(__sz, __c);
>>
>> -    bool __realloc = this->_M_requires_reallocation(__sz);
>> -    if (__sz < this->size())
>> -      this->_M_invalidate_after_nth(__sz);
>> +    _Guard __guard(*this);
>>     _Base::resize(__sz, __c);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    this->_M_update_guaranteed_capacity();
>> +    __guard._M_active = true;
>>       }
>> #else
>>       void
>>       resize(size_type __sz, _Tp __c = _Tp())
>>       {
>> -    bool __realloc = this->_M_requires_reallocation(__sz);
>> -    if (__sz < this->size())
>> -      this->_M_invalidate_after_nth(__sz);
>> +    _Guard __guard(*this);
>>     _Base::resize(__sz, __c);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    this->_M_update_guaranteed_capacity();
>> +    __guard._M_active = true;
>>       }
>> #endif
>>
>> @@ -506,7 +561,7 @@ namespace __debug
>>     if (std::__is_constant_evaluated())
>>       return _Base::reserve(__n);
>>
>> -    bool __realloc = this->_M_requires_reallocation(__n);
>> +    bool __realloc = __n > capacity();
>>     _Base::reserve(__n);
>>     if (__n > this->_M_guaranteed_capacity)
>>       this->_M_guaranteed_capacity = __n;
>> @@ -583,11 +638,9 @@ namespace __debug
>>     if (std::__is_constant_evaluated())
>>       return _Base::push_back(__x);
>>
>> -    bool __realloc = this->_M_requires_reallocation(this->size() + 1);
>> +    _Guard __guard(*this);
>>     _Base::push_back(__x);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    this->_M_update_guaranteed_capacity();
>> +    __guard._M_active = true;
>>       }
>>
>> #if __cplusplus >= 201103L
>> @@ -610,11 +663,9 @@ namespace __debug
>>       if (std::__is_constant_evaluated())
>>         return _Base::emplace_back(std::forward<_Args>(__args)...);
>>
>> -      bool __realloc = this->_M_requires_reallocation(this->size() + 
>> 1);
>> +      _Guard __guard(*this);
>>       _Base::emplace_back(std::forward<_Args>(__args)...);
>> -      if (__realloc)
>> -        this->_M_invalidate_all();
>> -      this->_M_update_guaranteed_capacity();
>> +      __guard._M_active = true;
>> #if __cplusplus > 201402L
>>       return back();
>> #endif
>> @@ -645,15 +696,14 @@ namespace __debug
>>                 this);
>>
>>       __glibcxx_check_insert(__position);
>> -      bool __realloc = this->_M_requires_reallocation(this->size() + 
>> 1);
>> +      _Base_iterator __res;
>> +      {
>>         difference_type __offset = __position.base() - _Base::cbegin();
>> -      _Base_iterator __res = _Base::emplace(__position.base(),
>> +        _InsertGuard __guard(*this, __offset);
>> +        __res = _Base::emplace(__position.base(),
>> std::forward<_Args>(__args)...);
>> -      if (__realloc)
>> -        this->_M_invalidate_all();
>> -      else
>> -        this->_M_invalidate_after_nth(__offset);
>> -      this->_M_update_guaranteed_capacity();
>> +        __guard._M_active = true;
>> +      }
>>       return { __res, this };
>>     }
>> #endif
>> @@ -670,14 +720,13 @@ namespace __debug
>>       return iterator(_Base::insert(__position.base(), __x), this);
>>
>>     __glibcxx_check_insert(__position);
>> -    bool __realloc = this->_M_requires_reallocation(this->size() + 1);
>> +    _Base_iterator __res;
>> +    {
>>       difference_type __offset = __position.base() - _Base::begin();
>> -    _Base_iterator __res = _Base::insert(__position.base(), __x);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    else
>> -      this->_M_invalidate_after_nth(__offset);
>> -    this->_M_update_guaranteed_capacity();
>> +      _InsertGuard __guard(*this, __offset);
>> +      __res = _Base::insert(__position.base(), __x);
>> +      __guard._M_active = true;
>> +    }
>>     return iterator(__res, this);
>>       }
>>
>> @@ -704,14 +753,13 @@ namespace __debug
>>       return iterator(_Base::insert(__position.base(), __n, __x), this);
>>
>>     __glibcxx_check_insert(__position);
>> -    bool __realloc = this->_M_requires_reallocation(this->size() + 
>> __n);
>> +    _Base_iterator __res;
>> +    {
>>       difference_type __offset = __position.base() - _Base::cbegin();
>> -    _Base_iterator __res = _Base::insert(__position.base(), __n, __x);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    else
>> -      this->_M_invalidate_after_nth(__offset);
>> -    this->_M_update_guaranteed_capacity();
>> +      _InsertGuard __guard(*this, __offset);
>> +      __res = _Base::insert(__position.base(), __n, __x);
>> +      __guard._M_active = true;
>> +    }
>>     return { __res, this };
>>       }
>> #else
>> @@ -719,14 +767,10 @@ namespace __debug
>>       insert(iterator __position, size_type __n, const _Tp& __x)
>>       {
>>     __glibcxx_check_insert(__position);
>> -    bool __realloc = this->_M_requires_reallocation(this->size() + 
>> __n);
>>     difference_type __offset = __position.base() - _Base::begin();
>> +    _InsertGuard __guard(*this, __offset);
>>     _Base::insert(__position.base(), __n, __x);
>> -    if (__realloc)
>> -      this->_M_invalidate_all();
>> -    else
>> -      this->_M_invalidate_after_nth(__offset);
>> -    this->_M_update_guaranteed_capacity();
>> +    __guard._M_active = true;
>>       }
>> #endif
>>
>> @@ -746,12 +790,10 @@ namespace __debug
>>       typename __gnu_debug::_Distance_traits<_InputIterator>::__type 
>> __dist;
>>       __glibcxx_check_insert_range(__position, __first, __last, __dist);
>>
>> -      /* Hard to guess if invalidation will occur, because __last
>> -         - __first can't be calculated in all cases, so we just
>> -         punt here by checking if it did occur. */
>> -      _Base_iterator __old_begin = _M_base().begin();
>> -      difference_type __offset = __position.base() - _Base::cbegin();
>>       _Base_iterator __res;
>> +      {
>> +        difference_type __offset = __position.base() - _Base::cbegin();
>> +        _InsertGuard __guard(*this, __offset);
>>         if (__dist.second >= __gnu_debug::__dp_sign)
>>           __res = _Base::insert(__position.base(),
>>                     __gnu_debug::__unsafe(__first),
>> @@ -759,11 +801,8 @@ namespace __debug
>>         else
>>           __res = _Base::insert(__position.base(), __first, __last);
>>
>> -      if (_M_base().begin() != __old_begin)
>> -        this->_M_invalidate_all();
>> -      else
>> -        this->_M_invalidate_after_nth(__offset);
>> -      this->_M_update_guaranteed_capacity();
>> +        __guard._M_active = true;
>> +      }
>>       return { __res, this };
>>     }
>> #else
>> @@ -775,22 +814,15 @@ namespace __debug
>>       typename __gnu_debug::_Distance_traits<_InputIterator>::__type 
>> __dist;
>>       __glibcxx_check_insert_range(__position, __first, __last, __dist);
>>
>> -      /* Hard to guess if invalidation will occur, because __last
>> -         - __first can't be calculated in all cases, so we just
>> -         punt here by checking if it did occur. */
>> -      _Base_iterator __old_begin = _M_base().begin();
>>       difference_type __offset = __position.base() - _Base::begin();
>> +      _InsertGuard __guard(*this, __offset);
>>       if (__dist.second >= __gnu_debug::__dp_sign)
>>         _Base::insert(__position.base(), __gnu_debug::__unsafe(__first),
>>                          __gnu_debug::__unsafe(__last));
>>       else
>>         _Base::insert(__position.base(), __first, __last);
>>
>> -      if (_M_base().begin() != __old_begin)
>> -        this->_M_invalidate_all();
>> -      else
>> -        this->_M_invalidate_after_nth(__offset);
>> -      this->_M_update_guaranteed_capacity();
>> +      __guard._M_active = true;
>>     }
>> #endif
>>
>> @@ -876,30 +908,20 @@ namespace __debug
>>     constexpr void
>>     assign_range(_Rg&& __rg)
>>     {
>> -      auto __old_capacity = _Base::capacity();
>> -      auto __old_size = _Base::size();
>> +      _Guard __guard(*this);
>>       _Base::assign_range(__rg);
>> -      if (!std::__is_constant_evaluated())
>> -        {
>> -          if (_Base::capacity() != __old_capacity)
>> -        this->_M_invalidate_all();
>> -          else if (_Base::size() < __old_size)
>> -        this->_M_invalidate_after_nth(_Base::size());
>> -          this->_M_update_guaranteed_capacity();
>> -        }
>> +      __guard._M_active = true;
>>     }
>>
>>       template<__detail::__container_compatible_range<_Tp> _Rg>
>>     constexpr iterator
>>     insert_range(const_iterator __pos, _Rg&& __rg)
>>     {
>> -      auto __old_capacity = _Base::capacity();
>> -      auto __res = _Base::insert_range(__pos.base(), __rg);
>> -      if (!std::__is_constant_evaluated())
>> +      _Base_iterator __res;
>>       {
>> -          if (_Base::capacity() != __old_capacity)
>> -        this->_M_invalidate_all();
>> -          this->_M_update_guaranteed_capacity();
>> +        _Guard __guard(*this);
>> +        __res = _Base::insert_range(__pos.base(), __rg);
>> +        __guard._M_active = true;
>>       }
>>       return iterator(__res, this);
>>     }
>> @@ -908,14 +930,9 @@ namespace __debug
>>     constexpr void
>>     append_range(_Rg&& __rg)
>>     {
>> -      auto __old_capacity = _Base::capacity();
>> +      _Guard __guard(*this);
>>       _Base::append_range(__rg);
>> -      if (!std::__is_constant_evaluated())
>> -        {
>> -          if (_Base::capacity() != __old_capacity)
>> -        this->_M_invalidate_all();
>> -          this->_M_update_guaranteed_capacity();
>> -        }
>> +      __guard._M_active = true;
>>     }
>> #endif
>>
>> @@ -1057,6 +1074,84 @@ _GLIBCXX_END_NAMESPACE_VERSION
>>
>> namespace __gnu_debug
>> {
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorGuard<_Tp, _Allocator>::_SafeVectorGuard(
>> + std::__debug::vector<_Tp, _Allocator>& __vector)
>> +    : _M_vect(__vector)
>> +    , _M_capacity(__vector.capacity())
>> +    , _M_size(__vector.size())
>> +    , _M_active(false)
>> +    { }
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorGuard<_Tp, _Allocator>::~_SafeVectorGuard()
>> +    {
>> +      if (!std::__is_constant_evaluated() && _M_active)
>> +    {
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +      if (_M_vect.size() > _M_capacity)
>> +#else
>> +      if (_M_vect.capacity() != _M_capacity)
>> +#endif
>
> I don't understand this condition. What difference does it make, and
> why do we want it?
>
> For resize and emplace_back/push_back the two conditions are
> equivalent, right? _M_vect.capacity() would not change (i.e.
> reallocation would not occur) unless v.size() == v.capacity(),
> and in that case v.size() would become greater than capacity.
>
> When are these different?

When _GLIBCXX_DEBUG_PEDANTIC _M_vect.capacity() returns a 'guaranteed 
capacity' and not the actual underlying vector capacity.

This guaranteed capacity can change on emplace_back/push_back. The 
problem is that it is updated later in the guard destructor, on the call 
to _M_update_guaranteed_capacity().

Thinking about it to reply I realized that I should not preserve 
original code order and just update guaranteed capacity first so that I 
only need (_M_vect.capacity() != _M_capacity) check.

>
>> +        _M_vect._M_invalidate_all();
>> +      else if (_M_vect.size() < _M_size)
>> +        _M_vect._M_invalidate_after_nth(_M_vect.size());
>> +
>> +      _M_vect._M_update_guaranteed_capacity();
>> +    }
>> +    }
>> +
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorPedanticGuard<_Tp, 
>> _Allocator>::_SafeVectorPedanticGuard(
>> + std::__debug::vector<_Tp, _Allocator>& __vector)
>> +    : _M_vect(__vector), _M_active(false)
>> +    { }
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorPedanticGuard<_Tp, 
>> _Allocator>::~_SafeVectorPedanticGuard()
>> +    {
>> +      if (!std::__is_constant_evaluated() && _M_active)
>> +    {
>> +      _M_vect._M_invalidate_all();
>> +      _M_vect._M_update_guaranteed_capacity();
>> +    }
>> +    }
>> +#endif
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorInsertGuard<_Tp, _Allocator>::_SafeVectorInsertGuard(
>> + std::__debug::vector<_Tp, _Allocator>& __vector, difference_type 
>> __offset)
>> +    : _M_vect(__vector)
>> +    , _M_capacity(__vector.capacity())
>> +    , _M_offset(__offset)
>> +    , _M_active(false)
>> +    { }
>> +
>> +  template<class _Tp, typename _Allocator>
>> +    _GLIBCXX20_CONSTEXPR
>> +    _SafeVectorInsertGuard<_Tp, _Allocator>::~_SafeVectorInsertGuard()
>> +    {
>> +      if (!std::__is_constant_evaluated() && _M_active)
>> +    {
>> +#ifdef _GLIBCXX_DEBUG_PEDANTIC
>> +      if (_M_vect.size() > _M_capacity)
>> +#else
>> +      if (_M_vect.capacity() != _M_capacity)
>> +#endif
>
> Same question here.
>
>> +        _M_vect._M_invalidate_all();
>> +      else
>> +        _M_vect._M_invalidate_after_nth(_M_offset);
>> +
>> +      _M_vect._M_update_guaranteed_capacity();
>> +    }
>> +    }
>> +
>>   template<typename _Tp, typename _Alloc>
>>     struct _Is_contiguous_sequence<std::__debug::vector<_Tp, _Alloc> >
>>     : std::__true_type
>> diff --git 
>> a/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc 
>> b/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc 
>>
>> index 2c9abcdbb91..0a29c0432e6 100644
>> --- 
>> a/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc
>> +++ 
>> b/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc
>> @@ -47,4 +47,4 @@ test02()
>> // { dg-error "PrivateDtor.* is private" "" { target *-*-* } 0 }
>>
>> // In Debug Mode the "required from here" errors come from 
>> <debug/vector>
>> -// { dg-error "required from here" "" { target *-*-* } 182 }
>> +// { dg-error "required from here" "" { target *-*-* } 258 }
>> diff --git 
>> a/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc 
>> b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc
>> new file mode 100644
>> index 00000000000..318f6b98de9
>> --- /dev/null
>> +++ 
>> b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc
>> @@ -0,0 +1,26 @@
>> +// { dg-do compile { target c++11 } }
>> +
>> +#include <debug/vector>
>> +#include <testsuite_hooks.h>
>> +
>> +void
>> +test01()
>> +{
>> +  __gnu_debug::vector<int> v = { 0, 1, 2, 3, 4, 5 };
>> +
>> +  auto it1 = v.begin() + 1;
>> +  auto it4 = v.begin() + 4;
>> +
>> +  VERIFY( !it1._M_singular() );
>> +  VERIFY( !it4._M_singular() );
>> +
>> +  v = { 10, 11, 12, 13 };
>> +
>> +  VERIFY( !it1._M_singular() );
>> +  VERIFY( it4._M_singular() );
>> +}
>> +
>> +int main()
>> +{
>> +  test01();
>> +}
>> diff --git 
>> a/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc 
>> b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc 
>>
>> new file mode 100644
>> index 00000000000..c8b3af25bd1
>> --- /dev/null
>> +++ 
>> b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc
>> @@ -0,0 +1,27 @@
>> +// { dg-do compile { target c++11 } }
>> +// { dg-options "-D_GLIBCXX_DEBUG_PEDANTIC" }
>> +
>> +#include <debug/vector>
>> +#include <testsuite_hooks.h>
>> +
>> +void
>> +test01()
>> +{
>> +  __gnu_debug::vector<int> v = { 0, 1, 2, 3, 4, 5 };
>> +
>> +  auto it1 = v.begin() + 1;
>> +  auto it4 = v.begin() + 4;
>> +
>> +  VERIFY( !it1._M_singular() );
>> +  VERIFY( !it4._M_singular() );
>> +
>> +  v = { 10, 11, 12, 13 };
>> +
>> +  VERIFY( it1._M_singular() );
>> +  VERIFY( it4._M_singular() );
>> +}
>> +
>> +int main()
>> +{
>> +  test01();
>> +}
>
Here is the new patch.

Tested under Linux x86_64 w/o _GLIBCXX_DEBUG mode.

Ok to commit ?

François

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diff --git a/libstdc++-v3/include/debug/assertions.h b/libstdc++-v3/include/debug/assertions.h
index c4993dc8596..768a7f05228 100644
--- a/libstdc++-v3/include/debug/assertions.h
+++ b/libstdc++-v3/include/debug/assertions.h
@@ -31,6 +31,12 @@
 
 #include <bits/c++config.h>
 
+#ifdef _GLIBCXX_DEBUG
+# define _GLIBCXX_DEBUG_STD_C ::std::_GLIBCXX_STD_C
+#else
+# define _GLIBCXX_DEBUG_STD_C ::std
+#endif
+
 #ifndef _GLIBCXX_DEBUG
 // Verify that [_First, _Last) forms a non-empty iterator range.
 # define __glibcxx_requires_non_empty_range(_First,_Last)	\
diff --git a/libstdc++-v3/include/debug/vector b/libstdc++-v3/include/debug/vector
index 56645d1e92c..ad79ec00919 100644
--- a/libstdc++-v3/include/debug/vector
+++ b/libstdc++-v3/include/debug/vector
@@ -51,11 +51,12 @@ namespace __gnu_debug
    * detecting code which relies on non-portable implementation details of
    * the libstdc++ reallocation policy.
    */
-  template<typename _SafeSequence,
-	   typename _BaseSequence>
+  template<class _Tp, typename _Allocator>
     class _Safe_vector
     {
-      typedef typename _BaseSequence::size_type size_type;
+      typedef std::__debug::vector<_Tp, _Allocator> _SafeSequence;
+      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
+      typedef typename _StdVector::size_type size_type;
 
       _GLIBCXX20_CONSTEXPR
       const _SafeSequence&
@@ -103,10 +104,6 @@ namespace __gnu_debug
 
       size_type _M_guaranteed_capacity;
 
-      bool
-      _M_requires_reallocation(size_type __elements) const _GLIBCXX_NOEXCEPT
-      { return __elements > _M_seq().capacity(); }
-
       _GLIBCXX20_CONSTEXPR
       void
       _M_update_guaranteed_capacity() _GLIBCXX_NOEXCEPT
@@ -115,6 +112,81 @@ namespace __gnu_debug
 	  _M_guaranteed_capacity = _M_seq().size();
       }
     };
+
+  /** @brief Guard class for iterators of vector invalidation.
+   *
+   * This guard invalidates on destroy, if activated, all iterators if
+   * reallocation took place and all iterators after updated size otherwise.
+   */
+  template<class _Tp, typename _Allocator>
+    class _SafeVectorGuard
+    {
+      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
+      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
+      typedef typename _StdVector::size_type size_type;
+
+      _SafeVector& _M_vect;
+      size_type _M_capacity, _M_size;
+
+    public:
+      bool _M_active;
+
+      _GLIBCXX20_CONSTEXPR
+      _SafeVectorGuard(_SafeVector& __vector);
+
+      _GLIBCXX20_CONSTEXPR
+      ~_SafeVectorGuard();
+    };
+
+#ifdef _GLIBCXX_DEBUG_PEDANTIC
+  /** @brief Guard class for iterators of vector invalidation.
+   *
+   * This guard invalidates on destroy, if activated, all iterators
+   * no matter what happenned to the vector.
+   */
+  template<class _Tp, typename _Allocator>
+    class _SafeVectorPedanticGuard
+    {
+      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
+      _SafeVector& _M_vect;
+
+    public:
+      bool _M_active;
+
+      _GLIBCXX20_CONSTEXPR
+      _SafeVectorPedanticGuard(_SafeVector& __vector);
+
+      _GLIBCXX20_CONSTEXPR
+      ~_SafeVectorPedanticGuard();
+    };
+#endif
+
+  /** @brief Guard class for iterators of vector invalidation.
+   *
+   * This guard invalidates on destroy, if activated, all iterators if
+   * reallocation took place and all iterators after an offset otherwise.
+   */
+  template<class _Tp, typename _Allocator>
+    class _SafeVectorInsertGuard
+    {
+      typedef std::__debug::vector<_Tp, _Allocator> _SafeVector;
+      typedef _GLIBCXX_DEBUG_STD_C::vector<_Tp, _Allocator> _StdVector;
+      typedef typename _StdVector::size_type size_type;
+      typedef typename _StdVector::difference_type difference_type;
+
+      _SafeVector& _M_vect;
+      size_type _M_capacity;
+      difference_type _M_offset;
+
+    public:
+      bool _M_active;
+
+      _GLIBCXX20_CONSTEXPR
+      _SafeVectorInsertGuard(_SafeVector& __vector, difference_type __offset);
+
+      _GLIBCXX20_CONSTEXPR
+      ~_SafeVectorInsertGuard();
+    };
 }
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -128,14 +200,33 @@ namespace __debug
     : public __gnu_debug::_Safe_container<
 	vector<_Tp, _Allocator>, _Allocator, __gnu_debug::_Safe_sequence>,
       public _GLIBCXX_STD_C::vector<_Tp, _Allocator>,
-      public __gnu_debug::_Safe_vector<
-	vector<_Tp, _Allocator>,
-	_GLIBCXX_STD_C::vector<_Tp, _Allocator> >
+      public __gnu_debug::_Safe_vector<_Tp, _Allocator>
     {
       typedef _GLIBCXX_STD_C::vector<_Tp, _Allocator>		_Base;
       typedef __gnu_debug::_Safe_container<
 	vector, _Allocator, __gnu_debug::_Safe_sequence>	_Safe;
-      typedef __gnu_debug::_Safe_vector<vector, _Base>		_Safe_vector;
+      typedef __gnu_debug::_Safe_vector<_Tp, _Allocator>	_Safe_vector;
+
+      typedef __gnu_debug::_SafeVectorGuard<_Tp, _Allocator> _ResizeGuard;
+#ifdef _GLIBCXX_DEBUG_PEDANTIC
+      typedef __gnu_debug::_SafeVectorPedanticGuard<_Tp, _Allocator>
+	_AssignGuard;
+#else
+      typedef _ResizeGuard _AssignGuard;
+#endif
+      typedef __gnu_debug::_SafeVectorInsertGuard<_Tp, _Allocator>
+	_InsertGuard;
+
+      template<typename _Tp2, typename _Alloc2>
+	friend class ::__gnu_debug::_SafeVectorGuard;
+
+#ifdef _GLIBCXX_DEBUG_PEDANTIC
+      template<typename _Tp2, typename _Alloc2>
+	friend class ::__gnu_debug::_SafeVectorPedanticGuard;
+#endif
+
+      template<typename _Tp2, typename _Alloc2>
+	friend class ::__gnu_debug::_SafeVectorInsertGuard;
 
       typedef typename _Base::iterator		_Base_iterator;
       typedef typename _Base::const_iterator	_Base_const_iterator;
@@ -276,12 +367,9 @@ namespace __debug
       vector&
       operator=(initializer_list<value_type> __l)
       {
+	_AssignGuard __guard(*this);
 	_Base::operator=(__l);
-	if (!std::__is_constant_evaluated())
-	  {
-	    this->_M_invalidate_all();
-	    this->_M_update_guaranteed_capacity();
-	  }
+	__guard._M_active = true;
 	return *this;
       }
 #endif
@@ -303,26 +391,23 @@ namespace __debug
 	  typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
 	  __glibcxx_check_valid_range2(__first, __last, __dist);
 
+	  _AssignGuard __guard(*this);
 	  if (__dist.second >= __gnu_debug::__dp_sign)
 	    _Base::assign(__gnu_debug::__unsafe(__first),
 			  __gnu_debug::__unsafe(__last));
 	  else
 	    _Base::assign(__first, __last);
 
-	  this->_M_invalidate_all();
-	  this->_M_update_guaranteed_capacity();
+	  __guard._M_active = true;
 	}
 
       _GLIBCXX20_CONSTEXPR
       void
       assign(size_type __n, const _Tp& __u)
       {
+	_AssignGuard __guard(*this);
 	_Base::assign(__n, __u);
-	if (!std::__is_constant_evaluated())
-	  {
-	    this->_M_invalidate_all();
-	    this->_M_update_guaranteed_capacity();
-	  }
+	__guard._M_active = true;
       }
 
 #if __cplusplus >= 201103L
@@ -330,12 +415,9 @@ namespace __debug
       void
       assign(initializer_list<value_type> __l)
       {
+	_AssignGuard __guard(*this);
 	_Base::assign(__l);
-	if (!std::__is_constant_evaluated())
-	  {
-	    this->_M_invalidate_all();
-	    this->_M_update_guaranteed_capacity();
-	  }
+	__guard._M_active = true;
       }
 #endif
 
@@ -428,13 +510,9 @@ namespace __debug
 	if (std::__is_constant_evaluated())
 	  return _Base::resize(__sz);
 
-	bool __realloc = this->_M_requires_reallocation(__sz);
-	if (__sz < this->size())
-	  this->_M_invalidate_after_nth(__sz);
+	_ResizeGuard __guard(*this);
 	_Base::resize(__sz);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	this->_M_update_guaranteed_capacity();
+	__guard._M_active = true;
       }
 
       _GLIBCXX20_CONSTEXPR
@@ -444,25 +522,17 @@ namespace __debug
 	if (std::__is_constant_evaluated())
 	  return _Base::resize(__sz, __c);
 
-	bool __realloc = this->_M_requires_reallocation(__sz);
-	if (__sz < this->size())
-	  this->_M_invalidate_after_nth(__sz);
+	_ResizeGuard __guard(*this);
 	_Base::resize(__sz, __c);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	this->_M_update_guaranteed_capacity();
+	__guard._M_active = true;
       }
 #else
       void
       resize(size_type __sz, _Tp __c = _Tp())
       {
-	bool __realloc = this->_M_requires_reallocation(__sz);
-	if (__sz < this->size())
-	  this->_M_invalidate_after_nth(__sz);
+	_ResizeGuard __guard(*this);
 	_Base::resize(__sz, __c);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	this->_M_update_guaranteed_capacity();
+	__guard._M_active = true;
       }
 #endif
 
@@ -506,7 +576,7 @@ namespace __debug
 	if (std::__is_constant_evaluated())
 	  return _Base::reserve(__n);
 
-	bool __realloc = this->_M_requires_reallocation(__n);
+	bool __realloc = __n > capacity();
 	_Base::reserve(__n);
 	if (__n > this->_M_guaranteed_capacity)
 	  this->_M_guaranteed_capacity = __n;
@@ -583,11 +653,9 @@ namespace __debug
 	if (std::__is_constant_evaluated())
 	  return _Base::push_back(__x);
 
-	bool __realloc = this->_M_requires_reallocation(this->size() + 1);
+	_ResizeGuard __guard(*this);
 	_Base::push_back(__x);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	this->_M_update_guaranteed_capacity();
+	__guard._M_active = true;
       }
 
 #if __cplusplus >= 201103L
@@ -610,11 +678,9 @@ namespace __debug
 	  if (std::__is_constant_evaluated())
 	    return _Base::emplace_back(std::forward<_Args>(__args)...);
 
-	  bool __realloc = this->_M_requires_reallocation(this->size() + 1);
+	  _ResizeGuard __guard(*this);
 	  _Base::emplace_back(std::forward<_Args>(__args)...);
-	  if (__realloc)
-	    this->_M_invalidate_all();
-	  this->_M_update_guaranteed_capacity();
+	  __guard._M_active = true;
 #if __cplusplus > 201402L
 	  return back();
 #endif
@@ -645,15 +711,14 @@ namespace __debug
 			    this);
 
 	  __glibcxx_check_insert(__position);
-	  bool __realloc = this->_M_requires_reallocation(this->size() + 1);
-	  difference_type __offset = __position.base() - _Base::cbegin();
-	  _Base_iterator __res = _Base::emplace(__position.base(),
-						std::forward<_Args>(__args)...);
-	  if (__realloc)
-	    this->_M_invalidate_all();
-	  else
-	    this->_M_invalidate_after_nth(__offset);
-	  this->_M_update_guaranteed_capacity();
+	  _Base_iterator __res;
+	  {
+	    difference_type __offset = __position.base() - _Base::cbegin();
+	    _InsertGuard __guard(*this, __offset);
+	    __res = _Base::emplace(__position.base(),
+				   std::forward<_Args>(__args)...);
+	    __guard._M_active = true;
+	  }
 	  return { __res, this };
 	}
 #endif
@@ -670,14 +735,13 @@ namespace __debug
 	  return iterator(_Base::insert(__position.base(), __x), this);
 
 	__glibcxx_check_insert(__position);
-	bool __realloc = this->_M_requires_reallocation(this->size() + 1);
-	difference_type __offset = __position.base() - _Base::begin();
-	_Base_iterator __res = _Base::insert(__position.base(), __x);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	else
-	  this->_M_invalidate_after_nth(__offset);
-	this->_M_update_guaranteed_capacity();
+	_Base_iterator __res;
+	{
+	  difference_type __offset = __position.base() - _Base::begin();
+	  _InsertGuard __guard(*this, __offset);
+	  __res = _Base::insert(__position.base(), __x);
+	  __guard._M_active = true;
+	}
 	return iterator(__res, this);
       }
 
@@ -704,14 +768,13 @@ namespace __debug
 	  return iterator(_Base::insert(__position.base(), __n, __x), this);
 
 	__glibcxx_check_insert(__position);
-	bool __realloc = this->_M_requires_reallocation(this->size() + __n);
-	difference_type __offset = __position.base() - _Base::cbegin();
-	_Base_iterator __res = _Base::insert(__position.base(), __n, __x);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	else
-	  this->_M_invalidate_after_nth(__offset);
-	this->_M_update_guaranteed_capacity();
+	_Base_iterator __res;
+	{
+	  difference_type __offset = __position.base() - _Base::cbegin();
+	  _InsertGuard __guard(*this, __offset);
+	  __res = _Base::insert(__position.base(), __n, __x);
+	  __guard._M_active = true;
+	}
 	return { __res, this };
       }
 #else
@@ -719,14 +782,10 @@ namespace __debug
       insert(iterator __position, size_type __n, const _Tp& __x)
       {
 	__glibcxx_check_insert(__position);
-	bool __realloc = this->_M_requires_reallocation(this->size() + __n);
 	difference_type __offset = __position.base() - _Base::begin();
+	_InsertGuard __guard(*this, __offset);
 	_Base::insert(__position.base(), __n, __x);
-	if (__realloc)
-	  this->_M_invalidate_all();
-	else
-	  this->_M_invalidate_after_nth(__offset);
-	this->_M_update_guaranteed_capacity();
+	__guard._M_active = true;
       }
 #endif
 
@@ -746,24 +805,19 @@ namespace __debug
 	  typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
 	  __glibcxx_check_insert_range(__position, __first, __last, __dist);
 
-	  /* Hard to guess if invalidation will occur, because __last
-	     - __first can't be calculated in all cases, so we just
-	     punt here by checking if it did occur. */
-	  _Base_iterator __old_begin = _M_base().begin();
-	  difference_type __offset = __position.base() - _Base::cbegin();
 	  _Base_iterator __res;
-	  if (__dist.second >= __gnu_debug::__dp_sign)
-	    __res = _Base::insert(__position.base(),
-				  __gnu_debug::__unsafe(__first),
-				  __gnu_debug::__unsafe(__last));
-	  else
-	    __res = _Base::insert(__position.base(), __first, __last);
-
-	  if (_M_base().begin() != __old_begin)
-	    this->_M_invalidate_all();
-	  else
-	    this->_M_invalidate_after_nth(__offset);
-	  this->_M_update_guaranteed_capacity();
+	  {
+	    difference_type __offset = __position.base() - _Base::cbegin();
+	    _InsertGuard __guard(*this, __offset);
+	    if (__dist.second >= __gnu_debug::__dp_sign)
+	      __res = _Base::insert(__position.base(),
+				    __gnu_debug::__unsafe(__first),
+				    __gnu_debug::__unsafe(__last));
+	    else
+	      __res = _Base::insert(__position.base(), __first, __last);
+
+	    __guard._M_active = true;
+	  }
 	  return { __res, this };
 	}
 #else
@@ -775,22 +829,15 @@ namespace __debug
 	  typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
 	  __glibcxx_check_insert_range(__position, __first, __last, __dist);
 
-	  /* Hard to guess if invalidation will occur, because __last
-	     - __first can't be calculated in all cases, so we just
-	     punt here by checking if it did occur. */
-	  _Base_iterator __old_begin = _M_base().begin();
 	  difference_type __offset = __position.base() - _Base::begin();
+	  _InsertGuard __guard(*this, __offset);
 	  if (__dist.second >= __gnu_debug::__dp_sign)
 	    _Base::insert(__position.base(), __gnu_debug::__unsafe(__first),
 					     __gnu_debug::__unsafe(__last));
 	  else
 	    _Base::insert(__position.base(), __first, __last);
 
-	  if (_M_base().begin() != __old_begin)
-	    this->_M_invalidate_all();
-	  else
-	    this->_M_invalidate_after_nth(__offset);
-	  this->_M_update_guaranteed_capacity();
+	  __guard._M_active = true;
 	}
 #endif
 
@@ -876,31 +923,22 @@ namespace __debug
 	constexpr void
 	assign_range(_Rg&& __rg)
 	{
-	  auto __old_capacity = _Base::capacity();
-	  auto __old_size = _Base::size();
+	  _AssignGuard __guard(*this);
 	  _Base::assign_range(__rg);
-	  if (!std::__is_constant_evaluated())
-	    {
-	      if (_Base::capacity() != __old_capacity)
-		this->_M_invalidate_all();
-	      else if (_Base::size() < __old_size)
-		this->_M_invalidate_after_nth(_Base::size());
-	      this->_M_update_guaranteed_capacity();
-	    }
+	  __guard._M_active = true;
 	}
 
       template<__detail::__container_compatible_range<_Tp> _Rg>
 	constexpr iterator
 	insert_range(const_iterator __pos, _Rg&& __rg)
 	{
-	  auto __old_capacity = _Base::capacity();
-	  auto __res = _Base::insert_range(__pos.base(), __rg);
-	  if (!std::__is_constant_evaluated())
-	    {
-	      if (_Base::capacity() != __old_capacity)
-		this->_M_invalidate_all();
-	      this->_M_update_guaranteed_capacity();
-	    }
+	  _Base_iterator __res;
+	  {
+	    difference_type __offset = __pos.base() - _Base::cbegin();
+	    _InsertGuard __guard(*this, __offset);
+	    __res = _Base::insert_range(__pos.base(), __rg);
+	    __guard._M_active = true;
+	  }
 	  return iterator(__res, this);
 	}
 
@@ -908,14 +946,9 @@ namespace __debug
 	constexpr void
 	append_range(_Rg&& __rg)
 	{
-	  auto __old_capacity = _Base::capacity();
+	  _ResizeGuard __guard(*this);
 	  _Base::append_range(__rg);
-	  if (!std::__is_constant_evaluated())
-	    {
-	      if (_Base::capacity() != __old_capacity)
-		this->_M_invalidate_all();
-	      this->_M_update_guaranteed_capacity();
-	    }
+	  __guard._M_active = true;
 	}
 #endif
 
@@ -1057,6 +1090,76 @@ _GLIBCXX_END_NAMESPACE_VERSION
 
 namespace __gnu_debug
 {
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorGuard<_Tp, _Allocator>::_SafeVectorGuard(
+      std::__debug::vector<_Tp, _Allocator>& __vector)
+    : _M_vect(__vector)
+    , _M_capacity(__vector.capacity())
+    , _M_size(__vector.size())
+    , _M_active(false)
+    { }
+
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorGuard<_Tp, _Allocator>::~_SafeVectorGuard()
+    {
+      if (!std::__is_constant_evaluated() && _M_active)
+	{
+	  _M_vect._M_update_guaranteed_capacity();
+
+	  if (_M_vect.capacity() != _M_capacity)
+	    _M_vect._M_invalidate_all();
+	  else if (_M_vect.size() < _M_size)
+	    _M_vect._M_invalidate_after_nth(_M_vect.size());
+	}
+    }
+
+#ifdef _GLIBCXX_DEBUG_PEDANTIC
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorPedanticGuard<_Tp, _Allocator>::_SafeVectorPedanticGuard(
+      std::__debug::vector<_Tp, _Allocator>& __vector)
+    : _M_vect(__vector), _M_active(false)
+    { }
+
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorPedanticGuard<_Tp, _Allocator>::~_SafeVectorPedanticGuard()
+    {
+      if (!std::__is_constant_evaluated() && _M_active)
+	{
+	  _M_vect._M_update_guaranteed_capacity();
+	  _M_vect._M_invalidate_all();
+	}
+    }
+#endif
+
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorInsertGuard<_Tp, _Allocator>::_SafeVectorInsertGuard(
+      std::__debug::vector<_Tp, _Allocator>& __vector, difference_type __offset)
+    : _M_vect(__vector)
+    , _M_capacity(__vector.capacity())
+    , _M_offset(__offset)
+    , _M_active(false)
+    { }
+
+  template<class _Tp, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
+    _SafeVectorInsertGuard<_Tp, _Allocator>::~_SafeVectorInsertGuard()
+    {
+      if (!std::__is_constant_evaluated() && _M_active)
+	{
+	  _M_vect._M_update_guaranteed_capacity();
+
+	  if (_M_vect.capacity() != _M_capacity)
+	    _M_vect._M_invalidate_all();
+	  else
+	    _M_vect._M_invalidate_after_nth(_M_offset);
+	}
+    }
+
   template<typename _Tp, typename _Alloc>
     struct _Is_contiguous_sequence<std::__debug::vector<_Tp, _Alloc> >
     : std::__true_type
diff --git a/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc b/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc
index 2c9abcdbb91..ccd5dc38e33 100644
--- a/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/cons/destructible_debug_neg.cc
@@ -47,4 +47,4 @@ test02()
 // { dg-error "PrivateDtor.* is private" "" { target *-*-* } 0 }
 
 // In Debug Mode the "required from here" errors come from <debug/vector>
-// { dg-error "required from here" "" { target *-*-* } 182 }
+// { dg-error "required from here" "" { target *-*-* } 273 }
diff --git a/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc
new file mode 100644
index 00000000000..318f6b98de9
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list.cc
@@ -0,0 +1,26 @@
+// { dg-do compile { target c++11 } }
+
+#include <debug/vector>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  __gnu_debug::vector<int> v = { 0, 1, 2, 3, 4, 5 };
+
+  auto it1 = v.begin() + 1;
+  auto it4 = v.begin() + 4;
+
+  VERIFY( !it1._M_singular() );
+  VERIFY( !it4._M_singular() );
+
+  v = { 10, 11, 12, 13 };
+
+  VERIFY( !it1._M_singular() );
+  VERIFY( it4._M_singular() );
+}
+
+int main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc
new file mode 100644
index 00000000000..c8b3af25bd1
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/debug/initializer_list_pedantic.cc
@@ -0,0 +1,27 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-D_GLIBCXX_DEBUG_PEDANTIC" }
+
+#include <debug/vector>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  __gnu_debug::vector<int> v = { 0, 1, 2, 3, 4, 5 };
+
+  auto it1 = v.begin() + 1;
+  auto it4 = v.begin() + 4;
+
+  VERIFY( !it1._M_singular() );
+  VERIFY( !it4._M_singular() );
+
+  v = { 10, 11, 12, 13 };
+
+  VERIFY( it1._M_singular() );
+  VERIFY( it4._M_singular() );
+}
+
+int main()
+{
+  test01();
+}


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