Re: [PATCH][_GLIBCXX_DEBUG] Fix std::__niter_base behavior

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Mon, 19 Feb 2024, 07:08 Stephan Bergmann, <sberg.fun@gmail.com> wrote:

> On 2/17/24 15:14, François Dumont wrote:
> > Thanks for the link, tested and committed.
>
> I assume this is the cause for the below failure now,
>

Yes, the new >= C++11 overload of __niter_base recursively unwraps multiple
layers of wrapping, so that a safe iterator wrapping a normal iterator
wrapping a pointer is unwrapped to just a pointer. But then __niter_wrap
doesn't restore both layers.

I think the change might need to be reverted.




> > $ cat test.cc
> > #include <algorithm>
> > #include <vector>
> > void f(std::vector<void const *> &v, void const * p) {
> >     std::erase(v, p);
> > }
>
> > $ ~/gcc/inst/bin/g++ -std=c++20 -D_GLIBCXX_DEBUG -fsyntax-only test.cc
> > In file included from ~/gcc/inst/include/c++/14.0.1/algorithm:60,
> >                  from test.cc:1:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h: In instantiation of
> ‘constexpr _From std::__niter_wrap(_From, _To) [with _From =
> __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<const void**,
> __cxx1998::vector<const void*, allocator<const void*> > >,
> __debug::vector<const void*>, random_access_iterator_tag>; _To =
> __gnu_cxx::__normal_iterator<const void**, __cxx1998::vector<const void*,
> allocator<const void*> > >]’:
> > ~/gcc/inst/include/c++/14.0.1/vector:144:29:   required from ‘constexpr
> typename std::__debug::vector<_Tp, _Allocator>::size_type
> std::erase(__debug::vector<_Tp, _Alloc>&, const _Up&) [with _Tp = const
> void*; _Alloc = allocator<const void*>; _Up = const void*; typename
> __debug::vector<_Tp, _Allocator>::size_type = long unsigned int]’
> >   144 |           __cont.erase(__niter_wrap(__cont.begin(), __removed),
> >       |                        ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
> > test.cc:4:15:   required from here
> >     4 |     std::erase(v, p);
> >       |     ~~~~~~~~~~^~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:30: error: no
> match for ‘operator-’ (operand types are
> ‘__gnu_cxx::__normal_iterator<const void**, std::__cxx1998::vector<const
> void*, std::allocator<const void*> > >’ and ‘const void**’)
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > In file included from
> ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:67:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1148:7: note:
> candidate: ‘constexpr __gnu_cxx::__normal_iterator<_Iterator, _Container>
> __gnu_cxx::__normal_iterator<_Iterator,
> _Container>::operator-(difference_type) const [with _Iterator = const
> void**; _Container = std::__cxx1998::vector<const void*,
> std::allocator<const void*> >; difference_type = long int]’ (near match)
> >  1148 |       operator-(difference_type __n) const _GLIBCXX_NOEXCEPT
> >       |       ^~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1148:7: note:
>  conversion of argument 1 would be ill-formed:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:49: error: invalid
> conversion from ‘const void**’ to ‘__gnu_cxx::__normal_iterator<const
> void**, std::__cxx1998::vector<const void*, std::allocator<const void*> >
> >::difference_type’ {aka ‘long int’} [-fpermissive]
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                                ~~~~~~~~~~~~~~~~~^~~~~~~~
> >       |                                                 |
> >       |                                                 const void**
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:618:5: note:
> candidate: ‘template<class _IteratorL, class _IteratorR> constexpr decltype
> ((__y.base() - __x.base())) std::operator-(const
> reverse_iterator<_IteratorL>&, const reverse_iterator<_IteratorR>&)’
> >   618 |     operator-(const reverse_iterator<_IteratorL>& __x,
> >       |     ^~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:618:5: note:
>  template argument deduction/substitution failed:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:30: note:
>  ‘__gnu_cxx::__normal_iterator<const void**, std::__cxx1998::vector<const
> void*, std::allocator<const void*> > >’ is not derived from ‘const
> std::reverse_iterator<_IteratorL>’
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1789:5: note:
> candidate: ‘template<class _IteratorL, class _IteratorR> constexpr decltype
> ((__x.base() - __y.base())) std::operator-(const
> move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)’
> >  1789 |     operator-(const move_iterator<_IteratorL>& __x,
> >       |     ^~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1789:5: note:
>  template argument deduction/substitution failed:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:30: note:
>  ‘__gnu_cxx::__normal_iterator<const void**, std::__cxx1998::vector<const
> void*, std::allocator<const void*> > >’ is not derived from ‘const
> std::move_iterator<_IteratorL>’
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1312:5: note:
> candidate: ‘template<class _IteratorL, class _IteratorR, class _Container>
> constexpr decltype ((__lhs.base() - __rhs.base()))
> __gnu_cxx::operator-(const __normal_iterator<_IteratorL, _Container>&,
> const __normal_iterator<_IteratorR, _Container>&)’
> >  1312 |     operator-(const __normal_iterator<_IteratorL, _Container>&
> __lhs,
> >       |     ^~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1312:5: note:
>  template argument deduction/substitution failed:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:30: note:
>  mismatched types ‘const __gnu_cxx::__normal_iterator<_IteratorR,
> _Container>’ and ‘const void**’
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1325:5: note:
> candidate: ‘template<class _Iterator, class _Container> constexpr typename
> __gnu_cxx::__normal_iterator<_Iterator, _Container>::difference_type
> __gnu_cxx::operator-(const __normal_iterator<_Iterator, _Container>&, const
> __normal_iterator<_Iterator, _Container>&)’
> >  1325 |     operator-(const __normal_iterator<_Iterator, _Container>&
> __lhs,
> >       |     ^~~~~~~~
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_iterator.h:1325:5: note:
>  template argument deduction/substitution failed:
> > ~/gcc/inst/include/c++/14.0.1/bits/stl_algobase.h:347:30: note:
>  mismatched types ‘const __gnu_cxx::__normal_iterator<_Iterator,
> _Container>’ and ‘const void**’
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |                       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>