libstdc++: [_GLIBCXX_DEBUG] Fix std::__niter_wrap behavior

     In _GLIBCXX_DEBUG mode the std::__niter_base can remove 2 layers, the
     __gnu_debug::_Safe_iterator<> and the __gnu_cxx::__normal_iterator<>.
     When std::__niter_wrap is called to build a 
__gnu_debug::_Safe_iterator<>
     from a __gnu_cxx::__normal_iterator<> we then have a consistency issue
     as the difference between the 2 iterators will done on a 
__normal_iterator
     on one side and a C pointer on the other. To avoid this problem call
     std::__niter_base on both input iterators.

     libstdc++-v3/ChangeLog:

             * include/bits/stl_algobase.h (std::__niter_wrap): Add a 
call to
             std::__niter_base on res iterator.

Tested under Linux x86_64 normal and _GLIBCXX_DEBUG modes in c++98, 
c++11, c++17.

Ok to commit ?

François


On 19/02/2024 09:21, Jonathan Wakely wrote:
>
>
> On Mon, 19 Feb 2024, 08:12 Jonathan Wakely, <jwakely.gcc@gmail.com> wrote:
>
>
>
>     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.
>
>
>
> Actually that's not the problem. __niter_wrap would restore both 
> layers, except that it uses __niter_base itself:
>
> >   347 |     { return __from + (__res - std::__niter_base(__from)); }
> >       |  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> And it seems to be getting called with the wrong types. Maybe that's 
> just a bug in std:: erase or maybe niter_wrap needs adjusting.
>
> I'll check in a couple of hours if François doesn't get to it first.
>
> I have to wonder how this wasn't caught by existing tests though.
>
>
>