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

["François Dumont" <frs.dumont@gmail.com>]

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Turns out that 23_containers/vector/erasure.cc was showing the problem 
in _GLIBCXX_DEBUG mode.

I had only run 25_algorithms tests in _GLIBCXX_DEBUG mode.

This is what I'm testing, I 'll let you know tomorrow morning if all 
successful.

Of course feel free to do or ask for a revert instead.

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.
>
>
>

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diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 0f73da13172..d534e02871f 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -344,7 +344,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX20_CONSTEXPR
     inline _From
     __niter_wrap(_From __from, _To __res)
-    { return __from + (__res - std::__niter_base(__from)); }
+    { return __from + (std::__niter_base(__res) - std::__niter_base(__from)); }
 
   // No need to wrap, iterator already has the right type.
   template<typename _Iterator>