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

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

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On 14/02/2024 20:44, Jonathan Wakely wrote:
>
>
> On Wed, 14 Feb 2024 at 18:39, François Dumont <frs.dumont@gmail.com> 
> wrote:
>
>     libstdc++: [_GLIBCXX_DEBUG] Fix std::__niter_base behavior
>
>     std::__niter_base is used in _GLIBCXX_DEBUG mode to remove
>     _Safe_iterator<>
>     wrapper on random access iterators. But doing so it should also
>     preserve
>     original
>     behavior to remove __normal_iterator wrapper.
>
>     libstdc++-v3/ChangeLog:
>
>          * include/bits/stl_algobase.h (std::__niter_base): Redefine the
>     overload
>          definitions for __gnu_debug::_Safe_iterator.
>          * include/debug/safe_iterator.tcc (std::__niter_base): Adapt
>     declarations.
>
>     Ok to commit once all tests completed (still need to check
>     pre-c++11) ?
>
>
>
> The declaration in  include/bits/stl_algobase.h has a 
> noexcept-specifier but the definition in 
> include/debug/safe_iterator.tcc does not have one - that seems wrong 
> (I'm surprised it even compiles).

It does ! I thought it was only necessary at declaration, and I also had 
troubles doing it right at definition because of the interaction with 
the auto and ->. Now simplified and consistent in this new proposal.


> Just using std::is_nothrow_copy_constructible<_Ite> seems simpler, 
> that will be true for __normal_iterator<I, C> if 
> is_nothrow_copy_constructible<I> is true.
>
Ok


> The definition in include/debug/safe_iterator.tcc should use 
> std::declval<_Ite>() not declval<_Ite>(). Is there any reason why the 
> definition uses a late-specified-return-type (i.e. auto and ->) when 
> the declaration doesn't?
>
>
I initially plan to use '-> 
std::decltype(std::__niter_base(__it.base()))' but this did not compile, 
ambiguity issue. So I resort to using std::declval and I could have then 
done it the same way as declaration, done now.

Attached is what I'm testing, ok to commit once fully tested ?

François


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diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index e7207f67266..0f73da13172 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -317,12 +317,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<_Iterator>::value)
     { return __it; }
 
+#if __cplusplus < 201103L
   template<typename _Ite, typename _Seq>
-    _GLIBCXX20_CONSTEXPR
     _Ite
     __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
 		 std::random_access_iterator_tag>&);
 
+ template<typename _Ite, typename _Cont, typename _Seq>
+    _Ite
+    __niter_base(const ::__gnu_debug::_Safe_iterator<
+		 ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _Seq,
+		 std::random_access_iterator_tag>&);
+#else
+  template<typename _Ite, typename _Seq>
+    _GLIBCXX20_CONSTEXPR
+    decltype(std::__niter_base(std::declval<_Ite>()))
+    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
+		 std::random_access_iterator_tag>&)
+    noexcept(std::is_nothrow_copy_constructible<_Ite>::value);
+#endif
+
   // Reverse the __niter_base transformation to get a
   // __normal_iterator back again (this assumes that __normal_iterator
   // is only used to wrap random access iterators, like pointers).
diff --git a/libstdc++-v3/include/debug/safe_iterator.tcc b/libstdc++-v3/include/debug/safe_iterator.tcc
index 6eb70cbda04..a8b24233e85 100644
--- a/libstdc++-v3/include/debug/safe_iterator.tcc
+++ b/libstdc++-v3/include/debug/safe_iterator.tcc
@@ -235,13 +235,29 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
+#if __cplusplus < 201103L
   template<typename _Ite, typename _Seq>
-    _GLIBCXX20_CONSTEXPR
     _Ite
     __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
 		 std::random_access_iterator_tag>& __it)
     { return __it.base(); }
 
+  template<typename _Ite, typename _Cont, typename _DbgSeq>
+    _Ite
+    __niter_base(const ::__gnu_debug::_Safe_iterator<
+		 ::__gnu_cxx::__normal_iterator<_Ite, _Cont>, _DbgSeq,
+		 std::random_access_iterator_tag>& __it)
+    { return __it.base().base(); }
+#else
+  template<typename _Ite, typename _Seq>
+    _GLIBCXX20_CONSTEXPR
+    decltype(std::__niter_base(std::declval<_Ite>()))
+    __niter_base(const ::__gnu_debug::_Safe_iterator<_Ite, _Seq,
+		 std::random_access_iterator_tag>& __it)
+    noexcept(std::is_nothrow_copy_constructible<_Ite>::value)
+    { return std::__niter_base(__it.base()); }
+#endif
+
   template<bool _IsMove,
 	   typename _Ite, typename _Seq, typename _Cat, typename _OI>
     _GLIBCXX20_CONSTEXPR