Re: _LIBCXX_DEBUG value initialized singular iterators assert failures in std algorithms [PR104316]

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

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I was a little bit too confident below. After review of all _M_singular 
usages I found another necessary fix.

After this one for sure we will be able to define 
__cpp_lib_null_iterators even in Debug mode.

     libstdc++: Fix N3344 behavior on _Safe_iterator::_M_can_advance

     We shall be able to advance from a 0 offset a value-initialized 
iterator.

     libstdc++-v3/ChangeLog:

             * include/debug/safe_iterator.tcc 
(_Safe_iterator<>::_M_can_advance):
             Accept 0 offset advance on value-initialized iterator.
             * testsuite/23_containers/vector/debug/n3644.cc: New test case.

Ok to commit ?

François


On 17/03/2024 17:52, François Dumont wrote:
>
>>
>> OK for trunk, thanks!
>>
>> I think this is OK to backport to 13 too.
>>
>> Maybe after this we can define the __cpp_lib_null_itetators macro for 
>> debug mode?
>>
> After this fix of local_iterator I think we can indeed.
>
> In fact the added 11316.cc was already passing for 
> unordered_set<>::local_iterator but simply because we were missing the 
> singular check. Both issues solved with this patch.
>
> I found the version.def file to cleanup but no idea how to regenerate 
> version.h from it so I'll let you do it, ok ?
>
>     libstdc++: Fix _Safe_local_iterator<>::_M_valid_range
>
>     Unordered container local_iterator range shall not contain any 
> singular
>     iterator unless both iterators are value-initialized.
>
>     libstdc++-v3/ChangeLog:
>
>             * include/debug/safe_local_iterator.tcc
>             (_Safe_local_iterator::_M_valid_range): Add 
> _M_value_initialized and
>             _M_singular checks.
>             * testsuite/23_containers/unordered_set/debug/114316.cc: 
> New test case.
>
>
> Ok to commit ?
>
> François

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diff --git a/libstdc++-v3/include/debug/safe_iterator.tcc b/libstdc++-v3/include/debug/safe_iterator.tcc
index 4b2baf2980e..deaa84d0a1f 100644
--- a/libstdc++-v3/include/debug/safe_iterator.tcc
+++ b/libstdc++-v3/include/debug/safe_iterator.tcc
@@ -86,6 +86,9 @@ namespace __gnu_debug
     _Safe_iterator<_Iterator, _Sequence, _Category>::
     _M_can_advance(difference_type __n, bool __strict) const
     {
+      if (this->_M_value_initialized() && __n == 0)
+	return true;
+
       if (this->_M_singular())
 	return false;
 
diff --git a/libstdc++-v3/testsuite/23_containers/vector/debug/n3644.cc b/libstdc++-v3/testsuite/23_containers/vector/debug/n3644.cc
new file mode 100644
index 00000000000..052c52f26b7
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/debug/n3644.cc
@@ -0,0 +1,16 @@
+// { dg-do run { target c++11 } }
+// { dg-require-debug-mode "" }
+
+#include <vector>
+#include <algorithm>
+
+#include <testsuite_hooks.h>
+
+int main()
+{
+  std::vector<int>::iterator it{};
+  auto cpy = it;
+  std::advance(it, 0);
+  VERIFY( it == cpy );
+  return 0;
+}