[gcc r13-8471] libstdc++: Fix N3344 behavior on _Safe_iterator::_M_can_advance

[Francois Dumont <fdumont@gcc.gnu.org>]

https://gcc.gnu.org/g:51e2f7a22e82a7cb2d321b82613b477b58ee4c60

commit r13-8471-g51e2f7a22e82a7cb2d321b82613b477b58ee4c60
Author: François Dumont <fdumont@gcc.gnu.org>
Date:   Sun Mar 17 19:06:55 2024 +0100

    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.
    
    (cherry picked from commit dda96a9d942d73a587e174dd5efe061208a195af)

Diff:
---
 libstdc++-v3/include/debug/safe_iterator.tcc             |  3 +++
 .../testsuite/23_containers/vector/debug/n3644.cc        | 16 ++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/libstdc++-v3/include/debug/safe_iterator.tcc b/libstdc++-v3/include/debug/safe_iterator.tcc
index 2640fc8a473..4e213c4a0f6 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;
+}