From 0c472257dc7196c9aadce29143a7116da9205fd8 Mon Sep 17 00:00:00 2001
From: Boris Dalstein <dalboris@gmail.com>
Date: Tue, 8 Oct 2024 17:34:07 +0200
Subject: [PATCH 1/2] Fix #5399: iterator increment operator does not skip
 first item

---
 include/pybind11/pytypes.h | 16 +++++++++++-----
 tests/test_pytypes.cpp     |  5 +++++
 tests/test_pytypes.py      |  6 ++++++
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index 7aafab6dcc..e763ce3efc 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -1470,27 +1470,26 @@ class iterator : public object {
     PYBIND11_OBJECT_DEFAULT(iterator, object, PyIter_Check)
 
     iterator &operator++() {
+        init();
         advance();
         return *this;
     }
 
     iterator operator++(int) {
         auto rv = *this;
+        init();
         advance();
         return rv;
     }
 
     // NOLINTNEXTLINE(readability-const-return-type) // PR #3263
     reference operator*() const {
-        if (m_ptr && !value.ptr()) {
-            auto &self = const_cast<iterator &>(*this);
-            self.advance();
-        }
+        init();
         return value;
     }
 
     pointer operator->() const {
-        operator*();
+        init();
         return &value;
     }
 
@@ -1513,6 +1512,13 @@ class iterator : public object {
     friend bool operator!=(const iterator &a, const iterator &b) { return a->ptr() != b->ptr(); }
 
 private:
+    void init() const {
+        if (m_ptr && !value.ptr()) {
+            auto &self = const_cast<iterator &>(*this);
+            self.advance();
+        }
+    }
+
     void advance() {
         value = reinterpret_steal<object>(PyIter_Next(m_ptr));
         if (value.ptr() == nullptr && PyErr_Occurred()) {
diff --git a/tests/test_pytypes.cpp b/tests/test_pytypes.cpp
index 19f65ce7eb..759f7ff500 100644
--- a/tests/test_pytypes.cpp
+++ b/tests/test_pytypes.cpp
@@ -150,6 +150,11 @@ TEST_SUBMODULE(pytypes, m) {
     m.def("get_iterator", [] { return py::iterator(); });
     // test_iterable
     m.def("get_iterable", [] { return py::iterable(); });
+    m.def("get_second_item_from_iterable", [](const py::iterable &iter) {
+        py::iterator it = iter.begin();
+        ++it;
+        return *it;
+    });
     m.def("get_frozenset_from_iterable",
           [](const py::iterable &iter) { return py::frozenset(iter); });
     m.def("get_list_from_iterable", [](const py::iterable &iter) { return py::list(iter); });
diff --git a/tests/test_pytypes.py b/tests/test_pytypes.py
index 39d0b619b8..f1273c7657 100644
--- a/tests/test_pytypes.py
+++ b/tests/test_pytypes.py
@@ -54,6 +54,12 @@ def test_iterable(doc):
     assert doc(m.get_iterable) == "get_iterable() -> Iterable"
 
 
+def test_get_second_item_from_iterable():
+    lins = [1, 2]
+    i = m.get_second_item_from_iterable(lins)
+    assert i == 2
+
+
 def test_float(doc):
     assert doc(m.get_float) == "get_float() -> float"
 

From 5a7af3f1e7f2c3a6c2a77664eb3a9da28b550198 Mon Sep 17 00:00:00 2001
From: Boris Dalstein <dalboris@gmail.com>
Date: Tue, 8 Oct 2024 18:59:46 +0200
Subject: [PATCH 2/2] Fix postfix increment operator: init() must be called
 before copying *this

---
 include/pybind11/pytypes.h | 6 +++++-
 tests/test_pytypes.cpp     | 7 +++++++
 tests/test_pytypes.py      | 7 +++----
 3 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index e763ce3efc..027e36098b 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -1476,8 +1476,12 @@ class iterator : public object {
     }
 
     iterator operator++(int) {
-        auto rv = *this;
+        // Note: We must call init() first so that rv.value is
+        // the same as this->value just before calling advance().
+        // Otherwise, dereferencing the returned iterator may call
+        // advance() again and return the 3rd item instead of the 1st.
         init();
+        auto rv = *this;
         advance();
         return rv;
     }
diff --git a/tests/test_pytypes.cpp b/tests/test_pytypes.cpp
index 759f7ff500..8df4cdd3f6 100644
--- a/tests/test_pytypes.cpp
+++ b/tests/test_pytypes.cpp
@@ -150,7 +150,14 @@ TEST_SUBMODULE(pytypes, m) {
     m.def("get_iterator", [] { return py::iterator(); });
     // test_iterable
     m.def("get_iterable", [] { return py::iterable(); });
+    m.def("get_first_item_from_iterable", [](const py::iterable &iter) {
+        // This tests the postfix increment operator
+        py::iterator it = iter.begin();
+        py::iterator it2 = it++;
+        return *it2;
+    });
     m.def("get_second_item_from_iterable", [](const py::iterable &iter) {
+        // This tests the prefix increment operator
         py::iterator it = iter.begin();
         ++it;
         return *it;
diff --git a/tests/test_pytypes.py b/tests/test_pytypes.py
index f1273c7657..9fd24b34f1 100644
--- a/tests/test_pytypes.py
+++ b/tests/test_pytypes.py
@@ -52,10 +52,9 @@ def test_from_iterable(pytype, from_iter_func):
 
 def test_iterable(doc):
     assert doc(m.get_iterable) == "get_iterable() -> Iterable"
-
-
-def test_get_second_item_from_iterable():
-    lins = [1, 2]
+    lins = [1, 2, 3]
+    i = m.get_first_item_from_iterable(lins)
+    assert i == 1
     i = m.get_second_item_from_iterable(lins)
     assert i == 2