From 716a2952d264c648da0815ef16740ad8e9bd4836 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 08:52:59 -0800
Subject: [PATCH 01/15] Content of PR #4374 applied on top of smart_holder
 branch.

---
 tests/CMakeLists.txt    |  1 +
 tests/test_mi_debug.cpp | 53 +++++++++++++++++++++++++++++++++++++++++
 tests/test_mi_debug.py  |  8 +++++++
 3 files changed, 62 insertions(+)
 create mode 100644 tests/test_mi_debug.cpp
 create mode 100644 tests/test_mi_debug.py

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index d958a8c5ca..fa185a2f8e 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -158,6 +158,7 @@ set(PYBIND11_TEST_FILES
     test_kwargs_and_defaults
     test_local_bindings
     test_methods_and_attributes
+    test_mi_debug
     test_modules
     test_multiple_inheritance
     test_numpy_array
diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
new file mode 100644
index 0000000000..c475314ca3
--- /dev/null
+++ b/tests/test_mi_debug.cpp
@@ -0,0 +1,53 @@
+#include <pybind11/pybind11.h>
+
+#include "pybind11_tests.h"
+
+#include <iostream>
+#include <memory>
+#include <vector>
+
+// The first base class.
+struct Base0 {
+    virtual ~Base0() = default;
+};
+
+using Base0Ptr = std::shared_ptr<Base0>;
+
+// The second base class.
+struct Base1 {
+    virtual ~Base1() = default;
+    std::vector<int> vec = {1, 2, 3, 4, 5};
+};
+
+using Base1Ptr = std::shared_ptr<Base1>;
+
+// The derived class.
+struct Derived : Base1, Base0 {
+    ~Derived() override = default;
+};
+
+using DerivedPtr = std::shared_ptr<Derived>;
+
+TEST_SUBMODULE(mi_debug, m) {
+    // Expose the bases.
+    pybind11::class_<Base0, Base0Ptr> bs0(m, "Base0");
+    pybind11::class_<Base1, Base1Ptr> bs1(m, "Base1");
+    // Expose the derived class.
+    pybind11::class_<Derived, DerivedPtr, Base0, Base1>(m, "Derived").def(pybind11::init<>());
+
+    // A helper that returns a pointer to base.
+    m.def("make_object", []() -> Base0Ptr {
+        auto ret_der = std::make_shared<Derived>();
+        std::cout << "ret der ptr: " << ret_der.get() << std::endl;
+        auto ret = Base0Ptr(ret_der);
+        std::cout << "ret base ptr: " << ret.get() << std::endl;
+        return ret;
+    });
+
+    // A helper that accepts in input a pointer to derived.
+    m.def("get_object_vec_size", [](const DerivedPtr &object) {
+        std::cout << "der ptr: " << object.get() << std::endl;
+        std::cout << object->vec.size() << std::endl;
+        return object->vec.size();
+    });
+}
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
new file mode 100644
index 0000000000..d4acc87653
--- /dev/null
+++ b/tests/test_mi_debug.py
@@ -0,0 +1,8 @@
+import pytest
+
+m = pytest.importorskip("pybind11_tests.mi_debug")
+
+
+def test_vec():
+    o = m.make_object()
+    assert 5 == m.get_object_vec_size(o)

From a2d5b9e44e346fce632893e951ce9f5cb442d139 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 11:16:12 -0800
Subject: [PATCH 02/15] More tests, with USE_SH switch. [ci skip]

---
 tests/test_mi_debug.cpp | 72 +++++++++++++++++++++++++++--------------
 tests/test_mi_debug.py  | 18 ++++++++---
 2 files changed, 60 insertions(+), 30 deletions(-)

diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index c475314ca3..7079efb9c8 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -1,53 +1,75 @@
 #include <pybind11/pybind11.h>
 
+#define USE_SH
+
+#if defined(USE_SH)
+#    include <pybind11/smart_holder.h>
+#endif
+
 #include "pybind11_tests.h"
 
-#include <iostream>
+#include <cstddef>
 #include <memory>
 #include <vector>
 
-// The first base class.
+namespace test_mi_debug {
+
 struct Base0 {
     virtual ~Base0() = default;
 };
 
-using Base0Ptr = std::shared_ptr<Base0>;
-
-// The second base class.
 struct Base1 {
     virtual ~Base1() = default;
     std::vector<int> vec = {1, 2, 3, 4, 5};
 };
 
-using Base1Ptr = std::shared_ptr<Base1>;
-
-// The derived class.
 struct Derived : Base1, Base0 {
     ~Derived() override = default;
 };
 
-using DerivedPtr = std::shared_ptr<Derived>;
+} // namespace test_mi_debug
+
+#if defined(USE_SH)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base0)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base1)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Derived)
+#endif
 
 TEST_SUBMODULE(mi_debug, m) {
-    // Expose the bases.
-    pybind11::class_<Base0, Base0Ptr> bs0(m, "Base0");
-    pybind11::class_<Base1, Base1Ptr> bs1(m, "Base1");
-    // Expose the derived class.
-    pybind11::class_<Derived, DerivedPtr, Base0, Base1>(m, "Derived").def(pybind11::init<>());
-
-    // A helper that returns a pointer to base.
-    m.def("make_object", []() -> Base0Ptr {
+    using namespace test_mi_debug;
+
+#if defined(USE_SH)
+    py::classh<Base0> bs0(m, "Base0");
+    py::classh<Base1> bs1(m, "Base1");
+    py::classh<Derived, Base1, Base0>(m, "Derived").def(py::init<>());
+#else
+    py::class_<Base0, std::shared_ptr<Base0>> bs0(m, "Base0");
+    py::class_<Base1, std::shared_ptr<Base1>> bs1(m, "Base1");
+    py::class_<Derived, std::shared_ptr<Derived>, Base1, Base0>(m, "Derived").def(py::init<>());
+#endif
+
+    m.def("make_derived_as_base0", []() -> std::shared_ptr<Base0> {
         auto ret_der = std::make_shared<Derived>();
-        std::cout << "ret der ptr: " << ret_der.get() << std::endl;
-        auto ret = Base0Ptr(ret_der);
-        std::cout << "ret base ptr: " << ret.get() << std::endl;
+        auto ret = std::shared_ptr<Base0>(ret_der);
         return ret;
     });
 
-    // A helper that accepts in input a pointer to derived.
-    m.def("get_object_vec_size", [](const DerivedPtr &object) {
-        std::cout << "der ptr: " << object.get() << std::endl;
-        std::cout << object->vec.size() << std::endl;
-        return object->vec.size();
+    // class_ OK
+    // classh FAIL
+    m.def("get_vec_size_raw_ptr_base0", [](const Base0 *obj) -> std::size_t {
+        auto obj_der = dynamic_cast<const Derived *>(obj);
+        if (obj_der == nullptr) {
+            return 0;
+        }
+        return obj_der->vec.size();
     });
+
+    // class_ OK
+    // classh FAIL
+    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) { return obj->vec.size(); });
+
+    // class_ OK
+    // classh FAIL
+    m.def("get_vec_size_shared_ptr_derived",
+          [](const std::shared_ptr<Derived> &obj) { return obj->vec.size(); });
 }
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index d4acc87653..d2bbe26f2d 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -1,8 +1,16 @@
-import pytest
+from pybind11_tests import mi_debug as m
 
-m = pytest.importorskip("pybind11_tests.mi_debug")
 
+def test_get_vec_size_raw_ptr_base0():
+    obj = m.make_derived_as_base0()
+    assert m.get_vec_size_raw_ptr_base0(obj) == 5
 
-def test_vec():
-    o = m.make_object()
-    assert 5 == m.get_object_vec_size(o)
+
+def test_get_vec_size_raw_ptr_derived():
+    obj = m.make_derived_as_base0()
+    assert m.get_vec_size_raw_ptr_derived(obj) == 5
+
+
+def test_get_vec_size_shared_ptr_derived():
+    obj = m.make_derived_as_base0()
+    assert m.get_vec_size_shared_ptr_derived(obj) == 5

From 551eabe8e71c8ea704e938988a2c0e17d6f76a57 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 12:05:38 -0800
Subject: [PATCH 03/15] Use `std::dynamic_pointer_cast<Base0>` [ci skip]

---
 tests/test_mi_debug.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index 7079efb9c8..0285b6440b 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -50,7 +50,7 @@ TEST_SUBMODULE(mi_debug, m) {
 
     m.def("make_derived_as_base0", []() -> std::shared_ptr<Base0> {
         auto ret_der = std::make_shared<Derived>();
-        auto ret = std::shared_ptr<Base0>(ret_der);
+        auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
         return ret;
     });
 

From af08ec6c66911694b9c482c4cd94949e76ff704d Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 14:00:31 -0800
Subject: [PATCH 04/15] All tests pass when using
 `m.make_derived_as_base0_raw_ptr()`, with `USE_SH` defined or not defined.
 [ci skip]

---
 tests/test_mi_debug.cpp | 29 ++++++++++++++++++++++++++++-
 tests/test_mi_debug.py  |  6 +++---
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index 0285b6440b..7b57c9d7c0 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -16,15 +16,21 @@ namespace test_mi_debug {
 
 struct Base0 {
     virtual ~Base0() = default;
+    Base0() = default;
+    Base0(const Base0 &) = delete;
 };
 
 struct Base1 {
     virtual ~Base1() = default;
     std::vector<int> vec = {1, 2, 3, 4, 5};
+    Base1() = default;
+    Base1(const Base1 &) = delete;
 };
 
 struct Derived : Base1, Base0 {
     ~Derived() override = default;
+    Derived() = default;
+    Derived(const Derived &) = delete;
 };
 
 } // namespace test_mi_debug
@@ -50,10 +56,27 @@ TEST_SUBMODULE(mi_debug, m) {
 
     m.def("make_derived_as_base0", []() -> std::shared_ptr<Base0> {
         auto ret_der = std::make_shared<Derived>();
+        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret_der.get(), __FILE__, __LINE__);
+        fflush(stdout);
         auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
+        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret.get(), __FILE__, __LINE__);
+        fflush(stdout);
         return ret;
     });
 
+    m.def(
+        "make_derived_as_base0_raw_ptr",
+        []() {
+            auto ret_der = new Derived{};
+            printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret_der, __FILE__, __LINE__);
+            fflush(stdout);
+            auto ret = dynamic_cast<Base0 *>(ret_der);
+            printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret, __FILE__, __LINE__);
+            fflush(stdout);
+            return ret;
+        },
+        py::return_value_policy::take_ownership);
+
     // class_ OK
     // classh FAIL
     m.def("get_vec_size_raw_ptr_base0", [](const Base0 *obj) -> std::size_t {
@@ -66,7 +89,11 @@ TEST_SUBMODULE(mi_debug, m) {
 
     // class_ OK
     // classh FAIL
-    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) { return obj->vec.size(); });
+    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) {
+        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) obj, __FILE__, __LINE__);
+        fflush(stdout);
+        return obj->vec.size();
+    });
 
     // class_ OK
     // classh FAIL
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index d2bbe26f2d..c3ae5eddbe 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -2,15 +2,15 @@
 
 
 def test_get_vec_size_raw_ptr_base0():
-    obj = m.make_derived_as_base0()
+    obj = m.make_derived_as_base0_raw_ptr()
     assert m.get_vec_size_raw_ptr_base0(obj) == 5
 
 
 def test_get_vec_size_raw_ptr_derived():
-    obj = m.make_derived_as_base0()
+    obj = m.make_derived_as_base0_raw_ptr()
     assert m.get_vec_size_raw_ptr_derived(obj) == 5
 
 
 def test_get_vec_size_shared_ptr_derived():
-    obj = m.make_derived_as_base0()
+    obj = m.make_derived_as_base0_raw_ptr()
     assert m.get_vec_size_shared_ptr_derived(obj) == 5

From 4e5ee456f4a9c54bc8a1d90b08cf6fbec9242c71 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 17:35:08 -0800
Subject: [PATCH 05/15] WIP

---
 tests/test_mi_debug.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index c3ae5eddbe..901f1864d7 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -7,7 +7,10 @@ def test_get_vec_size_raw_ptr_base0():
 
 
 def test_get_vec_size_raw_ptr_derived():
+    obj = m.make_derived_as_base0()
+    print("\nLOOOK", obj)
     obj = m.make_derived_as_base0_raw_ptr()
+    print("\nLOOOK", obj)
     assert m.get_vec_size_raw_ptr_derived(obj) == 5
 
 

From e313d86e8fe87ae03d340611e11747a765477bf2 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 22:48:54 -0800
Subject: [PATCH 06/15] Debug LOOOK & one-line bug fix:

```diff
-        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(src);
+        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(std::shared_ptr<void>(src, const_cast<void *>(st.first)));
```
---
 include/pybind11/detail/smart_holder_poc.h          | 5 +++++
 include/pybind11/detail/smart_holder_type_casters.h | 9 ++++++++-
 tests/test_mi_debug.py                              | 5 ++++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/pybind11/detail/smart_holder_poc.h b/include/pybind11/detail/smart_holder_poc.h
index de92f4156f..774d2654b2 100644
--- a/include/pybind11/detail/smart_holder_poc.h
+++ b/include/pybind11/detail/smart_holder_poc.h
@@ -220,6 +220,7 @@ struct smart_holder {
     }
 
     static smart_holder from_raw_ptr_unowned(void *raw_ptr) {
+  printf("\nLOOOK %s:%d\n", __FILE__, __LINE__); fflush(stdout);
         smart_holder hld;
         hld.vptr.reset(raw_ptr, [](void *) {});
         hld.vptr_is_using_noop_deleter = true;
@@ -250,6 +251,7 @@ struct smart_holder {
 
     template <typename T>
     static smart_holder from_raw_ptr_take_ownership(T *raw_ptr, bool void_cast_raw_ptr = false) {
+  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) raw_ptr, __FILE__, __LINE__); fflush(stdout);
         ensure_pointee_is_destructible<T>("from_raw_ptr_take_ownership");
         smart_holder hld;
         auto gd = make_guarded_builtin_delete<T>(true);
@@ -302,6 +304,7 @@ struct smart_holder {
     template <typename T, typename D>
     static smart_holder from_unique_ptr(std::unique_ptr<T, D> &&unq_ptr,
                                         bool void_cast_raw_ptr = false) {
+  printf("\nLOOOK %s:%d\n", __FILE__, __LINE__); fflush(stdout);
         smart_holder hld;
         hld.rtti_uqp_del = &typeid(D);
         hld.vptr_is_using_builtin_delete = is_std_default_delete<T>(*hld.rtti_uqp_del);
@@ -333,6 +336,8 @@ struct smart_holder {
 
     template <typename T>
     static smart_holder from_shared_ptr(std::shared_ptr<T> shd_ptr) {
+  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) shd_ptr.get(), __FILE__, __LINE__); fflush(stdout);
+  // long *BAD = nullptr; *BAD = 101;
         smart_holder hld;
         hld.vptr = std::static_pointer_cast<void>(shd_ptr);
         hld.vptr_is_external_shared_ptr = true;
diff --git a/include/pybind11/detail/smart_holder_type_casters.h b/include/pybind11/detail/smart_holder_type_casters.h
index 9f95e8c72b..b070103bef 100644
--- a/include/pybind11/detail/smart_holder_type_casters.h
+++ b/include/pybind11/detail/smart_holder_type_casters.h
@@ -356,6 +356,8 @@ struct smart_holder_type_caster_class_hooks : smart_holder_type_caster_base_tag
         auto *holder_void_ptr = const_cast<void *>(holder_const_void_ptr);
 
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(WrappedType)));
+  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) v_h.value_ptr(), __FILE__, __LINE__); fflush(stdout);
+  // long *BAD = nullptr; *BAD = 101;
         if (!v_h.instance_registered()) {
             register_instance(inst, v_h.value_ptr(), v_h.type);
             v_h.set_instance_registered();
@@ -650,6 +652,7 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
     }
 
     static handle cast(T const *src, return_value_policy policy, handle parent) {
+  printf("\nLOOOK [%lu] IsBase0Still %s:%d\n", (std::size_t) src, __FILE__, __LINE__); fflush(stdout);
         auto st = type_caster_base<T>::src_and_type(src);
         if (policy == return_value_policy::_clif_automatic) {
             policy = return_value_policy::copy;
@@ -702,6 +705,7 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
                                                        void *(*copy_constructor)(const void *),
                                                        void *(*move_constructor)(const void *),
                                                        const void *existing_holder = nullptr) {
+  printf("\nLOOOK [%lu] IsDerivedAlready %s:%d\n", (std::size_t) _src, __FILE__, __LINE__); fflush(stdout);
         if (!tinfo) { // no type info: error will be set already
             return handle();
         }
@@ -794,6 +798,7 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
     static constexpr auto name = const_name<T>();
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
+  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) src.get(), __FILE__, __LINE__); fflush(stdout);
         switch (policy) {
             case return_value_policy::automatic:
             case return_value_policy::automatic_reference:
@@ -835,8 +840,10 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
         inst_raw_ptr->owned = true;
         void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
         valueptr = src_raw_void_ptr;
+  printf("\nLOOOK [%lu] IsBase0 %s:%d\n", (std::size_t) valueptr, __FILE__, __LINE__); fflush(stdout);
+  printf("\nLOOOK [%lu] IsDerived %s:%d\n", (std::size_t) st.first, __FILE__, __LINE__); fflush(stdout);
 
-        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(src);
+        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(std::shared_ptr<void>(src, const_cast<void *>(st.first)));
         tinfo->init_instance(inst_raw_ptr, static_cast<const void *>(&smhldr));
 
         if (policy == return_value_policy::reference_internal) {
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index 901f1864d7..218aea34f1 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -6,9 +6,12 @@ def test_get_vec_size_raw_ptr_base0():
     assert m.get_vec_size_raw_ptr_base0(obj) == 5
 
 
-def test_get_vec_size_raw_ptr_derived():
+def test_get_vec_size_raw_ptr_derived_from_shared():
     obj = m.make_derived_as_base0()
     print("\nLOOOK", obj)
+    assert m.get_vec_size_raw_ptr_derived(obj) == 5
+
+def test_get_vec_size_raw_ptr_derived():
     obj = m.make_derived_as_base0_raw_ptr()
     print("\nLOOOK", obj)
     assert m.get_vec_size_raw_ptr_derived(obj) == 5

From 7a5ea90832c631a9706e8f6a162a36d8fb4370fb Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 2 Dec 2022 22:58:37 -0800
Subject: [PATCH 07/15] Remove all print LOOOK and clang-format the fix.

---
 include/pybind11/detail/smart_holder_poc.h         |  5 -----
 .../pybind11/detail/smart_holder_type_casters.h    | 10 ++--------
 tests/test_mi_debug.cpp                            | 14 +-------------
 tests/test_mi_debug.py                             |  3 +--
 4 files changed, 4 insertions(+), 28 deletions(-)

diff --git a/include/pybind11/detail/smart_holder_poc.h b/include/pybind11/detail/smart_holder_poc.h
index 774d2654b2..de92f4156f 100644
--- a/include/pybind11/detail/smart_holder_poc.h
+++ b/include/pybind11/detail/smart_holder_poc.h
@@ -220,7 +220,6 @@ struct smart_holder {
     }
 
     static smart_holder from_raw_ptr_unowned(void *raw_ptr) {
-  printf("\nLOOOK %s:%d\n", __FILE__, __LINE__); fflush(stdout);
         smart_holder hld;
         hld.vptr.reset(raw_ptr, [](void *) {});
         hld.vptr_is_using_noop_deleter = true;
@@ -251,7 +250,6 @@ struct smart_holder {
 
     template <typename T>
     static smart_holder from_raw_ptr_take_ownership(T *raw_ptr, bool void_cast_raw_ptr = false) {
-  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) raw_ptr, __FILE__, __LINE__); fflush(stdout);
         ensure_pointee_is_destructible<T>("from_raw_ptr_take_ownership");
         smart_holder hld;
         auto gd = make_guarded_builtin_delete<T>(true);
@@ -304,7 +302,6 @@ struct smart_holder {
     template <typename T, typename D>
     static smart_holder from_unique_ptr(std::unique_ptr<T, D> &&unq_ptr,
                                         bool void_cast_raw_ptr = false) {
-  printf("\nLOOOK %s:%d\n", __FILE__, __LINE__); fflush(stdout);
         smart_holder hld;
         hld.rtti_uqp_del = &typeid(D);
         hld.vptr_is_using_builtin_delete = is_std_default_delete<T>(*hld.rtti_uqp_del);
@@ -336,8 +333,6 @@ struct smart_holder {
 
     template <typename T>
     static smart_holder from_shared_ptr(std::shared_ptr<T> shd_ptr) {
-  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) shd_ptr.get(), __FILE__, __LINE__); fflush(stdout);
-  // long *BAD = nullptr; *BAD = 101;
         smart_holder hld;
         hld.vptr = std::static_pointer_cast<void>(shd_ptr);
         hld.vptr_is_external_shared_ptr = true;
diff --git a/include/pybind11/detail/smart_holder_type_casters.h b/include/pybind11/detail/smart_holder_type_casters.h
index b070103bef..56155add86 100644
--- a/include/pybind11/detail/smart_holder_type_casters.h
+++ b/include/pybind11/detail/smart_holder_type_casters.h
@@ -356,8 +356,6 @@ struct smart_holder_type_caster_class_hooks : smart_holder_type_caster_base_tag
         auto *holder_void_ptr = const_cast<void *>(holder_const_void_ptr);
 
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(WrappedType)));
-  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) v_h.value_ptr(), __FILE__, __LINE__); fflush(stdout);
-  // long *BAD = nullptr; *BAD = 101;
         if (!v_h.instance_registered()) {
             register_instance(inst, v_h.value_ptr(), v_h.type);
             v_h.set_instance_registered();
@@ -652,7 +650,6 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
     }
 
     static handle cast(T const *src, return_value_policy policy, handle parent) {
-  printf("\nLOOOK [%lu] IsBase0Still %s:%d\n", (std::size_t) src, __FILE__, __LINE__); fflush(stdout);
         auto st = type_caster_base<T>::src_and_type(src);
         if (policy == return_value_policy::_clif_automatic) {
             policy = return_value_policy::copy;
@@ -705,7 +702,6 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
                                                        void *(*copy_constructor)(const void *),
                                                        void *(*move_constructor)(const void *),
                                                        const void *existing_holder = nullptr) {
-  printf("\nLOOOK [%lu] IsDerivedAlready %s:%d\n", (std::size_t) _src, __FILE__, __LINE__); fflush(stdout);
         if (!tinfo) { // no type info: error will be set already
             return handle();
         }
@@ -798,7 +794,6 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
     static constexpr auto name = const_name<T>();
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
-  printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) src.get(), __FILE__, __LINE__); fflush(stdout);
         switch (policy) {
             case return_value_policy::automatic:
             case return_value_policy::automatic_reference:
@@ -840,10 +835,9 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
         inst_raw_ptr->owned = true;
         void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
         valueptr = src_raw_void_ptr;
-  printf("\nLOOOK [%lu] IsBase0 %s:%d\n", (std::size_t) valueptr, __FILE__, __LINE__); fflush(stdout);
-  printf("\nLOOOK [%lu] IsDerived %s:%d\n", (std::size_t) st.first, __FILE__, __LINE__); fflush(stdout);
 
-        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(std::shared_ptr<void>(src, const_cast<void *>(st.first)));
+        auto smhldr = pybindit::memory::smart_holder::from_shared_ptr(
+            std::shared_ptr<void>(src, const_cast<void *>(st.first)));
         tinfo->init_instance(inst_raw_ptr, static_cast<const void *>(&smhldr));
 
         if (policy == return_value_policy::reference_internal) {
diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index 7b57c9d7c0..759524fb3b 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -56,11 +56,7 @@ TEST_SUBMODULE(mi_debug, m) {
 
     m.def("make_derived_as_base0", []() -> std::shared_ptr<Base0> {
         auto ret_der = std::make_shared<Derived>();
-        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret_der.get(), __FILE__, __LINE__);
-        fflush(stdout);
         auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
-        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret.get(), __FILE__, __LINE__);
-        fflush(stdout);
         return ret;
     });
 
@@ -68,11 +64,7 @@ TEST_SUBMODULE(mi_debug, m) {
         "make_derived_as_base0_raw_ptr",
         []() {
             auto ret_der = new Derived{};
-            printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret_der, __FILE__, __LINE__);
-            fflush(stdout);
             auto ret = dynamic_cast<Base0 *>(ret_der);
-            printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) ret, __FILE__, __LINE__);
-            fflush(stdout);
             return ret;
         },
         py::return_value_policy::take_ownership);
@@ -89,11 +81,7 @@ TEST_SUBMODULE(mi_debug, m) {
 
     // class_ OK
     // classh FAIL
-    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) {
-        printf("\nLOOOK [%lu] %s:%d\n", (std::size_t) obj, __FILE__, __LINE__);
-        fflush(stdout);
-        return obj->vec.size();
-    });
+    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) { return obj->vec.size(); });
 
     // class_ OK
     // classh FAIL
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index 218aea34f1..15db3c32bb 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -8,12 +8,11 @@ def test_get_vec_size_raw_ptr_base0():
 
 def test_get_vec_size_raw_ptr_derived_from_shared():
     obj = m.make_derived_as_base0()
-    print("\nLOOOK", obj)
     assert m.get_vec_size_raw_ptr_derived(obj) == 5
 
+
 def test_get_vec_size_raw_ptr_derived():
     obj = m.make_derived_as_base0_raw_ptr()
-    print("\nLOOOK", obj)
     assert m.get_vec_size_raw_ptr_derived(obj) == 5
 
 

From 8aa790b7ff0c6af17173832645b0bd4abf61fbed Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sun, 4 Dec 2022 03:30:31 -0800
Subject: [PATCH 08/15] Resolve clang-tidy errors.

---
 tests/test_mi_debug.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index 759524fb3b..857f617deb 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -63,8 +63,8 @@ TEST_SUBMODULE(mi_debug, m) {
     m.def(
         "make_derived_as_base0_raw_ptr",
         []() {
-            auto ret_der = new Derived{};
-            auto ret = dynamic_cast<Base0 *>(ret_der);
+            auto *ret_der = new Derived{};
+            auto *ret = dynamic_cast<Base0 *>(ret_der);
             return ret;
         },
         py::return_value_policy::take_ownership);
@@ -72,7 +72,7 @@ TEST_SUBMODULE(mi_debug, m) {
     // class_ OK
     // classh FAIL
     m.def("get_vec_size_raw_ptr_base0", [](const Base0 *obj) -> std::size_t {
-        auto obj_der = dynamic_cast<const Derived *>(obj);
+        const auto *obj_der = dynamic_cast<const Derived *>(obj);
         if (obj_der == nullptr) {
             return 0;
         }

From f96201a6c4e46bfb2e5cb22e7fbdf72beff69e5c Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 9 Dec 2022 14:59:23 -0800
Subject: [PATCH 09/15] Systematic test matrix.

---
 tests/test_mi_debug.cpp | 62 ++++++++++++++++++++---------------------
 tests/test_mi_debug.py  | 57 +++++++++++++++++++++++++------------
 2 files changed, 69 insertions(+), 50 deletions(-)

diff --git a/tests/test_mi_debug.cpp b/tests/test_mi_debug.cpp
index 857f617deb..354a8b0b6b 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_mi_debug.cpp
@@ -1,10 +1,5 @@
 #include <pybind11/pybind11.h>
-
-#define USE_SH
-
-#if defined(USE_SH)
-#    include <pybind11/smart_holder.h>
-#endif
+#include <pybind11/smart_holder.h>
 
 #include "pybind11_tests.h"
 
@@ -35,33 +30,19 @@ struct Derived : Base1, Base0 {
 
 } // namespace test_mi_debug
 
-#if defined(USE_SH)
 PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base0)
 PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base1)
 PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Derived)
-#endif
 
 TEST_SUBMODULE(mi_debug, m) {
     using namespace test_mi_debug;
 
-#if defined(USE_SH)
     py::classh<Base0> bs0(m, "Base0");
     py::classh<Base1> bs1(m, "Base1");
-    py::classh<Derived, Base1, Base0>(m, "Derived").def(py::init<>());
-#else
-    py::class_<Base0, std::shared_ptr<Base0>> bs0(m, "Base0");
-    py::class_<Base1, std::shared_ptr<Base1>> bs1(m, "Base1");
-    py::class_<Derived, std::shared_ptr<Derived>, Base1, Base0>(m, "Derived").def(py::init<>());
-#endif
-
-    m.def("make_derived_as_base0", []() -> std::shared_ptr<Base0> {
-        auto ret_der = std::make_shared<Derived>();
-        auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
-        return ret;
-    });
+    py::classh<Derived, Base1, Base0>(m, "Derived");
 
     m.def(
-        "make_derived_as_base0_raw_ptr",
+        "get_derived_as_base0_raw_ptr",
         []() {
             auto *ret_der = new Derived{};
             auto *ret = dynamic_cast<Base0 *>(ret_der);
@@ -69,9 +50,19 @@ TEST_SUBMODULE(mi_debug, m) {
         },
         py::return_value_policy::take_ownership);
 
-    // class_ OK
-    // classh FAIL
-    m.def("get_vec_size_raw_ptr_base0", [](const Base0 *obj) -> std::size_t {
+    m.def("get_derived_as_base0_shared_ptr", []() -> std::shared_ptr<Base0> {
+        auto ret_der = std::make_shared<Derived>();
+        auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
+        return ret;
+    });
+
+    m.def("get_derived_as_base0_unique_ptr", []() -> std::unique_ptr<Base0> {
+        auto ret_der = std::unique_ptr<Derived>(new Derived{});
+        auto ret = std::unique_ptr<Base0>(std::move(ret_der));
+        return ret;
+    });
+
+    m.def("vec_size_base0_raw_ptr", [](const Base0 *obj) -> std::size_t {
         const auto *obj_der = dynamic_cast<const Derived *>(obj);
         if (obj_der == nullptr) {
             return 0;
@@ -79,12 +70,19 @@ TEST_SUBMODULE(mi_debug, m) {
         return obj_der->vec.size();
     });
 
-    // class_ OK
-    // classh FAIL
-    m.def("get_vec_size_raw_ptr_derived", [](const Derived *obj) { return obj->vec.size(); });
+    m.def("vec_size_base0_shared_ptr", [](const std::shared_ptr<Base0> &obj) -> std::size_t {
+        const auto obj_der = std::dynamic_pointer_cast<Derived>(obj);
+        if (!obj_der) {
+            return 0;
+        }
+        return obj_der->vec.size();
+    });
 
-    // class_ OK
-    // classh FAIL
-    m.def("get_vec_size_shared_ptr_derived",
-          [](const std::shared_ptr<Derived> &obj) { return obj->vec.size(); });
+    m.def("vec_size_base0_unique_ptr", [](std::unique_ptr<Base0> obj) -> std::size_t {
+        const auto *obj_der = dynamic_cast<const Derived *>(obj.get());
+        if (obj_der == nullptr) {
+            return 0;
+        }
+        return obj_der->vec.size();
+    });
 }
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index 15db3c32bb..d09bf30275 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -1,21 +1,42 @@
-from pybind11_tests import mi_debug as m
-
-
-def test_get_vec_size_raw_ptr_base0():
-    obj = m.make_derived_as_base0_raw_ptr()
-    assert m.get_vec_size_raw_ptr_base0(obj) == 5
-
+import pytest
 
-def test_get_vec_size_raw_ptr_derived_from_shared():
-    obj = m.make_derived_as_base0()
-    assert m.get_vec_size_raw_ptr_derived(obj) == 5
-
-
-def test_get_vec_size_raw_ptr_derived():
-    obj = m.make_derived_as_base0_raw_ptr()
-    assert m.get_vec_size_raw_ptr_derived(obj) == 5
+from pybind11_tests import mi_debug as m
 
 
-def test_get_vec_size_shared_ptr_derived():
-    obj = m.make_derived_as_base0_raw_ptr()
-    assert m.get_vec_size_shared_ptr_derived(obj) == 5
+@pytest.mark.parametrize(
+    "vec_size_fn",
+    [
+        m.vec_size_base0_raw_ptr,
+        m.vec_size_base0_shared_ptr,
+        m.vec_size_base0_unique_ptr,
+    ],
+)
+@pytest.mark.parametrize(
+    "get_fn",
+    [
+        m.get_derived_as_base0_raw_ptr,
+        m.get_derived_as_base0_shared_ptr,
+        # *** FAILS *** m.get_derived_as_base0_unique_ptr,
+    ],
+)
+def test_get_vec_size(get_fn, vec_size_fn):
+    obj = get_fn()
+    if (
+        get_fn is m.get_derived_as_base0_shared_ptr
+        and vec_size_fn is m.vec_size_base0_unique_ptr
+    ):
+        with pytest.raises(ValueError) as exc_info:
+            vec_size_fn(obj)
+        assert (
+            str(exc_info.value)
+            == "Cannot disown external shared_ptr (loaded_as_unique_ptr)."
+        )
+    else:
+        assert vec_size_fn(obj) == 5
+        if vec_size_fn is m.vec_size_base0_unique_ptr:
+            with pytest.raises(ValueError) as exc_info:
+                vec_size_fn(obj)
+            assert (
+                str(exc_info.value)
+                == "Missing value for wrapped C++ type: Python instance was disowned."
+            )

From 8f18d7af28bcabf77a1e29f02a7e2c07005e6948 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Fri, 9 Dec 2022 22:35:32 -0800
Subject: [PATCH 10/15] Bug fix in `smart_holder_type_caster<std::unique_ptr<T,
 D>>::cast()`

---
 include/pybind11/detail/smart_holder_poc.h    |  6 +++---
 .../detail/smart_holder_type_casters.h        | 21 ++++++++++++-------
 tests/test_mi_debug.py                        |  2 +-
 3 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/include/pybind11/detail/smart_holder_poc.h b/include/pybind11/detail/smart_holder_poc.h
index de92f4156f..e169f2f26b 100644
--- a/include/pybind11/detail/smart_holder_poc.h
+++ b/include/pybind11/detail/smart_holder_poc.h
@@ -301,7 +301,7 @@ struct smart_holder {
 
     template <typename T, typename D>
     static smart_holder from_unique_ptr(std::unique_ptr<T, D> &&unq_ptr,
-                                        bool void_cast_raw_ptr = false) {
+                                        void *void_ptr = nullptr) {
         smart_holder hld;
         hld.rtti_uqp_del = &typeid(D);
         hld.vptr_is_using_builtin_delete = is_std_default_delete<T>(*hld.rtti_uqp_del);
@@ -311,8 +311,8 @@ struct smart_holder {
         } else {
             gd = make_guarded_custom_deleter<T, D>(true);
         }
-        if (void_cast_raw_ptr) {
-            hld.vptr.reset(static_cast<void *>(unq_ptr.get()), std::move(gd));
+        if (void_ptr != nullptr) {
+            hld.vptr.reset(void_ptr, std::move(gd));
         } else {
             hld.vptr.reset(unq_ptr.get(), std::move(gd));
         }
diff --git a/include/pybind11/detail/smart_holder_type_casters.h b/include/pybind11/detail/smart_holder_type_casters.h
index 56155add86..b3289773a7 100644
--- a/include/pybind11/detail/smart_holder_type_casters.h
+++ b/include/pybind11/detail/smart_holder_type_casters.h
@@ -386,8 +386,8 @@ struct smart_holder_type_caster_class_hooks : smart_holder_type_caster_base_tag
     template <typename T, typename D>
     static smart_holder smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&unq_ptr,
                                                      bool void_cast_raw_ptr) {
-        return pybindit::memory::smart_holder::from_unique_ptr(std::move(unq_ptr),
-                                                               void_cast_raw_ptr);
+        void *void_ptr = void_cast_raw_ptr ? static_cast<void *>(unq_ptr.get()) : nullptr;
+        return pybindit::memory::smart_holder::from_unique_ptr(std::move(unq_ptr), void_ptr);
     }
 
     template <typename T>
@@ -891,17 +891,16 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
             return none().release();
         }
 
-        auto src_raw_ptr = src.get();
-        auto st = type_caster_base<T>::src_and_type(src_raw_ptr);
+        auto st = type_caster_base<T>::src_and_type(src.get());
         if (st.second == nullptr) {
             return handle(); // no type info: error will be set already
         }
 
-        void *src_raw_void_ptr = static_cast<void *>(src_raw_ptr);
+        void *src_raw_void_ptr = const_cast<void *>(st.first);
         const detail::type_info *tinfo = st.second;
         if (handle existing_inst = find_registered_python_instance(src_raw_void_ptr, tinfo)) {
             auto *self_life_support
-                = dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(src_raw_ptr);
+                = dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(src.get());
             if (self_life_support != nullptr) {
                 value_and_holder &v_h = self_life_support->v_h;
                 if (v_h.inst != nullptr && v_h.vh != nullptr) {
@@ -927,8 +926,14 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
         void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
         valueptr = src_raw_void_ptr;
 
-        auto smhldr = pybindit::memory::smart_holder::from_unique_ptr(std::move(src),
-                                                                      /*void_cast_raw_ptr*/ false);
+        if (static_cast<void *>(src.get()) == src_raw_void_ptr) {
+            // This is a multiple-inheritance situation that is incompatible with the current
+            // shared_from_this handling (see PR #3023).
+            // SMART_HOLDER_WIP: IMPROVABLE: Is there a better solution?
+            src_raw_void_ptr = nullptr;
+        }
+        auto smhldr
+            = pybindit::memory::smart_holder::from_unique_ptr(std::move(src), src_raw_void_ptr);
         tinfo->init_instance(inst_raw_ptr, static_cast<const void *>(&smhldr));
 
         if (policy == return_value_policy::reference_internal) {
diff --git a/tests/test_mi_debug.py b/tests/test_mi_debug.py
index d09bf30275..06e96c5eaf 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_mi_debug.py
@@ -16,7 +16,7 @@
     [
         m.get_derived_as_base0_raw_ptr,
         m.get_derived_as_base0_shared_ptr,
-        # *** FAILS *** m.get_derived_as_base0_unique_ptr,
+        m.get_derived_as_base0_unique_ptr,
     ],
 )
 def test_get_vec_size(get_fn, vec_size_fn):

From 3c45051450b80b9d1d196f78067cf373b1cbcf91 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sat, 10 Dec 2022 10:45:17 -0800
Subject: [PATCH 11/15] Rename: test_mi_debug -> test_class_sh_mi_thunks

---
 tests/CMakeLists.txt                               |  2 +-
 ...st_mi_debug.cpp => test_class_sh_mi_thunks.cpp} | 14 +++++++-------
 ...test_mi_debug.py => test_class_sh_mi_thunks.py} |  2 +-
 3 files changed, 9 insertions(+), 9 deletions(-)
 rename tests/{test_mi_debug.cpp => test_class_sh_mi_thunks.cpp} (85%)
 rename tests/{test_mi_debug.py => test_class_sh_mi_thunks.py} (95%)

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index fa185a2f8e..e6970739ff 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -127,6 +127,7 @@ set(PYBIND11_TEST_FILES
     test_class_sh_disowning_mi
     test_class_sh_factory_constructors
     test_class_sh_inheritance
+    test_class_sh_mi_thunks
     test_class_sh_module_local.py
     test_class_sh_property
     test_class_sh_shared_ptr_copy_move
@@ -158,7 +159,6 @@ set(PYBIND11_TEST_FILES
     test_kwargs_and_defaults
     test_local_bindings
     test_methods_and_attributes
-    test_mi_debug
     test_modules
     test_multiple_inheritance
     test_numpy_array
diff --git a/tests/test_mi_debug.cpp b/tests/test_class_sh_mi_thunks.cpp
similarity index 85%
rename from tests/test_mi_debug.cpp
rename to tests/test_class_sh_mi_thunks.cpp
index 354a8b0b6b..5d256a3263 100644
--- a/tests/test_mi_debug.cpp
+++ b/tests/test_class_sh_mi_thunks.cpp
@@ -7,7 +7,7 @@
 #include <memory>
 #include <vector>
 
-namespace test_mi_debug {
+namespace test_class_sh_mi_thunks {
 
 struct Base0 {
     virtual ~Base0() = default;
@@ -28,14 +28,14 @@ struct Derived : Base1, Base0 {
     Derived(const Derived &) = delete;
 };
 
-} // namespace test_mi_debug
+} // namespace test_class_sh_mi_thunks
 
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base0)
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Base1)
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_mi_debug::Derived)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Base0)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Base1)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Derived)
 
-TEST_SUBMODULE(mi_debug, m) {
-    using namespace test_mi_debug;
+TEST_SUBMODULE(class_sh_mi_thunks, m) {
+    using namespace test_class_sh_mi_thunks;
 
     py::classh<Base0> bs0(m, "Base0");
     py::classh<Base1> bs1(m, "Base1");
diff --git a/tests/test_mi_debug.py b/tests/test_class_sh_mi_thunks.py
similarity index 95%
rename from tests/test_mi_debug.py
rename to tests/test_class_sh_mi_thunks.py
index 06e96c5eaf..13a77dc8c1 100644
--- a/tests/test_mi_debug.py
+++ b/tests/test_class_sh_mi_thunks.py
@@ -1,6 +1,6 @@
 import pytest
 
-from pybind11_tests import mi_debug as m
+from pybind11_tests import class_sh_mi_thunks as m
 
 
 @pytest.mark.parametrize(

From 73a905235c72a5490022e2f6c3929fe9bc9bab54 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sat, 10 Dec 2022 14:02:54 -0800
Subject: [PATCH 12/15] Add `test_ptrdiff_derived_base0()`

---
 tests/test_class_sh_mi_thunks.cpp | 7 +++++++
 tests/test_class_sh_mi_thunks.py  | 9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/tests/test_class_sh_mi_thunks.cpp b/tests/test_class_sh_mi_thunks.cpp
index 5d256a3263..34ac1835fb 100644
--- a/tests/test_class_sh_mi_thunks.cpp
+++ b/tests/test_class_sh_mi_thunks.cpp
@@ -37,6 +37,13 @@ PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Derived)
 TEST_SUBMODULE(class_sh_mi_thunks, m) {
     using namespace test_class_sh_mi_thunks;
 
+    m.def("ptrdiff_derived_base0", []() {
+        auto drvd = std::unique_ptr<Derived>(new Derived{});
+        auto base0 = dynamic_cast<Base0 *>(drvd.get());
+        return std::ptrdiff_t(reinterpret_cast<char *>(drvd.get())
+                              - reinterpret_cast<char *>(base0));
+    });
+
     py::classh<Base0> bs0(m, "Base0");
     py::classh<Base1> bs1(m, "Base1");
     py::classh<Derived, Base1, Base0>(m, "Derived");
diff --git a/tests/test_class_sh_mi_thunks.py b/tests/test_class_sh_mi_thunks.py
index 13a77dc8c1..f11e94aa2a 100644
--- a/tests/test_class_sh_mi_thunks.py
+++ b/tests/test_class_sh_mi_thunks.py
@@ -3,6 +3,15 @@
 from pybind11_tests import class_sh_mi_thunks as m
 
 
+def test_ptrdiff_derived_base0():
+    ptrdiff = m.ptrdiff_derived_base0()
+    # A failure here does not (necessarily) mean that there is a bug, but that
+    # test_class_sh_mi_thunks is not exercising what it is supposed to.
+    # If this ever fails on some platforms: use pytest.skip()
+    # If this ever fails on all platforms: don't know, seems extremely unlikely.
+    assert ptrdiff != 0
+
+
 @pytest.mark.parametrize(
     "vec_size_fn",
     [

From f31dafae96d48cc3a5d856657fc1e21a4106a9c4 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sat, 10 Dec 2022 14:23:42 -0800
Subject: [PATCH 13/15] Miscellaneous polishing (naming, comments). No
 functional changes.

---
 tests/test_class_sh_mi_thunks.cpp | 45 +++++++++++++++++--------------
 tests/test_class_sh_mi_thunks.py  | 12 ++++-----
 2 files changed, 31 insertions(+), 26 deletions(-)

diff --git a/tests/test_class_sh_mi_thunks.cpp b/tests/test_class_sh_mi_thunks.cpp
index 34ac1835fb..b2e6e661a2 100644
--- a/tests/test_class_sh_mi_thunks.cpp
+++ b/tests/test_class_sh_mi_thunks.cpp
@@ -9,6 +9,10 @@
 
 namespace test_class_sh_mi_thunks {
 
+// For general background: https://shaharmike.com/cpp/vtable-part2/
+// C++ vtables - Part 2 - Multiple Inheritance
+// ... the compiler creates a 'thunk' method that corrects `this` ...
+
 struct Base0 {
     virtual ~Base0() = default;
     Base0() = default;
@@ -17,6 +21,7 @@ struct Base0 {
 
 struct Base1 {
     virtual ~Base1() = default;
+    // Using `vector` here because it is known to make this test very sensitive to bugs.
     std::vector<int> vec = {1, 2, 3, 4, 5};
     Base1() = default;
     Base1(const Base1 &) = delete;
@@ -37,42 +42,42 @@ PYBIND11_SMART_HOLDER_TYPE_CASTERS(test_class_sh_mi_thunks::Derived)
 TEST_SUBMODULE(class_sh_mi_thunks, m) {
     using namespace test_class_sh_mi_thunks;
 
-    m.def("ptrdiff_derived_base0", []() {
-        auto drvd = std::unique_ptr<Derived>(new Derived{});
+    m.def("ptrdiff_drvd_base0", []() {
+        auto drvd = std::unique_ptr<Derived>(new Derived);
         auto base0 = dynamic_cast<Base0 *>(drvd.get());
         return std::ptrdiff_t(reinterpret_cast<char *>(drvd.get())
                               - reinterpret_cast<char *>(base0));
     });
 
-    py::classh<Base0> bs0(m, "Base0");
-    py::classh<Base1> bs1(m, "Base1");
+    py::classh<Base0>(m, "Base0");
+    py::classh<Base1>(m, "Base1");
     py::classh<Derived, Base1, Base0>(m, "Derived");
 
     m.def(
-        "get_derived_as_base0_raw_ptr",
+        "get_drvd_as_base0_raw_ptr",
         []() {
-            auto *ret_der = new Derived{};
-            auto *ret = dynamic_cast<Base0 *>(ret_der);
-            return ret;
+            auto *drvd = new Derived;
+            auto *base0 = dynamic_cast<Base0 *>(drvd);
+            return base0;
         },
         py::return_value_policy::take_ownership);
 
-    m.def("get_derived_as_base0_shared_ptr", []() -> std::shared_ptr<Base0> {
-        auto ret_der = std::make_shared<Derived>();
-        auto ret = std::dynamic_pointer_cast<Base0>(ret_der);
-        return ret;
+    m.def("get_drvd_as_base0_shared_ptr", []() {
+        auto drvd = std::make_shared<Derived>();
+        auto base0 = std::dynamic_pointer_cast<Base0>(drvd);
+        return base0;
     });
 
-    m.def("get_derived_as_base0_unique_ptr", []() -> std::unique_ptr<Base0> {
-        auto ret_der = std::unique_ptr<Derived>(new Derived{});
-        auto ret = std::unique_ptr<Base0>(std::move(ret_der));
-        return ret;
+    m.def("get_drvd_as_base0_unique_ptr", []() {
+        auto drvd = std::unique_ptr<Derived>(new Derived);
+        auto base0 = std::unique_ptr<Base0>(std::move(drvd));
+        return base0;
     });
 
-    m.def("vec_size_base0_raw_ptr", [](const Base0 *obj) -> std::size_t {
+    m.def("vec_size_base0_raw_ptr", [](const Base0 *obj) {
         const auto *obj_der = dynamic_cast<const Derived *>(obj);
         if (obj_der == nullptr) {
-            return 0;
+            return std::size_t(0);
         }
         return obj_der->vec.size();
     });
@@ -80,7 +85,7 @@ TEST_SUBMODULE(class_sh_mi_thunks, m) {
     m.def("vec_size_base0_shared_ptr", [](const std::shared_ptr<Base0> &obj) -> std::size_t {
         const auto obj_der = std::dynamic_pointer_cast<Derived>(obj);
         if (!obj_der) {
-            return 0;
+            return std::size_t(0);
         }
         return obj_der->vec.size();
     });
@@ -88,7 +93,7 @@ TEST_SUBMODULE(class_sh_mi_thunks, m) {
     m.def("vec_size_base0_unique_ptr", [](std::unique_ptr<Base0> obj) -> std::size_t {
         const auto *obj_der = dynamic_cast<const Derived *>(obj.get());
         if (obj_der == nullptr) {
-            return 0;
+            return std::size_t(0);
         }
         return obj_der->vec.size();
     });
diff --git a/tests/test_class_sh_mi_thunks.py b/tests/test_class_sh_mi_thunks.py
index f11e94aa2a..9fec3c4b6f 100644
--- a/tests/test_class_sh_mi_thunks.py
+++ b/tests/test_class_sh_mi_thunks.py
@@ -3,8 +3,8 @@
 from pybind11_tests import class_sh_mi_thunks as m
 
 
-def test_ptrdiff_derived_base0():
-    ptrdiff = m.ptrdiff_derived_base0()
+def test_ptrdiff_drvd_base0():
+    ptrdiff = m.ptrdiff_drvd_base0()
     # A failure here does not (necessarily) mean that there is a bug, but that
     # test_class_sh_mi_thunks is not exercising what it is supposed to.
     # If this ever fails on some platforms: use pytest.skip()
@@ -23,15 +23,15 @@ def test_ptrdiff_derived_base0():
 @pytest.mark.parametrize(
     "get_fn",
     [
-        m.get_derived_as_base0_raw_ptr,
-        m.get_derived_as_base0_shared_ptr,
-        m.get_derived_as_base0_unique_ptr,
+        m.get_drvd_as_base0_raw_ptr,
+        m.get_drvd_as_base0_shared_ptr,
+        m.get_drvd_as_base0_unique_ptr,
     ],
 )
 def test_get_vec_size(get_fn, vec_size_fn):
     obj = get_fn()
     if (
-        get_fn is m.get_derived_as_base0_shared_ptr
+        get_fn is m.get_drvd_as_base0_shared_ptr
         and vec_size_fn is m.vec_size_base0_unique_ptr
     ):
         with pytest.raises(ValueError) as exc_info:

From 6512c02ed2aef16cd34ecd45e00a9b3a33acbd25 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sat, 10 Dec 2022 14:42:56 -0800
Subject: [PATCH 14/15] Improve test_class_sh_mi_thunks.py implementation. No
 change in test coverage.

---
 tests/test_class_sh_mi_thunks.py | 47 ++++++++++++++++++--------------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/tests/test_class_sh_mi_thunks.py b/tests/test_class_sh_mi_thunks.py
index 9fec3c4b6f..7d6e3849a9 100644
--- a/tests/test_class_sh_mi_thunks.py
+++ b/tests/test_class_sh_mi_thunks.py
@@ -17,7 +17,6 @@ def test_ptrdiff_drvd_base0():
     [
         m.vec_size_base0_raw_ptr,
         m.vec_size_base0_shared_ptr,
-        m.vec_size_base0_unique_ptr,
     ],
 )
 @pytest.mark.parametrize(
@@ -28,24 +27,30 @@ def test_ptrdiff_drvd_base0():
         m.get_drvd_as_base0_unique_ptr,
     ],
 )
-def test_get_vec_size(get_fn, vec_size_fn):
+def test_get_vec_size_raw_shared(get_fn, vec_size_fn):
     obj = get_fn()
-    if (
-        get_fn is m.get_drvd_as_base0_shared_ptr
-        and vec_size_fn is m.vec_size_base0_unique_ptr
-    ):
-        with pytest.raises(ValueError) as exc_info:
-            vec_size_fn(obj)
-        assert (
-            str(exc_info.value)
-            == "Cannot disown external shared_ptr (loaded_as_unique_ptr)."
-        )
-    else:
-        assert vec_size_fn(obj) == 5
-        if vec_size_fn is m.vec_size_base0_unique_ptr:
-            with pytest.raises(ValueError) as exc_info:
-                vec_size_fn(obj)
-            assert (
-                str(exc_info.value)
-                == "Missing value for wrapped C++ type: Python instance was disowned."
-            )
+    assert vec_size_fn(obj) == 5
+
+
+@pytest.mark.parametrize(
+    "get_fn", [m.get_drvd_as_base0_raw_ptr, m.get_drvd_as_base0_unique_ptr]
+)
+def test_get_vec_size_unique(get_fn):
+    obj = get_fn()
+    assert m.vec_size_base0_unique_ptr(obj) == 5
+    with pytest.raises(ValueError) as exc_info:
+        m.vec_size_base0_unique_ptr(obj)
+    assert (
+        str(exc_info.value)
+        == "Missing value for wrapped C++ type: Python instance was disowned."
+    )
+
+
+def test_get_shared_vec_size_unique():
+    obj = m.get_drvd_as_base0_shared_ptr()
+    with pytest.raises(ValueError) as exc_info:
+        m.vec_size_base0_unique_ptr(obj)
+    assert (
+        str(exc_info.value)
+        == "Cannot disown external shared_ptr (loaded_as_unique_ptr)."
+    )

From c1b2b6c54cbeae22590ef2368808511c43bf78bd Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Sat, 10 Dec 2022 15:06:47 -0800
Subject: [PATCH 15/15] Resolve clang-tidy error.

---
 tests/test_class_sh_mi_thunks.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_class_sh_mi_thunks.cpp b/tests/test_class_sh_mi_thunks.cpp
index b2e6e661a2..c4f430e864 100644
--- a/tests/test_class_sh_mi_thunks.cpp
+++ b/tests/test_class_sh_mi_thunks.cpp
@@ -44,7 +44,7 @@ TEST_SUBMODULE(class_sh_mi_thunks, m) {
 
     m.def("ptrdiff_drvd_base0", []() {
         auto drvd = std::unique_ptr<Derived>(new Derived);
-        auto base0 = dynamic_cast<Base0 *>(drvd.get());
+        auto *base0 = dynamic_cast<Base0 *>(drvd.get());
         return std::ptrdiff_t(reinterpret_cast<char *>(drvd.get())
                               - reinterpret_cast<char *>(base0));
     });