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)));
```

Author: rwgk

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;

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();
@@ -642,6 +644,7 @@ 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);
         return cast_const_raw_ptr( // Originally type_caster_generic::cast.
             st.first,
@@ -688,6 +691,7 @@ 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();
         }
@@ -780,6 +784,7 @@ 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:
@@ -821,8 +826,10 @@ 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) {

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