Eliminate cross-DSO RTTI reliance in `smart_holder` functionality (for platforms like macOS). (#5728)

* Revert PR #5700 production code change (pybind11/detail/struct_smart_holder.h).

```
git checkout b19489145b2c7a117138632d624809dfb3b380bb~1 include/pybind11/detail/struct_smart_holder.h
```

* Introduce `get_internals().get_memory_guarded_delete()`

* [skip ci] Only pass around `memory::get_guarded_delete` function pointer.

* [skip ci] Change a variable name for internal consistency. Add 3 x NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.

* Add comment: get_internals().get_memory_guarded_delete does not need with_internals()

* Traverse all DSOs to find memory::guarded_delete with matching RTTI.

* Add nullptr check to dynamic_cast overload.

Suggested by ChatGPT for these reasons:

* Prevents runtime RTTI lookups on nullptr.

* Helps avoid undefined behavior if users pass in nulls from failed casts or optional paths.

* Ensures consistent return value semantics and no accidental access to vtable.

* Improve smart_holder unique_ptr deleter compatibility checks across DSOs:

* Replace RTTI-based detection of std::default_delete<T> with a constexpr check to avoid RTTI reliance

* Add type_info_equal_across_dso_boundaries() fallback using type_info::name() for RTTI equality across macOS DSOs

* Rename related flags and functions for clarity (e.g., builtin → std_default)

* Improves ABI robustness and clarity of ownership checks in smart_holder

* Trivial renaming for internal consistency: builtin_delete → std_default_delete

* Add get_trampoline_self_life_support to detail::type_info (passes local testing).

* Polish previous commit slightly.

* [skip ci] Store memory::get_guarded_delete in `detail::type_info` instead of `detail::internals` (no searching across DSOs required).

* Revert change suggested by ChatGPT. After double-checking, ChatGPT agrees this isn't needed.

* Minor polishing.

include/pybind11/attr.h

@@ -12,6 +12,7 @@
 
 #include "detail/common.h"
 #include "cast.h"
+#include "trampoline_self_life_support.h"
 
 #include <functional>
 
@@ -312,6 +313,12 @@ struct type_record {
     /// Function pointer to class_<..>::dealloc
     void (*dealloc)(detail::value_and_holder &) = nullptr;
 
+    /// Function pointer for casting alias class (aka trampoline) pointer to
+    /// trampoline_self_life_support pointer. Sidesteps cross-DSO RTTI issues
+    /// on platforms like macOS (see PR #5728 for details).
+    get_trampoline_self_life_support_fn get_trampoline_self_life_support
+        = [](void *) -> trampoline_self_life_support * { return nullptr; };
+
     /// List of base classes of the newly created type
     list bases;
 

include/pybind11/cast.h

@@ -980,7 +980,7 @@ public:
 
     explicit operator std::shared_ptr<type> &() {
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
-            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(value);
+            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(typeinfo, value);
         }
         return shared_ptr_storage;
     }
@@ -989,7 +989,8 @@ public:
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
             // Reusing shared_ptr code to minimize code complexity.
             shared_ptr_storage
-                = sh_load_helper.load_as_shared_ptr(value,
+                = sh_load_helper.load_as_shared_ptr(typeinfo,
+                                                    value,
                                                     /*responsible_parent=*/nullptr,
                                                     /*force_potentially_slicing_shared_ptr=*/true);
         }
@@ -1019,7 +1020,8 @@ public:
         copyable_holder_caster loader;
         loader.load(responsible_parent, /*convert=*/false);
         assert(loader.typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder);
-        return loader.sh_load_helper.load_as_shared_ptr(loader.value, responsible_parent);
+        return loader.sh_load_helper.load_as_shared_ptr(
+            loader.typeinfo, loader.value, responsible_parent);
     }
 
 protected:
@@ -1240,7 +1242,7 @@ public:
 
     explicit operator std::unique_ptr<type, deleter>() {
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
-            return sh_load_helper.template load_as_unique_ptr<deleter>(value);
+            return sh_load_helper.template load_as_unique_ptr<deleter>(typeinfo, value);
         }
         pybind11_fail("Expected to be UNREACHABLE: " __FILE__ ":" PYBIND11_TOSTRING(__LINE__));
     }
@@ -1248,12 +1250,12 @@ public:
     explicit operator const std::unique_ptr<type, deleter> &() {
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
             // Get shared_ptr to ensure that the Python object is not disowned elsewhere.
-            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(value);
+            shared_ptr_storage = sh_load_helper.load_as_shared_ptr(typeinfo, value);
             // Build a temporary unique_ptr that is meant to never expire.
             unique_ptr_storage = std::shared_ptr<std::unique_ptr<type, deleter>>(
                 new std::unique_ptr<type, deleter>{
                     sh_load_helper.template load_as_const_unique_ptr<deleter>(
-                        shared_ptr_storage.get())},
+                        typeinfo, shared_ptr_storage.get())},
                 [](std::unique_ptr<type, deleter> *ptr) {
                     if (!ptr) {
                         pybind11_fail("FATAL: `const std::unique_ptr<T, D> &` was disowned "

include/pybind11/detail/internals.h

@@ -12,8 +12,10 @@
 #include <pybind11/conduit/pybind11_platform_abi_id.h>
 #include <pybind11/gil_simple.h>
 #include <pybind11/pytypes.h>
+#include <pybind11/trampoline_self_life_support.h>
 
 #include "common.h"
+#include "struct_smart_holder.h"
 
 #include <atomic>
 #include <exception>
@@ -35,11 +37,11 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 10
+#    define PYBIND11_INTERNALS_VERSION 11
 #endif
 
-#if PYBIND11_INTERNALS_VERSION < 10
-#    error "PYBIND11_INTERNALS_VERSION 10 is the minimum for all platforms for pybind11v3."
+#if PYBIND11_INTERNALS_VERSION < 11
+#    error "PYBIND11_INTERNALS_VERSION 11 is the minimum for all platforms for pybind11v3."
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -308,6 +310,12 @@ struct type_info {
     void *(*operator_new)(size_t);
     void (*init_instance)(instance *, const void *);
     void (*dealloc)(value_and_holder &v_h);
+
+    // Cross-DSO-safe function pointers, to sidestep cross-DSO RTTI issues
+    // on platforms like macOS (see PR #5728 for details):
+    memory::get_guarded_delete_fn get_memory_guarded_delete = memory::get_guarded_delete;
+    get_trampoline_self_life_support_fn get_trampoline_self_life_support = nullptr;
+
     std::vector<PyObject *(*) (PyObject *, PyTypeObject *)> implicit_conversions;
     std::vector<std::pair<const std::type_info *, void *(*) (void *)>> implicit_casts;
     std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;

include/pybind11/detail/struct_smart_holder.h

@@ -50,6 +50,7 @@ Details:
 
 #include "pybind11_namespace_macros.h"
 
+#include <cstring>
 #include <functional>
 #include <memory>
 #include <stdexcept>
@@ -58,19 +59,6 @@ Details:
 #include <typeinfo>
 #include <utility>
 
-// IMPORTANT: This code block must stay BELOW the #include <stdexcept> above.
-// This is only required on some builds with libc++ (one of three implementations
-// in
-// https://github.com/llvm/llvm-project/blob/a9b64bb3180dab6d28bf800a641f9a9ad54d2c0c/libcxx/include/typeinfo#L271-L276
-// require it)
-#if !defined(PYBIND11_EXPORT_GUARDED_DELETE)
-#    if defined(_LIBCPP_VERSION) && !defined(WIN32) && !defined(_WIN32)
-#        define PYBIND11_EXPORT_GUARDED_DELETE __attribute__((visibility("default")))
-#    else
-#        define PYBIND11_EXPORT_GUARDED_DELETE
-#    endif
-#endif
-
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(memory)
 
@@ -91,7 +79,8 @@ static constexpr bool type_has_shared_from_this(const void *) {
     return false;
 }
 
-struct PYBIND11_EXPORT_GUARDED_DELETE guarded_delete {
+struct guarded_delete {
+    // NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.
     std::weak_ptr<void> released_ptr;    // Trick to keep the smart_holder memory footprint small.
     std::function<void(void *)> del_fun; // Rare case.
     void (*del_ptr)(void *);             // Common case.
@@ -113,13 +102,19 @@ struct PYBIND11_EXPORT_GUARDED_DELETE guarded_delete {
     }
 };
 
+inline guarded_delete *get_guarded_delete(const std::shared_ptr<void> &ptr) {
+    return std::get_deleter<guarded_delete>(ptr);
+}
+
+using get_guarded_delete_fn = guarded_delete *(*) (const std::shared_ptr<void> &);
+
 template <typename T, typename std::enable_if<std::is_destructible<T>::value, int>::type = 0>
-inline void builtin_delete_if_destructible(void *raw_ptr) {
+inline void std_default_delete_if_destructible(void *raw_ptr) {
     std::default_delete<T>{}(static_cast<T *>(raw_ptr));
 }
 
 template <typename T, typename std::enable_if<!std::is_destructible<T>::value, int>::type = 0>
-inline void builtin_delete_if_destructible(void *) {
+inline void std_default_delete_if_destructible(void *) {
     // This noop operator is needed to avoid a compilation error (for `delete raw_ptr;`), but
     // throwing an exception from a destructor will std::terminate the process. Therefore the
     // runtime check for lifetime-management correctness is implemented elsewhere (in
@@ -127,12 +122,13 @@ inline void builtin_delete_if_destructible(void *) {
 }
 
 template <typename T>
-guarded_delete make_guarded_builtin_delete(bool armed_flag) {
-    return guarded_delete(builtin_delete_if_destructible<T>, armed_flag);
+guarded_delete make_guarded_std_default_delete(bool armed_flag) {
+    return guarded_delete(std_default_delete_if_destructible<T>, armed_flag);
 }
 
 template <typename T, typename D>
 struct custom_deleter {
+    // NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.
     D deleter;
     explicit custom_deleter(D &&deleter) : deleter{std::forward<D>(deleter)} {}
     void operator()(void *raw_ptr) { deleter(static_cast<T *>(raw_ptr)); }
@@ -144,17 +140,25 @@ guarded_delete make_guarded_custom_deleter(D &&uqp_del, bool armed_flag) {
         std::function<void(void *)>(custom_deleter<T, D>(std::forward<D>(uqp_del))), armed_flag);
 }
 
-template <typename T>
-inline bool is_std_default_delete(const std::type_info &rtti_deleter) {
-    return rtti_deleter == typeid(std::default_delete<T>)
-           || rtti_deleter == typeid(std::default_delete<T const>);
+template <typename T, typename D>
+constexpr bool uqp_del_is_std_default_delete() {
+    return std::is_same<D, std::default_delete<T>>::value
+           || std::is_same<D, std::default_delete<T const>>::value;
+}
+
+inline bool type_info_equal_across_dso_boundaries(const std::type_info &a,
+                                                  const std::type_info &b) {
+    // RTTI pointer comparison may fail across DSOs (e.g., macOS libc++).
+    // Fallback to name comparison, which is generally safe and ABI-stable enough for our use.
+    return a == b || std::strcmp(a.name(), b.name()) == 0;
 }
 
 struct smart_holder {
+    // NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.
     const std::type_info *rtti_uqp_del = nullptr;
     std::shared_ptr<void> vptr;
     bool vptr_is_using_noop_deleter : 1;
-    bool vptr_is_using_builtin_delete : 1;
+    bool vptr_is_using_std_default_delete : 1;
     bool vptr_is_external_shared_ptr : 1;
     bool is_populated : 1;
     bool is_disowned : 1;
@@ -166,7 +170,7 @@ struct smart_holder {
     smart_holder &operator=(const smart_holder &) = delete;
 
     smart_holder()
-        : vptr_is_using_noop_deleter{false}, vptr_is_using_builtin_delete{false},
+        : vptr_is_using_noop_deleter{false}, vptr_is_using_std_default_delete{false},
           vptr_is_external_shared_ptr{false}, is_populated{false}, is_disowned{false} {}
 
     bool has_pointee() const { return vptr != nullptr; }
@@ -191,7 +195,7 @@ struct smart_holder {
         }
     }
 
-    void ensure_vptr_is_using_builtin_delete(const char *context) const {
+    void ensure_vptr_is_using_std_default_delete(const char *context) const {
         if (vptr_is_external_shared_ptr) {
             throw std::invalid_argument(std::string("Cannot disown external shared_ptr (")
                                         + context + ").");
@@ -200,24 +204,26 @@ struct smart_holder {
             throw std::invalid_argument(std::string("Cannot disown non-owning holder (") + context
                                         + ").");
         }
-        if (!vptr_is_using_builtin_delete) {
+        if (!vptr_is_using_std_default_delete) {
             throw std::invalid_argument(std::string("Cannot disown custom deleter (") + context
                                         + ").");
         }
     }
 
     template <typename T, typename D>
-    void ensure_compatible_rtti_uqp_del(const char *context) const {
-        const std::type_info *rtti_requested = &typeid(D);
+    void ensure_compatible_uqp_del(const char *context) const {
         if (!rtti_uqp_del) {
-            if (!is_std_default_delete<T>(*rtti_requested)) {
+            if (!uqp_del_is_std_default_delete<T, D>()) {
                 throw std::invalid_argument(std::string("Missing unique_ptr deleter (") + context
                                             + ").");
             }
-            ensure_vptr_is_using_builtin_delete(context);
-        } else if (!(*rtti_requested == *rtti_uqp_del)
-                   && !(vptr_is_using_builtin_delete
-                        && is_std_default_delete<T>(*rtti_requested))) {
+            ensure_vptr_is_using_std_default_delete(context);
+            return;
+        }
+        if (uqp_del_is_std_default_delete<T, D>() && vptr_is_using_std_default_delete) {
+            return;
+        }
+        if (!type_info_equal_across_dso_boundaries(typeid(D), *rtti_uqp_del)) {
             throw std::invalid_argument(std::string("Incompatible unique_ptr deleter (") + context
                                         + ").");
         }
@@ -244,19 +250,20 @@ struct smart_holder {
         }
     }
 
-    void reset_vptr_deleter_armed_flag(bool armed_flag) const {
-        auto *vptr_del_ptr = std::get_deleter<guarded_delete>(vptr);
-        if (vptr_del_ptr == nullptr) {
+    void reset_vptr_deleter_armed_flag(const get_guarded_delete_fn ggd_fn, bool armed_flag) const {
+        auto *gd = ggd_fn(vptr);
+        if (gd == nullptr) {
             throw std::runtime_error(
                 "smart_holder::reset_vptr_deleter_armed_flag() called in an invalid context.");
         }
-        vptr_del_ptr->armed_flag = armed_flag;
+        gd->armed_flag = armed_flag;
     }
 
-    // Caller is responsible for precondition: ensure_compatible_rtti_uqp_del<T, D>() must succeed.
+    // Caller is responsible for precondition: ensure_compatible_uqp_del<T, D>() must succeed.
     template <typename T, typename D>
-    std::unique_ptr<D> extract_deleter(const char *context) const {
-        const auto *gd = std::get_deleter<guarded_delete>(vptr);
+    std::unique_ptr<D> extract_deleter(const char *context,
+                                       const get_guarded_delete_fn ggd_fn) const {
+        auto *gd = ggd_fn(vptr);
         if (gd && gd->use_del_fun) {
             const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter<T, D>>();
             if (custom_deleter_ptr == nullptr) {
@@ -288,28 +295,28 @@ struct smart_holder {
     static smart_holder from_raw_ptr_take_ownership(T *raw_ptr, bool void_cast_raw_ptr = false) {
         ensure_pointee_is_destructible<T>("from_raw_ptr_take_ownership");
         smart_holder hld;
-        auto gd = make_guarded_builtin_delete<T>(true);
+        auto gd = make_guarded_std_default_delete<T>(true);
         if (void_cast_raw_ptr) {
             hld.vptr.reset(static_cast<void *>(raw_ptr), std::move(gd));
         } else {
             hld.vptr.reset(raw_ptr, std::move(gd));
         }
-        hld.vptr_is_using_builtin_delete = true;
+        hld.vptr_is_using_std_default_delete = true;
         hld.is_populated = true;
         return hld;
     }
 
     // Caller is responsible for ensuring the complex preconditions
     // (see `smart_holder_type_caster_support::load_helper`).
-    void disown() {
-        reset_vptr_deleter_armed_flag(false);
+    void disown(const get_guarded_delete_fn ggd_fn) {
+        reset_vptr_deleter_armed_flag(ggd_fn, false);
         is_disowned = true;
     }
 
     // Caller is responsible for ensuring the complex preconditions
     // (see `smart_holder_type_caster_support::load_helper`).
-    void reclaim_disowned() {
-        reset_vptr_deleter_armed_flag(true);
+    void reclaim_disowned(const get_guarded_delete_fn ggd_fn) {
+        reset_vptr_deleter_armed_flag(ggd_fn, true);
         is_disowned = false;
     }
 
@@ -319,14 +326,14 @@ struct smart_holder {
 
     void ensure_can_release_ownership(const char *context = "ensure_can_release_ownership") const {
         ensure_is_not_disowned(context);
-        ensure_vptr_is_using_builtin_delete(context);
+        ensure_vptr_is_using_std_default_delete(context);
         ensure_use_count_1(context);
     }
 
     // Caller is responsible for ensuring the complex preconditions
     // (see `smart_holder_type_caster_support::load_helper`).
-    void release_ownership() {
-        reset_vptr_deleter_armed_flag(false);
+    void release_ownership(const get_guarded_delete_fn ggd_fn) {
+        reset_vptr_deleter_armed_flag(ggd_fn, false);
         release_disowned();
     }
 
@@ -335,10 +342,10 @@ struct smart_holder {
                                         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);
+        hld.vptr_is_using_std_default_delete = uqp_del_is_std_default_delete<T, D>();
         guarded_delete gd{nullptr, false};
-        if (hld.vptr_is_using_builtin_delete) {
-            gd = make_guarded_builtin_delete<T>(true);
+        if (hld.vptr_is_using_std_default_delete) {
+            gd = make_guarded_std_default_delete<T>(true);
         } else {
             gd = make_guarded_custom_deleter<T, D>(std::move(unq_ptr.get_deleter()), true);
         }

include/pybind11/detail/type_caster_base.h

@@ -531,8 +531,8 @@ struct value_and_holder_helper {
     }
 
     // have_holder() must be true or this function will fail.
-    void throw_if_instance_is_currently_owned_by_shared_ptr() const {
-        auto *vptr_gd_ptr = std::get_deleter<memory::guarded_delete>(holder().vptr);
+    void throw_if_instance_is_currently_owned_by_shared_ptr(const type_info *tinfo) const {
+        auto *vptr_gd_ptr = tinfo->get_memory_guarded_delete(holder().vptr);
         if (vptr_gd_ptr != nullptr && !vptr_gd_ptr->released_ptr.expired()) {
             throw value_error("Python instance is currently owned by a std::shared_ptr.");
         }
@@ -564,8 +564,7 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
     assert(st.second != nullptr);
     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.get());
+        auto *self_life_support = tinfo->get_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) {
@@ -576,7 +575,7 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
                 }
                 // Critical transfer-of-ownership section. This must stay together.
                 self_life_support->deactivate_life_support();
-                holder.reclaim_disowned();
+                holder.reclaim_disowned(tinfo->get_memory_guarded_delete);
                 (void) src.release();
                 // Critical section end.
                 return existing_inst;
@@ -742,7 +741,8 @@ struct load_helper : value_and_holder_helper {
         return std::shared_ptr<T>(raw_ptr, shared_ptr_parent_life_support(parent.ptr()));
     }
 
-    std::shared_ptr<T> load_as_shared_ptr(void *void_raw_ptr,
+    std::shared_ptr<T> load_as_shared_ptr(const type_info *tinfo,
+                                          void *void_raw_ptr,
                                           handle responsible_parent = nullptr,
                                           // to support py::potentially_slicing_weak_ptr
                                           // with minimal added code complexity:
@@ -763,7 +763,7 @@ struct load_helper : value_and_holder_helper {
         }
         auto *type_raw_ptr = static_cast<T *>(void_raw_ptr);
         if (python_instance_is_alias && !force_potentially_slicing_shared_ptr) {
-            auto *vptr_gd_ptr = std::get_deleter<memory::guarded_delete>(hld.vptr);
+            auto *vptr_gd_ptr = tinfo->get_memory_guarded_delete(holder().vptr);
             if (vptr_gd_ptr != nullptr) {
                 std::shared_ptr<void> released_ptr = vptr_gd_ptr->released_ptr.lock();
                 if (released_ptr) {
@@ -800,31 +800,32 @@ struct load_helper : value_and_holder_helper {
     }
 
     template <typename D>
-    std::unique_ptr<T, D> load_as_unique_ptr(void *raw_void_ptr,
+    std::unique_ptr<T, D> load_as_unique_ptr(const type_info *tinfo,
+                                             void *raw_void_ptr,
                                              const char *context = "load_as_unique_ptr") {
         if (!have_holder()) {
             return unique_with_deleter<T, D>(nullptr, std::unique_ptr<D>());
         }
         throw_if_uninitialized_or_disowned_holder(typeid(T));
-        throw_if_instance_is_currently_owned_by_shared_ptr();
+        throw_if_instance_is_currently_owned_by_shared_ptr(tinfo);
         holder().ensure_is_not_disowned(context);
-        holder().template ensure_compatible_rtti_uqp_del<T, D>(context);
+        holder().template ensure_compatible_uqp_del<T, D>(context);
         holder().ensure_use_count_1(context);
 
         T *raw_type_ptr = static_cast<T *>(raw_void_ptr);
 
-        auto *self_life_support
-            = dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(raw_type_ptr);
+        auto *self_life_support = tinfo->get_trampoline_self_life_support(raw_type_ptr);
         // This is enforced indirectly by a static_assert in the class_ implementation:
         assert(!python_instance_is_alias || self_life_support);
 
-        std::unique_ptr<D> extracted_deleter = holder().template extract_deleter<T, D>(context);
+        std::unique_ptr<D> extracted_deleter
+            = holder().template extract_deleter<T, D>(context, tinfo->get_memory_guarded_delete);
 
         // Critical transfer-of-ownership section. This must stay together.
         if (self_life_support != nullptr) {
-            holder().disown();
+            holder().disown(tinfo->get_memory_guarded_delete);
         } else {
-            holder().release_ownership();
+            holder().release_ownership(tinfo->get_memory_guarded_delete);
         }
         auto result = unique_with_deleter<T, D>(raw_type_ptr, std::move(extracted_deleter));
         if (self_life_support != nullptr) {
@@ -842,14 +843,17 @@ struct load_helper : value_and_holder_helper {
     // This assumes load_as_shared_ptr succeeded(), and the returned shared_ptr is still alive.
     // The returned unique_ptr is meant to never expire (the behavior is undefined otherwise).
     template <typename D>
-    std::unique_ptr<T, D>
-    load_as_const_unique_ptr(T *raw_type_ptr, const char *context = "load_as_const_unique_ptr") {
+    std::unique_ptr<T, D> load_as_const_unique_ptr(const type_info *tinfo,
+                                                   T *raw_type_ptr,
+                                                   const char *context
+                                                   = "load_as_const_unique_ptr") {
         if (!have_holder()) {
             return unique_with_deleter<T, D>(nullptr, std::unique_ptr<D>());
         }
-        holder().template ensure_compatible_rtti_uqp_del<T, D>(context);
-        return unique_with_deleter<T, D>(
-            raw_type_ptr, std::move(holder().template extract_deleter<T, D>(context)));
+        holder().template ensure_compatible_uqp_del<T, D>(context);
+        return unique_with_deleter<T, D>(raw_type_ptr,
+                                         std::move(holder().template extract_deleter<T, D>(
+                                             context, tinfo->get_memory_guarded_delete)));
     }
 };
 

include/pybind11/pybind11.h

@@ -1571,6 +1571,7 @@ protected:
         tinfo->holder_size_in_ptrs = size_in_ptrs(rec.holder_size);
         tinfo->init_instance = rec.init_instance;
         tinfo->dealloc = rec.dealloc;
+        tinfo->get_trampoline_self_life_support = rec.get_trampoline_self_life_support;
         tinfo->simple_type = true;
         tinfo->simple_ancestors = true;
         tinfo->module_local = rec.module_local;
@@ -2066,6 +2067,16 @@ public:
             record.dealloc = dealloc_without_manipulating_gil;
         }
 
+        if (std::is_base_of<trampoline_self_life_support, type_alias>::value) {
+            // Store a cross-DSO-safe getter.
+            // This lambda is defined in the same DSO that instantiates
+            // class_<type, alias_type>, but it can be called safely from any other DSO.
+            record.get_trampoline_self_life_support = [](void *type_ptr) {
+                return dynamic_raw_ptr_cast_if_possible<trampoline_self_life_support>(
+                    static_cast<type *>(type_ptr));
+            };
+        }
+
         generic_type::initialize(record);
 
         if (has_alias) {

include/pybind11/trampoline_self_life_support.h

@@ -19,6 +19,7 @@ PYBIND11_NAMESPACE_END(detail)
 // https://github.com/google/clif/blob/07f95d7e69dca2fcf7022978a55ef3acff506c19/clif/python/runtime.cc#L37
 // URL provided here mainly to give proper credit.
 struct trampoline_self_life_support {
+    // NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.
     detail::value_and_holder v_h;
 
     trampoline_self_life_support() = default;
@@ -57,4 +58,8 @@ struct trampoline_self_life_support {
     trampoline_self_life_support &operator=(trampoline_self_life_support &&) = delete;
 };
 
+PYBIND11_NAMESPACE_BEGIN(detail)
+using get_trampoline_self_life_support_fn = trampoline_self_life_support *(*) (void *);
+PYBIND11_NAMESPACE_END(detail)
+
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

tests/pure_cpp/smart_holder_poc.h

@@ -36,17 +36,17 @@ T *as_raw_ptr_release_ownership(smart_holder &hld,
                                 const char *context = "as_raw_ptr_release_ownership") {
     hld.ensure_can_release_ownership(context);
     T *raw_ptr = hld.as_raw_ptr_unowned<T>();
-    hld.release_ownership();
+    hld.release_ownership(get_guarded_delete);
     return raw_ptr;
 }
 
 template <typename T, typename D = std::default_delete<T>>
 std::unique_ptr<T, D> as_unique_ptr(smart_holder &hld) {
     static const char *context = "as_unique_ptr";
-    hld.ensure_compatible_rtti_uqp_del<T, D>(context);
+    hld.ensure_compatible_uqp_del<T, D>(context);
     hld.ensure_use_count_1(context);
     T *raw_ptr = hld.as_raw_ptr_unowned<T>();
-    hld.release_ownership();
+    hld.release_ownership(get_guarded_delete);
     // KNOWN DEFECT (see PR #4850): Does not copy the deleter.
     return std::unique_ptr<T, D>(raw_ptr);
 }

tests/pure_cpp/smart_holder_poc_test.cpp

@@ -14,6 +14,7 @@
 #define CATCH_CONFIG_MAIN
 #include "catch.hpp"
 
+using pybind11::memory::guarded_delete;
 using pybind11::memory::smart_holder;
 namespace poc = pybind11::memory::smart_holder_poc;
 
@@ -160,11 +161,12 @@ TEST_CASE("from_raw_ptr_take_ownership+as_shared_ptr", "[S]") {
 TEST_CASE("from_raw_ptr_take_ownership+disown+reclaim_disowned", "[S]") {
     auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
     std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
-    hld.disown();
+    hld.disown(pybind11::memory::get_guarded_delete);
     REQUIRE(poc::as_lvalue_ref<int>(hld) == 19);
     REQUIRE(*new_owner == 19);
-    hld.reclaim_disowned(); // Manually veriified: without this, clang++ -fsanitize=address reports
-                            // "detected memory leaks".
+    // Manually verified: without this, clang++ -fsanitize=address reports
+    // "detected memory leaks".
+    hld.reclaim_disowned(pybind11::memory::get_guarded_delete);
     // NOLINTNEXTLINE(bugprone-unused-return-value)
     (void) new_owner.release(); // Manually verified: without this, clang++ -fsanitize=address
                                 // reports "attempting double-free".
@@ -175,7 +177,7 @@ TEST_CASE("from_raw_ptr_take_ownership+disown+reclaim_disowned", "[S]") {
 TEST_CASE("from_raw_ptr_take_ownership+disown+release_disowned", "[S]") {
     auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
     std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
-    hld.disown();
+    hld.disown(pybind11::memory::get_guarded_delete);
     REQUIRE(poc::as_lvalue_ref<int>(hld) == 19);
     REQUIRE(*new_owner == 19);
     hld.release_disowned();
@@ -187,7 +189,7 @@ TEST_CASE("from_raw_ptr_take_ownership+disown+ensure_is_not_disowned", "[E]") {
     auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
     hld.ensure_is_not_disowned(context); // Does not throw.
     std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
-    hld.disown();
+    hld.disown(pybind11::memory::get_guarded_delete);
     REQUIRE_THROWS_WITH(hld.ensure_is_not_disowned(context),
                         "Holder was disowned already (test_case).");
 }