pybind
diff --git a/‎.codespell-ignore-lines
+11 b/‎.codespell-ignore-lines
+11
diff --git a/‎.github/workflows/ci.yml
+14 b/‎.github/workflows/ci.yml
+14
diff --git a/‎.pre-commit-config.yaml
+2 b/‎.pre-commit-config.yaml
+2
diff --git a/‎CMakeLists.txt
+4 b/‎CMakeLists.txt
+4
diff --git a/‎README.rst
+29-6 b/‎README.rst
+29-6
diff --git a/‎docs/advanced/cast/stl.rst
+2-2 b/‎docs/advanced/cast/stl.rst
+2-2
diff --git a/‎docs/advanced/classes.rst
+44-29 b/‎docs/advanced/classes.rst
+44-29
@@ -12,6 +12,17 @@ template <op_id id, op_type ot, typename L, typename R>
     template <detail::op_id id, detail::op_type ot, typename L, typename R, typename... Extra>
     class_ &def(const detail::op_<id, ot, L, R> &op, const Extra &...extra) {
     class_ &def_cast(const detail::op_<id, ot, L, R> &op, const Extra &...extra) {
+    int valu;
+    explicit movable_int(int v) : valu{v} {}
+    movable_int(movable_int &&other) noexcept : valu(other.valu) { other.valu = 91; }
+    explicit indestructible_int(int v) : valu{v} {}
+    REQUIRE(hld.as_raw_ptr_unowned<zombie>()->valu == 19);
+        REQUIRE(othr.valu == 19);
+        REQUIRE(orig.valu == 91);
+        (m.pass_valu, "Valu", "pass_valu:Valu(_MvCtor)*_CpCtor"),
+atyp_valu        rtrn_valu() { atyp_valu obj{"Valu"}; return obj; }
+    assert m.atyp_valu().get_mtxt() == "Valu"
+// valu(e), ref(erence), ptr or p (pointer), r = rvalue, m = mutable, c = const,
 @pytest.mark.parametrize("access", ["ro", "rw", "static_ro", "static_rw"])
 struct IntStruct {
     explicit IntStruct(int v) : value(v){};
 
@@ -87,6 +87,20 @@ jobs:
           # Extra ubuntu latest job
           - runs-on: ubuntu-latest
             python: '3.11'
+          # Run tests with py::smart_holder as the default holder
+          # with recent (or ideally latest) released Python version.
+          - runs-on: ubuntu-latest
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE"
+          - runs-on: macos-13
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="-DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE"
+          - runs-on: windows-2022
+            python: '3.12'
+            args: >
+              -DCMAKE_CXX_FLAGS="/DPYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE /GR /EHsc"
         exclude:
           # The setup-python action currently doesn't have graalpy for windows
           - python: 'graalpy-24.1'
 
@@ -76,6 +76,7 @@ repos:
   - id: mixed-line-ending
   - id: requirements-txt-fixer
   - id: trailing-whitespace
+    exclude: \.patch?$
 
 # Also code format the docs
 - repo: https://github.com/adamchainz/blacken-docs
@@ -90,6 +91,7 @@ repos:
   rev: "v1.5.5"
   hooks:
   - id: remove-tabs
+    exclude: (^docs/.*|\.patch)?$
 
 # Avoid directional quotes
 - repo: https://github.com/sirosen/texthooks
 
@@ -131,10 +131,13 @@ set(PYBIND11_HEADERS
     include/pybind11/detail/common.h
     include/pybind11/detail/cpp_conduit.h
     include/pybind11/detail/descr.h
+    include/pybind11/detail/dynamic_raw_ptr_cast_if_possible.h
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h
+    include/pybind11/detail/struct_smart_holder.h
     include/pybind11/detail/type_caster_base.h
     include/pybind11/detail/typeid.h
+    include/pybind11/detail/using_smart_holder.h
     include/pybind11/detail/value_and_holder.h
     include/pybind11/detail/exception_translation.h
     include/pybind11/attr.h
@@ -164,6 +167,7 @@ set(PYBIND11_HEADERS
     include/pybind11/stl.h
     include/pybind11/stl_bind.h
     include/pybind11/stl/filesystem.h
+    include/pybind11/trampoline_self_life_support.h
     include/pybind11/type_caster_pyobject_ptr.h
     include/pybind11/typing.h
     include/pybind11/warnings.h)
 
@@ -1,7 +1,7 @@
 .. figure:: https://github.com/pybind/pybind11/raw/master/docs/pybind11-logo.png
    :alt: pybind11 logo
 
-**pybind11 — Seamless operability between C++11 and Python**
+**pybind11 (v3)  — Seamless interoperability between C++ and Python**
 
 |Latest Documentation Status| |Stable Documentation Status| |Gitter chat| |GitHub Discussions| |CI| |Build status|
 
@@ -134,11 +134,34 @@ About
 
 This project was created by `Wenzel
 Jakob <http://rgl.epfl.ch/people/wjakob>`_. Significant features and/or
-improvements to the code were contributed by Jonas Adler, Lori A. Burns,
-Sylvain Corlay, Eric Cousineau, Aaron Gokaslan, Ralf Grosse-Kunstleve, Trent Houliston, Axel
-Huebl, @hulucc, Yannick Jadoul, Sergey Lyskov, Johan Mabille, Tomasz Miąsko,
-Dean Moldovan, Ben Pritchard, Jason Rhinelander, Boris Schäling, Pim
-Schellart, Henry Schreiner, Ivan Smirnov, Boris Staletic, and Patrick Stewart.
+improvements to the code were contributed by
+Jonas Adler,
+Lori A. Burns,
+Sylvain Corlay,
+Eric Cousineau,
+Aaron Gokaslan,
+Ralf Grosse-Kunstleve,
+Trent Houliston,
+Axel Huebl,
+@hulucc,
+Yannick Jadoul,
+Sergey Lyskov,
+Johan Mabille,
+Tomasz Miąsko,
+Dean Moldovan,
+Ben Pritchard,
+Jason Rhinelander,
+Boris Schäling,
+Pim Schellart,
+Henry Schreiner,
+Ivan Smirnov,
+Dustin Spicuzza,
+Boris Staletic,
+Ethan Steinberg,
+Patrick Stewart,
+Ivor Wanders,
+and
+Xiaofei Wang.
 
 We thank Google for a generous financial contribution to the continuous
 integration infrastructure used by this project.
 
@@ -169,8 +169,8 @@ macro must be specified at the top level (and outside of any namespaces), since
 it adds a template instantiation of ``type_caster``. If your binding code consists of
 multiple compilation units, it must be present in every file (typically via a
 common header) preceding any usage of ``std::vector<int>``. Opaque types must
-also have a corresponding ``class_`` declaration to associate them with a name
-in Python, and to define a set of available operations, e.g.:
+also have a corresponding ``py::class_`` declaration to associate them with a
+name in Python, and to define a set of available operations, e.g.:
 
 .. code-block:: cpp
 
 
@@ -64,7 +64,7 @@ helper class that is defined as follows:
 
 .. code-block:: cpp
 
-    class PyAnimal : public Animal {
+    class PyAnimal : public Animal, py::trampoline_self_life_support {
     public:
         /* Inherit the constructors */
         using Animal::Animal;
@@ -80,6 +80,18 @@ helper class that is defined as follows:
         }
     };
 
+The ``py::trampoline_self_life_support`` base class is needed to ensure
+that a ``std::unique_ptr`` can safely be passed between Python and C++. To
+steer clear of notorious pitfalls (e.g. inheritance slicing), it is best
+practice to always use the base class, in combination with
+``py::smart_holder``.
+
+.. note::
+    For completeness, the base class has no effect if a holder other than
+    ``py::smart_holder`` used, including the default ``std::unique_ptr<T>``.
+    Please think twice, though, the pitfalls are very real, and the overhead
+    for using the safer ``py::smart_holder`` is very likely to be in the noise.
+
 The macro :c:macro:`PYBIND11_OVERRIDE_PURE` should be used for pure virtual
 functions, and :c:macro:`PYBIND11_OVERRIDE` should be used for functions which have
 a default implementation.  There are also two alternate macros
@@ -95,18 +107,18 @@ The binding code also needs a few minor adaptations (highlighted):
     :emphasize-lines: 2,3
 
     PYBIND11_MODULE(example, m) {
-        py::class_<Animal, PyAnimal /* <--- trampoline*/>(m, "Animal")
+        py::class_<Animal, PyAnimal /* <--- trampoline */, py::smart_holder>(m, "Animal")
             .def(py::init<>())
             .def("go", &Animal::go);
 
-        py::class_<Dog, Animal>(m, "Dog")
+        py::class_<Dog, Animal, py::smart_holder>(m, "Dog")
             .def(py::init<>());
 
         m.def("call_go", &call_go);
     }
 
 Importantly, pybind11 is made aware of the trampoline helper class by
-specifying it as an extra template argument to :class:`class_`. (This can also
+specifying it as an extra template argument to ``py::class_``. (This can also
 be combined with other template arguments such as a custom holder type; the
 order of template types does not matter).  Following this, we are able to
 define a constructor as usual.
@@ -116,9 +128,9 @@ Bindings should be made against the actual class, not the trampoline helper clas
 .. code-block:: cpp
     :emphasize-lines: 3
 
-    py::class_<Animal, PyAnimal /* <--- trampoline*/>(m, "Animal");
+    py::class_<Animal, PyAnimal /* <--- trampoline */, py::smart_holder>(m, "Animal");
         .def(py::init<>())
-        .def("go", &PyAnimal::go); /* <--- THIS IS WRONG, use &Animal::go */
+        .def("go", &Animal::go); /* <--- DO NOT USE &PyAnimal::go HERE */
 
 Note, however, that the above is sufficient for allowing python classes to
 extend ``Animal``, but not ``Dog``: see :ref:`virtual_and_inheritance` for the
@@ -244,13 +256,13 @@ override the ``name()`` method):
 
 .. code-block:: cpp
 
-    class PyAnimal : public Animal {
+    class PyAnimal : public Animal, py::trampoline_self_life_support {
     public:
         using Animal::Animal; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, Animal, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, Animal, name, ); }
     };
-    class PyDog : public Dog {
+    class PyDog : public Dog, py::trampoline_self_life_support {
     public:
         using Dog::Dog; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE(std::string, Dog, go, n_times); }
@@ -272,7 +284,7 @@ declare or override any virtual methods itself:
 .. code-block:: cpp
 
     class Husky : public Dog {};
-    class PyHusky : public Husky {
+    class PyHusky : public Husky, py::trampoline_self_life_support {
     public:
         using Husky::Husky; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, Husky, go, n_times); }
@@ -287,13 +299,15 @@ follows:
 
 .. code-block:: cpp
 
-    template <class AnimalBase = Animal> class PyAnimal : public AnimalBase {
+    template <class AnimalBase = Animal>
+    class PyAnimal : public AnimalBase, py::trampoline_self_life_support {
     public:
         using AnimalBase::AnimalBase; // Inherit constructors
         std::string go(int n_times) override { PYBIND11_OVERRIDE_PURE(std::string, AnimalBase, go, n_times); }
         std::string name() override { PYBIND11_OVERRIDE(std::string, AnimalBase, name, ); }
     };
-    template <class DogBase = Dog> class PyDog : public PyAnimal<DogBase> {
+    template <class DogBase = Dog>
+    class PyDog : public PyAnimal<DogBase>, py::trampoline_self_life_support {
     public:
         using PyAnimal<DogBase>::PyAnimal; // Inherit constructors
         // Override PyAnimal's pure virtual go() with a non-pure one:
@@ -311,9 +325,9 @@ The classes are then registered with pybind11 using:
 
 .. code-block:: cpp
 
-    py::class_<Animal, PyAnimal<>> animal(m, "Animal");
-    py::class_<Dog, Animal, PyDog<>> dog(m, "Dog");
-    py::class_<Husky, Dog, PyDog<Husky>> husky(m, "Husky");
+    py::class_<Animal, PyAnimal<>, py::smart_holder> animal(m, "Animal");
+    py::class_<Dog, Animal, PyDog<>, py::smart_holder> dog(m, "Dog");
+    py::class_<Husky, Dog, PyDog<Husky>, py::smart_holder> husky(m, "Husky");
     // ... add animal, dog, husky definitions
 
 Note that ``Husky`` did not require a dedicated trampoline template class at
@@ -499,12 +513,12 @@ an alias:
         // ...
         virtual ~Example() = default;
     };
-    class PyExample : public Example {
+    class PyExample : public Example, py::trampoline_self_life_support {
     public:
         using Example::Example;
         PyExample(Example &&base) : Example(std::move(base)) {}
     };
-    py::class_<Example, PyExample>(m, "Example")
+    py::class_<Example, PyExample, py::smart_holder>(m, "Example")
         // Returns an Example pointer.  If a PyExample is needed, the Example
         // instance will be moved via the extra constructor in PyExample, above.
         .def(py::init([]() { return new Example(); }))
@@ -550,9 +564,10 @@ pybind11. The underlying issue is that the ``std::unique_ptr`` holder type that
 is responsible for managing the lifetime of instances will reference the
 destructor even if no deallocations ever take place. In order to expose classes
 with private or protected destructors, it is possible to override the holder
-type via a holder type argument to ``class_``. Pybind11 provides a helper class
-``py::nodelete`` that disables any destructor invocations. In this case, it is
-crucial that instances are deallocated on the C++ side to avoid memory leaks.
+type via a holder type argument to ``py::class_``. Pybind11 provides a helper
+class ``py::nodelete`` that disables any destructor invocations. In this case,
+it is crucial that instances are deallocated on the C++ side to avoid memory
+leaks.
 
 .. code-block:: cpp
 
@@ -871,7 +886,7 @@ Multiple Inheritance
 
 pybind11 can create bindings for types that derive from multiple base types
 (aka. *multiple inheritance*). To do so, specify all bases in the template
-arguments of the ``class_`` declaration:
+arguments of the ``py::class_`` declaration:
 
 .. code-block:: cpp
 
@@ -946,23 +961,23 @@ because of conflicting definitions on the external type:
     // dogs.cpp
 
     // Binding for external library class:
-    py::class<pets::Pet>(m, "Pet")
+    py::class_<pets::Pet>(m, "Pet")
         .def("name", &pets::Pet::name);
 
     // Binding for local extension class:
-    py::class<Dog, pets::Pet>(m, "Dog")
+    py::class_<Dog, pets::Pet>(m, "Dog")
         .def(py::init<std::string>());
 
 .. code-block:: cpp
 
     // cats.cpp, in a completely separate project from the above dogs.cpp.
 
     // Binding for external library class:
-    py::class<pets::Pet>(m, "Pet")
+    py::class_<pets::Pet>(m, "Pet")
         .def("get_name", &pets::Pet::name);
 
     // Binding for local extending class:
-    py::class<Cat, pets::Pet>(m, "Cat")
+    py::class_<Cat, pets::Pet>(m, "Cat")
         .def(py::init<std::string>());
 
 .. code-block:: pycon
@@ -980,13 +995,13 @@ the ``py::class_`` constructor:
 .. code-block:: cpp
 
     // Pet binding in dogs.cpp:
-    py::class<pets::Pet>(m, "Pet", py::module_local())
+    py::class_<pets::Pet>(m, "Pet", py::module_local())
         .def("name", &pets::Pet::name);
 
 .. code-block:: cpp
 
     // Pet binding in cats.cpp:
-    py::class<pets::Pet>(m, "Pet", py::module_local())
+    py::class_<pets::Pet>(m, "Pet", py::module_local())
         .def("get_name", &pets::Pet::name);
 
 This makes the Python-side ``dogs.Pet`` and ``cats.Pet`` into distinct classes,
@@ -1104,7 +1119,7 @@ described trampoline:
         virtual int foo() const { return 42; }
     };
 
-    class Trampoline : public A {
+    class Trampoline : public A, py::trampoline_self_life_support {
     public:
         int foo() const override { PYBIND11_OVERRIDE(int, A, foo, ); }
     };
@@ -1114,7 +1129,7 @@ described trampoline:
         using A::foo;
     };
 
-    py::class_<A, Trampoline>(m, "A") // <-- `Trampoline` here
+    py::class_<A, Trampoline, py::smart_holder>(m, "A") // <-- `Trampoline` here
         .def("foo", &Publicist::foo); // <-- `Publicist` here, not `Trampoline`!
 
 Binding final classes
@@ -1195,7 +1210,7 @@ but once again each instantiation must be explicitly specified:
         T fn(V v);
     };
 
-    py::class<MyClass<int>>(m, "MyClassT")
+    py::class_<MyClass<int>>(m, "MyClassT")
         .def("fn", &MyClass<int>::fn<std::string>);
 
 Custom automatic downcasters