Replacing detail::is_smart_holder<H> in cast.h with detail::is_smart_holder_type_caster<T>.

Moving get_local_load_function_ptr, init_instance_for_type to smart_holder_type_caster_class_hooks.
Expanding static_assert in py::type::handle_of<> to accommodate smart_holder_type_casters.

include/pybind11/cast.h

@@ -1628,7 +1628,6 @@ using type_caster_holder = conditional_t<is_copy_constructible<holder_type>::val
                                          copyable_holder_caster<type, holder_type>,
                                          move_only_holder_caster<type, holder_type>>;
 
-template <typename T, bool Value = false> struct is_smart_holder { static constexpr bool value = Value; };
 template <typename T, bool Value = false> struct always_construct_holder { static constexpr bool value = Value; };
 
 /// Create a specialization for custom holder types (silently ignores std::shared_ptr)
@@ -1643,8 +1642,7 @@ template <typename T, bool Value = false> struct always_construct_holder { stati
 
 // PYBIND11_DECLARE_HOLDER_TYPE holder types:
 template <typename base, typename holder> struct is_holder_type :
-    detail::any_of<std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>>,
-                   detail::is_smart_holder<holder>> {};
+    std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>> {};
 // Specialization for always-supported unique_ptr holders:
 template <typename base, typename deleter> struct is_holder_type<base, std::unique_ptr<base, deleter>> :
     std::true_type {};
@@ -2263,15 +2261,22 @@ object object_api<Derived>::call(Args &&...args) const {
     return operator()<policy>(std::forward<Args>(args)...);
 }
 
+template <typename T, typename SFINAE = void>
+struct is_smart_holder_type_caster : std::false_type {};
+template <typename T>
+struct is_smart_holder_type_caster<
+    T,
+    enable_if_t<type_caster<T>::is_smart_holder_type_caster::value, void>> : std::true_type {};
+
 PYBIND11_NAMESPACE_END(detail)
 
 
 template<typename T>
 handle type::handle_of() {
-   static_assert(
-      std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>::value,
-      "py::type::of<T> only supports the case where T is a registered C++ types."
-    );
+    static_assert(
+        detail::any_of<std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>,
+                       detail::is_smart_holder_type_caster<T>>::value,
+        "py::type::of<T> only supports the case where T is a registered C++ types.");
 
     return detail::get_type_handle(typeid(T), true);
 }

include/pybind11/detail/smart_holder_type_casters.h

@@ -228,6 +228,41 @@ public:
 };
 // clang-format on
 
+struct smart_holder_type_caster_class_hooks {
+    using is_smart_holder_type_caster = std::true_type;
+
+    static decltype(&modified_type_caster_generic_load_impl::local_load)
+    get_local_load_function_ptr() {
+        return &modified_type_caster_generic_load_impl::local_load;
+    }
+
+    template <typename T>
+    static void init_instance_for_type(detail::instance *inst, const void *holder_const_void_ptr) {
+        // Need for const_cast is a consequence of the type_info::init_instance type:
+        // void (*init_instance)(instance *, const void *);
+        auto holder_void_ptr = const_cast<void *>(holder_const_void_ptr);
+
+        auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(T)));
+        if (!v_h.instance_registered()) {
+            register_instance(inst, v_h.value_ptr(), v_h.type);
+            v_h.set_instance_registered();
+        }
+        using holder_type = pybindit::memory::smart_holder;
+        if (holder_void_ptr) {
+            // Note: inst->owned ignored.
+            auto holder_ptr = static_cast<holder_type *>(holder_void_ptr);
+            new (std::addressof(v_h.holder<holder_type>())) holder_type(std::move(*holder_ptr));
+        } else if (inst->owned) {
+            new (std::addressof(v_h.holder<holder_type>()))
+                holder_type(holder_type::from_raw_ptr_take_ownership(v_h.value_ptr<T>()));
+        } else {
+            new (std::addressof(v_h.holder<holder_type>()))
+                holder_type(holder_type::from_raw_ptr_unowned(v_h.value_ptr<T>()));
+        }
+        v_h.set_holder_constructed();
+    }
+};
+
 template <typename T>
 struct smart_holder_type_caster_load {
     using holder_type = pybindit::memory::smart_holder;
@@ -305,7 +340,8 @@ struct make_constructor : private type_caster_base<int> { // Any type, nothing s
 };
 
 template <typename T>
-struct smart_holder_type_caster : smart_holder_type_caster_load<T> {
+struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
+                                  smart_holder_type_caster_class_hooks {
     static constexpr auto name = _<T>();
 
     // static handle cast(T, ...)
@@ -462,7 +498,8 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T> {
 };
 
 template <typename T>
-struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_load<T> {
+struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_load<T>,
+                                                      smart_holder_type_caster_class_hooks {
     static constexpr auto name = _<std::shared_ptr<T>>();
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
@@ -506,7 +543,8 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
 };
 
 template <typename T>
-struct smart_holder_type_caster<std::shared_ptr<T const>> : smart_holder_type_caster_load<T> {
+struct smart_holder_type_caster<std::shared_ptr<T const>> : smart_holder_type_caster_load<T>,
+                                                            smart_holder_type_caster_class_hooks {
     static constexpr auto name = _<std::shared_ptr<T const>>();
 
     static handle
@@ -524,7 +562,8 @@ struct smart_holder_type_caster<std::shared_ptr<T const>> : smart_holder_type_ca
 };
 
 template <typename T>
-struct smart_holder_type_caster<std::unique_ptr<T>> : smart_holder_type_caster_load<T> {
+struct smart_holder_type_caster<std::unique_ptr<T>> : smart_holder_type_caster_load<T>,
+                                                      smart_holder_type_caster_class_hooks {
     static constexpr auto name = _<std::unique_ptr<T>>();
 
     static handle cast(std::unique_ptr<T> &&src, return_value_policy policy, handle parent) {
@@ -566,7 +605,8 @@ struct smart_holder_type_caster<std::unique_ptr<T>> : smart_holder_type_caster_l
 };
 
 template <typename T>
-struct smart_holder_type_caster<std::unique_ptr<T const>> : smart_holder_type_caster_load<T> {
+struct smart_holder_type_caster<std::unique_ptr<T const>> : smart_holder_type_caster_load<T>,
+                                                            smart_holder_type_caster_class_hooks {
     static constexpr auto name = _<std::unique_ptr<T const>>();
 
     static handle cast(std::unique_ptr<T const> &&src, return_value_policy policy, handle parent) {
@@ -582,42 +622,6 @@ struct smart_holder_type_caster<std::unique_ptr<T const>> : smart_holder_type_ca
     operator std::unique_ptr<T const>() { return this->loaded_as_unique_ptr(); }
 };
 
-template <>
-struct is_smart_holder<pybindit::memory::smart_holder>
-    : is_smart_holder<pybindit::memory::smart_holder, true> {
-
-    static decltype(&modified_type_caster_generic_load_impl::local_load)
-    get_type_caster_local_load_function_ptr() {
-        return &modified_type_caster_generic_load_impl::local_load;
-    }
-
-    template <typename T>
-    static void init_instance_for_type(detail::instance *inst, const void *holder_const_void_ptr) {
-        // Need for const_cast is a consequence of the type_info::init_instance type:
-        // void (*init_instance)(instance *, const void *);
-        auto holder_void_ptr = const_cast<void *>(holder_const_void_ptr);
-
-        auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(T)));
-        if (!v_h.instance_registered()) {
-            register_instance(inst, v_h.value_ptr(), v_h.type);
-            v_h.set_instance_registered();
-        }
-        using holder_type = pybindit::memory::smart_holder;
-        if (holder_void_ptr) {
-            // Note: inst->owned ignored.
-            auto holder_ptr = static_cast<holder_type *>(holder_void_ptr);
-            new (std::addressof(v_h.holder<holder_type>())) holder_type(std::move(*holder_ptr));
-        } else if (inst->owned) {
-            new (std::addressof(v_h.holder<holder_type>()))
-                holder_type(holder_type::from_raw_ptr_take_ownership(v_h.value_ptr<T>()));
-        } else {
-            new (std::addressof(v_h.holder<holder_type>()))
-                holder_type(holder_type::from_raw_ptr_unowned(v_h.value_ptr<T>()));
-        }
-        v_h.set_holder_constructed();
-    }
-};
-
 #define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T)                                                     \
     namespace pybind11 {                                                                          \
     namespace detail {                                                                            \

include/pybind11/pybind11.h

@@ -1242,9 +1242,13 @@ auto method_adaptor(Return (Class::*pmf)(Args...) const) -> Return (Derived::*)(
 
 template <typename type_, typename... options>
 class class_ : public detail::generic_type {
-    template <typename T> using is_holder = detail::is_holder_type<type_, T>;
     template <typename T> using is_subtype = detail::is_strict_base_of<type_, T>;
     template <typename T> using is_base = detail::is_strict_base_of<T, type_>;
+    template <typename T>
+    using is_holder = detail::any_of<detail::is_holder_type<type_, T>,
+                                     detail::all_of<detail::negation<is_base<T>>,
+                                                    detail::negation<is_subtype<T>>,
+                                                    detail::is_smart_holder_type_caster<type_>>>;
     // struct instead of using here to help MSVC:
     template <typename T> struct is_valid_class_option :
         detail::any_of<is_holder<T>, is_subtype<T>, is_base<T>> {};
@@ -1502,14 +1506,19 @@ public:
     }
 
 private:
-    template <typename H = holder_type, detail::enable_if_t<!detail::is_smart_holder<H>::value, int> = 0>
+    template <typename T = type,
+              detail::enable_if_t<
+                  std::is_base_of<detail::type_caster_generic, detail::type_caster<T>>::value,
+                  int> = 0>
     void generic_type_initialize(const detail::type_record &record) {
         generic_type::initialize(record, &detail::type_caster_generic::local_load);
     }
 
-    template <typename H = holder_type, detail::enable_if_t<detail::is_smart_holder<H>::value, int> = 0>
+    template <
+        typename T = type,
+        detail::enable_if_t<detail::type_caster<T>::is_smart_holder_type_caster::value, int> = 0>
     void generic_type_initialize(const detail::type_record &record) {
-        generic_type::initialize(record, detail::is_smart_holder<H>::get_type_caster_local_load_function_ptr());
+        generic_type::initialize(record, detail::type_caster<T>::get_local_load_function_ptr());
     }
 
     /// Initialize holder object, variant 1: object derives from enable_shared_from_this
@@ -1556,7 +1565,10 @@ private:
     /// instance.  Should be called as soon as the `type` value_ptr is set for an instance.  Takes an
     /// optional pointer to an existing holder to use; if not specified and the instance is
     /// `.owned`, a new holder will be constructed to manage the value pointer.
-    template <typename H = holder_type, detail::enable_if_t<!detail::is_smart_holder<H>::value, int> = 0>
+    template <typename T = type,
+              detail::enable_if_t<
+                  std::is_base_of<detail::type_caster_generic, detail::type_caster<T>>::value,
+                  int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(type)));
         if (!v_h.instance_registered()) {
@@ -1566,9 +1578,11 @@ private:
         init_holder(inst, v_h, (const holder_type *) holder_ptr, v_h.value_ptr<type>());
     }
 
-    template <typename H = holder_type, detail::enable_if_t<detail::is_smart_holder<H>::value, int> = 0>
+    template <
+        typename T = type,
+        detail::enable_if_t<detail::type_caster<T>::is_smart_holder_type_caster::value, int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
-        detail::is_smart_holder<H>::template init_instance_for_type<type>(inst, holder_ptr);
+        detail::type_caster<T>::template init_instance_for_type<type>(inst, holder_ptr);
     }
 
     /// Deallocates an instance; via holder, if constructed; otherwise via operator delete.

tests/test_class_sh_basic.cpp

@@ -93,6 +93,10 @@ TEST_SUBMODULE(class_sh_basic, m) {
     // These require selected functions above to work first, as indicated:
     m.def("get_mtxt", get_mtxt);                         // pass_cref_atyp
     m.def("unique_ptr_roundtrip", unique_ptr_roundtrip); // pass_uqmp_atyp, rtrn_uqmp_atyp
+
+    m.def("py_type_handle_of_atyp", []() {
+        return py::type::handle_of<atyp>(); // Exercises static_cast in this function.
+    });
 }
 
 } // namespace class_sh_basic

tests/test_class_sh_basic.py

@@ -5,12 +5,12 @@ from pybind11_tests import class_sh_basic as m
 
 
 def test_atyp_constructors():
-    e = m.atyp()
-    assert e.__class__.__name__ == "atyp"
-    e = m.atyp("")
-    assert e.__class__.__name__ == "atyp"
-    e = m.atyp("txtm")
-    assert e.__class__.__name__ == "atyp"
+    obj = m.atyp()
+    assert obj.__class__.__name__ == "atyp"
+    obj = m.atyp("")
+    assert obj.__class__.__name__ == "atyp"
+    obj = m.atyp("txtm")
+    assert obj.__class__.__name__ == "atyp"
 
 
 def test_cast():
@@ -80,3 +80,8 @@ def test_unique_ptr_roundtrip(num_round_trips=1000):
         # Ensure the returned object is a different Python instance.
         assert id_rtrn != id_orig
         id_orig = id_rtrn
+
+
+def test_py_type_handle_of_atyp():
+    obj = m.py_type_handle_of_atyp()
+    assert obj.__class__.__name__ == "pybind11_type"