snapshot of first attempt to replace `void *` with `any` (messy state)

include/pybind11/attr.h

@@ -245,7 +245,7 @@ struct type_record {
     void *(*operator_new)(size_t) = nullptr;
 
     /// Function pointer to class_<..>::init_instance
-    void (*init_instance)(instance *, const void *) = nullptr;
+    void (*init_instance)(instance *, std::any) = nullptr;
 
     /// Function pointer to class_<..>::dealloc
     void (*dealloc)(detail::value_and_holder &) = nullptr;

include/pybind11/cast.h

@@ -14,6 +14,7 @@
 #include "detail/typeid.h"
 #include "detail/descr.h"
 #include "detail/internals.h"
+#include <any>
 #include <array>
 #include <limits>
 #include <tuple>
@@ -232,6 +233,13 @@ struct value_and_holder {
     explicit operator bool() const { return xxx_value_ptr<void>(); }
 
     template <typename H> H &xxx_holder() const {
+#ifdef JUNK
+        to_cout(std::string("xxx_holder<") + typeid(H).name() + ">");
+if (std::string(typeid(H).name()) == "St10shared_ptrIN14pybind11_tests17holder_shared_ptr7pointeeEE") {
+  long *BAD = nullptr;
+  std::cout << *BAD;
+}
+#endif
         return reinterpret_cast<H &>(zzz_vh[1]);
     }
     bool holder_constructed() const {
@@ -500,7 +508,7 @@ public:
                                          const detail::type_info *tinfo,
                                          void *(*copy_constructor)(const void *),
                                          void *(*move_constructor)(const void *),
-                                         const void *existing_holder = nullptr) {
+                                         std::any existing_holder = std::any{}) {
         if (!tinfo) // no type info: error will be set already
             return handle();
 
@@ -586,7 +594,7 @@ public:
                 throw cast_error("unhandled return_value_policy: should not happen!");
         }
 
-        tinfo->init_instance(wrapper, existing_holder);
+        tinfo->init_instance(wrapper, existing_holder);  // HOLDER_SHARED_MAKE_UNIQUE STACK #4
 
         return inst.release();
     }
@@ -913,10 +921,10 @@ public:
             make_copy_constructor(src), make_move_constructor(src));
     }
 
-    static handle cast_holder(const itype *src, const void *holder) {
+    static handle cast_holder(const itype *src, std::any holder) {
         auto st = src_and_type(src);
-        return type_caster_generic::cast(
-            st.first, return_value_policy::take_ownership, {}, st.second,
+        return type_caster_generic::cast(  // HOLDER_SHARED_MAKE_UNIQUE STACK #5
+            st.first, return_value_policy::take_ownership, {}, st.second,  // tinfo=st.second  TODODODO return_value_policy::existing_holder
             nullptr, nullptr, holder);
     }
 
@@ -1582,7 +1590,7 @@ struct move_only_holder_caster {
 
     static handle cast(holder_type &&src, return_value_policy, handle) {
         auto *ptr = holder_helper<holder_type>::yyy_get(src);  // move_only_holder_caster::cast
-        return type_caster_base<type>::cast_holder(ptr, std::addressof(src));
+        return type_caster_base<type>::cast_holder(ptr, std::addressof(src));  // HOLDER_SHARED_MAKE_UNIQUE STACK #6
     }
     static constexpr auto name = type_caster_base<type>::name;
 };

include/pybind11/detail/internals.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "../pytypes.h"
+#include <any>
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
@@ -130,7 +131,7 @@ struct type_info {
     const std::type_info *cpptype;
     size_t type_size, type_align, holder_size_in_ptrs;
     void *(*operator_new)(size_t);
-    void (*init_instance)(instance *, const void *);
+    void (*init_instance)(instance *, std::any);
     void (*dealloc)(value_and_holder &v_h);
     std::vector<PyObject *(*)(PyObject *, PyTypeObject *)> implicit_conversions;
     std::vector<std::pair<const std::type_info *, void *(*)(void *)>> implicit_casts;

include/pybind11/pybind11.h

@@ -158,7 +158,7 @@ protected:
                       "The number of argument annotations does not match the number of function arguments");
 
         /* Dispatch code which converts function arguments and performs the actual function call */
-        rec->impl = [](function_call &call) -> handle {
+        rec->impl = [](function_call &call) -> handle {  // HOLDER_SHARED_MAKE_UNIQUE STACK #8
             cast_in args_converter;
 
             /* Try to cast the function arguments into the C++ domain */
@@ -180,7 +180,7 @@ protected:
             using Guard = extract_guard_t<Extra...>;
 
             /* Perform the function call */
-            handle result = cast_out::cast(
+            handle result = cast_out::cast(  // HOLDER_SHARED_MAKE_UNIQUE STACK #7
                 std::move(args_converter).template call<Return, Guard>(cap->f), policy, call.parent);
 
             /* Invoke call policy post-call hook */
@@ -713,7 +713,7 @@ protected:
 
                 // 6. Call the function.
                 try {
-                    loader_life_support guard{};
+                    loader_life_support guard{};  // HOLDER_SHARED_MAKE_UNIQUE STACK #9
                     result = func.impl(call);
                 } catch (reference_cast_error &) {
                     result = PYBIND11_TRY_NEXT_OVERLOAD;
@@ -1472,19 +1472,19 @@ private:
 
     static void init_holder_from_existing(const detail::value_and_holder &v_h,
             const holder_type *holder_ptr, std::true_type /*is_copy_constructible*/) {
-        new (std::addressof(v_h.xxx_holder<holder_type>())) holder_type(*reinterpret_cast<const holder_type *>(holder_ptr));
+        new (std::addressof(v_h.xxx_holder<holder_type>())) holder_type(*reinterpret_cast<const holder_type *>(holder_ptr));  // HOLDER_SHARED_MAKE_UNIQUE STACK #1
     }
 
     static void init_holder_from_existing(const detail::value_and_holder &v_h,
             const holder_type *holder_ptr, std::false_type /*is_copy_constructible*/) {
-        new (std::addressof(v_h.xxx_holder<holder_type>())) holder_type(std::move(*const_cast<holder_type *>(holder_ptr)));
+        new (std::addressof(v_h.xxx_holder<holder_type>())) holder_type(std::move(*const_cast<holder_type *>(holder_ptr)));  // DIRTY CONST_CAST
     }
 
     /// Initialize holder object, variant 2: try to construct from existing holder object, if possible
     static void init_holder(detail::instance *inst, detail::value_and_holder &v_h,
             const holder_type *holder_ptr, const void * /* dummy -- not enable_shared_from_this<T>) */) {
         if (holder_ptr) {
-            init_holder_from_existing(v_h, holder_ptr, std::is_copy_constructible<holder_type>());
+            init_holder_from_existing(v_h, holder_ptr, std::is_copy_constructible<holder_type>());  // HOLDER_SHARED_MAKE_UNIQUE STACK #2
             v_h.set_holder_constructed();
         } else if (inst->aaa_owned || detail::always_construct_holder<holder_type>::value) {
             new (std::addressof(v_h.xxx_holder<holder_type>())) holder_type(v_h.xxx_value_ptr<type>());  // init_holder
@@ -1496,13 +1496,33 @@ 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
     /// `.aaa_owned`, a new holder will be constructed to manage the value pointer.
-    static void init_instance(detail::instance *inst, const void *holder_ptr) {
+    static void init_instance(detail::instance *inst, std::any holder_ptr) {  // TODODODO std::any
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(type)));
         if (!v_h.instance_registered()) {
             register_instance(inst, v_h.xxx_value_ptr<void>(), v_h.type);  // before init_holder
             v_h.set_instance_registered();
         }
-        init_holder(inst, v_h, (const holder_type *) holder_ptr, v_h.xxx_value_ptr<type>());  // calling init_holder
+        const holder_type *holder_ptr_raw = nullptr;
+        if (holder_ptr.has_value() && std::any_cast<std::nullptr_t>(&holder_ptr) == nullptr) {
+            detail::to_cout("before any_cast");
+            std::string htn(holder_ptr.type().name());
+            detail::clean_type_id(htn);
+            detail::to_cout(std::string("holder_ptr.type().name() ")  + htn);
+            detail::to_cout(std::string("   typeid(holder_type *) ")  + type_id<holder_type *>());
+            auto cast_ptr_mutbl = std::any_cast<holder_type *>(&holder_ptr);
+            if (cast_ptr_mutbl != nullptr) {
+                holder_ptr_raw = *cast_ptr_mutbl;
+            } else {
+                auto cast_ptr_const = std::any_cast<const holder_type *>(&holder_ptr);
+                if (cast_ptr_const == nullptr) {
+                    detail::to_cout("any_cast mutbl + const failure");
+                    throw std::runtime_error("Incompatible holder types.");
+                }
+                holder_ptr_raw = *cast_ptr_const;
+            }
+            detail::to_cout("after any_cast");
+        }
+        init_holder(inst, v_h, holder_ptr_raw, v_h.xxx_value_ptr<type>());  // calling init_holder  // HOLDER_SHARED_MAKE_UNIQUE STACK #3
     }
 
     /// Deallocates an instance; via holder, if constructed; otherwise via operator delete.

tests/test_tagbased_polymorphic.cpp

@@ -80,6 +80,7 @@ std::vector<std::unique_ptr<Animal>> create_zoo()
     ret.emplace_back(new Chihuahua("Hertzl"));
     ret.emplace_back(new Cat("Tiger", Cat::Kind::Cat));
     ret.emplace_back(new Panther("Leo"));
+    py::detail::to_cout("before ret");
     return ret;
 }