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 /*
     pybind11/detail/type_caster_base.h (originally first part of pybind11/cast.h)
 
     Copyright (c) 2016 Wenzel Jakob <wenzel.jakob@epfl.ch>
 
     All rights reserved. Use of this source code is governed by a
     BSD-style license that can be found in the LICENSE file.
 */
 
 #pragma once
 
 #include <pybind11/gil.h>
 #include <pybind11/pytypes.h>
 #include <pybind11/trampoline_self_life_support.h>
 
 #include "common.h"
 #include "cpp_conduit.h"
 #include "descr.h"
 #include "dynamic_raw_ptr_cast_if_possible.h"
 #include "internals.h"
 #include "typeid.h"
 #include "using_smart_holder.h"
 #include "value_and_holder.h"
 
 #include <cstdint>
 #include <cstring>
 #include <iterator>
 #include <new>
 #include <stdexcept>
 #include <string>
 #include <type_traits>
 #include <typeindex>
 #include <typeinfo>
 #include <unordered_map>
 #include <utility>
 #include <vector>
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 /// A life support system for temporary objects created by `type_caster::load()`.
 /// Adding a patient will keep it alive up until the enclosing function returns.
 class loader_life_support {
 private:
     loader_life_support *parent = nullptr;
     std::unordered_set<PyObject *> keep_alive;
 
 public:
     /// A new patient frame is created when a function is entered
     loader_life_support() {
         auto &stack_top = get_internals().loader_life_support_tls;
         parent = stack_top.get();
         stack_top = this;
     }
 
     /// ... and destroyed after it returns
     ~loader_life_support() {
         auto &stack_top = get_internals().loader_life_support_tls;
         if (stack_top.get() != this) {
             pybind11_fail("loader_life_support: internal error");
         }
         stack_top = parent;
         for (auto *item : keep_alive) {
             Py_DECREF(item);
         }
     }
 
     /// This can only be used inside a pybind11-bound function, either by `argument_loader`
     /// at argument preparation time or by `py::cast()` at execution time.
     PYBIND11_NOINLINE static void add_patient(handle h) {
         loader_life_support *frame = get_internals().loader_life_support_tls.get();
         if (!frame) {
             // NOTE: It would be nice to include the stack frames here, as this indicates
             // use of pybind11::cast<> outside the normal call framework, finding such
             // a location is challenging. Developers could consider printing out
             // stack frame addresses here using something like __builtin_frame_address(0)
             throw cast_error("When called outside a bound function, py::cast() cannot "
                              "do Python -> C++ conversions which require the creation "
                              "of temporary values");
         }
 
         if (frame->keep_alive.insert(h.ptr()).second) {
             Py_INCREF(h.ptr());
         }
     }
 };
 
 // Gets the cache entry for the given type, creating it if necessary.  The return value is the pair
 // returned by emplace, i.e. an iterator for the entry and a bool set to `true` if the entry was
 // just created.
 inline std::pair<decltype(internals::registered_types_py)::iterator, bool>
 all_type_info_get_cache(PyTypeObject *type);
 
 // Band-aid workaround to fix a subtle but serious bug in a minimalistic fashion. See PR #4762.
 inline void all_type_info_add_base_most_derived_first(std::vector<type_info *> &bases,
                                                       type_info *addl_base) {
     for (auto it = bases.begin(); it != bases.end(); it++) {
         type_info *existing_base = *it;
         if (PyType_IsSubtype(addl_base->type, existing_base->type) != 0) {
             bases.insert(it, addl_base);
             return;
         }
     }
     bases.push_back(addl_base);
 }
 
 // Populates a just-created cache entry.
 PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_info *> &bases) {
     assert(bases.empty());
     std::vector<PyTypeObject *> check;
     for (handle parent : reinterpret_borrow<tuple>(t->tp_bases)) {
         check.push_back((PyTypeObject *) parent.ptr());
     }
     auto const &type_dict = get_internals().registered_types_py;
     for (size_t i = 0; i < check.size(); i++) {
         auto *type = check[i];
         // Ignore Python2 old-style class super type:
         if (!PyType_Check((PyObject *) type)) {
             continue;
         }
 
         // Check `type` in the current set of registered python types:
         auto it = type_dict.find(type);
         if (it != type_dict.end()) {
             // We found a cache entry for it, so it's either pybind-registered or has pre-computed
             // pybind bases, but we have to make sure we haven't already seen the type(s) before:
             // we want to follow Python/virtual C++ rules that there should only be one instance of
             // a common base.
             for (auto *tinfo : it->second) {
                 // NB: Could use a second set here, rather than doing a linear search, but since
                 // having a large number of immediate pybind11-registered types seems fairly
                 // unlikely, that probably isn't worthwhile.
                 bool found = false;
                 for (auto *known : bases) {
                     if (known == tinfo) {
                         found = true;
                         break;
                     }
                 }
                 if (!found) {
                     all_type_info_add_base_most_derived_first(bases, tinfo);
                 }
             }
         } else if (type->tp_bases) {
             // It's some python type, so keep follow its bases classes to look for one or more
             // registered types
             if (i + 1 == check.size()) {
                 // When we're at the end, we can pop off the current element to avoid growing
                 // `check` when adding just one base (which is typical--i.e. when there is no
                 // multiple inheritance)
                 check.pop_back();
                 i--;
             }
             for (handle parent : reinterpret_borrow<tuple>(type->tp_bases)) {
                 check.push_back((PyTypeObject *) parent.ptr());
             }
         }
     }
 }
 
 /**
  * Extracts vector of type_info pointers of pybind-registered roots of the given Python type.  Will
  * be just 1 pybind type for the Python type of a pybind-registered class, or for any Python-side
  * derived class that uses single inheritance.  Will contain as many types as required for a Python
  * class that uses multiple inheritance to inherit (directly or indirectly) from multiple
  * pybind-registered classes.  Will be empty if neither the type nor any base classes are
  * pybind-registered.
  *
  * The value is cached for the lifetime of the Python type.
  */
 inline const std::vector<detail::type_info *> &all_type_info(PyTypeObject *type) {
     return all_type_info_get_cache(type).first->second;
 }
 
 /**
  * Gets a single pybind11 type info for a python type.  Returns nullptr if neither the type nor any
  * ancestors are pybind11-registered.  Throws an exception if there are multiple bases--use
  * `all_type_info` instead if you want to support multiple bases.
  */
 PYBIND11_NOINLINE detail::type_info *get_type_info(PyTypeObject *type) {
     const auto &bases = all_type_info(type);
     if (bases.empty()) {
         return nullptr;
     }
     if (bases.size() > 1) {
         pybind11_fail(
             "pybind11::detail::get_type_info: type has multiple pybind11-registered bases");
     }
     return bases.front();
 }
 
 inline detail::type_info *get_local_type_info(const std::type_index &tp) {
     auto &locals = get_local_internals().registered_types_cpp;
     auto it = locals.find(tp);
     if (it != locals.end()) {
         return it->second;
     }
     return nullptr;
 }
 
 inline detail::type_info *get_global_type_info(const std::type_index &tp) {
     return with_internals([&](internals &internals) {
         detail::type_info *type_info = nullptr;
         auto &types = internals.registered_types_cpp;
         auto it = types.find(tp);
         if (it != types.end()) {
             type_info = it->second;
         }
         return type_info;
     });
 }
 
 /// Return the type info for a given C++ type; on lookup failure can either throw or return
 /// nullptr.
 PYBIND11_NOINLINE detail::type_info *get_type_info(const std::type_index &tp,
                                                    bool throw_if_missing = false) {
     if (auto *ltype = get_local_type_info(tp)) {
         return ltype;
     }
     if (auto *gtype = get_global_type_info(tp)) {
         return gtype;
     }
 
     if (throw_if_missing) {
         std::string tname = tp.name();
         detail::clean_type_id(tname);
         pybind11_fail("pybind11::detail::get_type_info: unable to find type info for \""
                       + std::move(tname) + '"');
     }
     return nullptr;
 }
 
 PYBIND11_NOINLINE handle get_type_handle(const std::type_info &tp, bool throw_if_missing) {
     detail::type_info *type_info = get_type_info(tp, throw_if_missing);
     return handle(type_info ? ((PyObject *) type_info->type) : nullptr);
 }
 
 inline bool try_incref(PyObject *obj) {
     // Tries to increment the reference count of an object if it's not zero.
     // TODO: Use PyUnstable_TryIncref when available.
     // See https://github.com/python/cpython/issues/128844
 #ifdef Py_GIL_DISABLED
     // See
     // https://github.com/python/cpython/blob/d05140f9f77d7dfc753dd1e5ac3a5962aaa03eff/Include/internal/pycore_object.h#L761
     uint32_t local = _Py_atomic_load_uint32_relaxed(&obj->ob_ref_local);
     local += 1;
     if (local == 0) {
         // immortal
         return true;
     }
     if (_Py_IsOwnedByCurrentThread(obj)) {
         _Py_atomic_store_uint32_relaxed(&obj->ob_ref_local, local);
 #    ifdef Py_REF_DEBUG
         _Py_INCREF_IncRefTotal();
 #    endif
         return true;
     }
     Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&obj->ob_ref_shared);
     for (;;) {
         // If the shared refcount is zero and the object is either merged
         // or may not have weak references, then we cannot incref it.
         if (shared == 0 || shared == _Py_REF_MERGED) {
             return false;
         }
 
         if (_Py_atomic_compare_exchange_ssize(
                 &obj->ob_ref_shared, &shared, shared + (1 << _Py_REF_SHARED_SHIFT))) {
 #    ifdef Py_REF_DEBUG
             _Py_INCREF_IncRefTotal();
 #    endif
             return true;
         }
     }
 #else
     assert(Py_REFCNT(obj) > 0);
     Py_INCREF(obj);
     return true;
 #endif
 }
 
 // Searches the inheritance graph for a registered Python instance, using all_type_info().
 PYBIND11_NOINLINE handle find_registered_python_instance(void *src,
                                                          const detail::type_info *tinfo) {
     return with_instance_map(src, [&](instance_map &instances) {
         auto it_instances = instances.equal_range(src);
         for (auto it_i = it_instances.first; it_i != it_instances.second; ++it_i) {
             for (auto *instance_type : detail::all_type_info(Py_TYPE(it_i->second))) {
                 if (instance_type && same_type(*instance_type->cpptype, *tinfo->cpptype)) {
                     auto *wrapper = reinterpret_cast<PyObject *>(it_i->second);
                     if (try_incref(wrapper)) {
                         return handle(wrapper);
                     }
                 }
             }
         }
         return handle();
     });
 }
 
 // Container for accessing and iterating over an instance's values/holders
 struct values_and_holders {
 private:
     instance *inst;
     using type_vec = std::vector<detail::type_info *>;
     const type_vec &tinfo;
 
 public:
     explicit values_and_holders(instance *inst)
         : inst{inst}, tinfo(all_type_info(Py_TYPE(inst))) {}
 
     explicit values_and_holders(PyObject *obj)
         : inst{nullptr}, tinfo(all_type_info(Py_TYPE(obj))) {
         if (!tinfo.empty()) {
             inst = reinterpret_cast<instance *>(obj);
         }
     }
 
     struct iterator {
     private:
         instance *inst = nullptr;
         const type_vec *types = nullptr;
         value_and_holder curr;
         friend struct values_and_holders;
         iterator(instance *inst, const type_vec *tinfo) : inst{inst}, types{tinfo} {
             if (inst != nullptr) {
                 assert(!types->empty());
                 curr = value_and_holder(
                     inst /* instance */,
                     (*types)[0] /* type info */,
                     0, /* vpos: (non-simple types only): the first vptr comes first */
                     0 /* index */);
             }
         }
         // Past-the-end iterator:
         explicit iterator(size_t end) : curr(end) {}
 
     public:
         bool operator==(const iterator &other) const { return curr.index == other.curr.index; }
         bool operator!=(const iterator &other) const { return curr.index != other.curr.index; }
         iterator &operator++() {
             if (!inst->simple_layout) {
                 curr.vh += 1 + (*types)[curr.index]->holder_size_in_ptrs;
             }
             ++curr.index;
             curr.type = curr.index < types->size() ? (*types)[curr.index] : nullptr;
             return *this;
         }
         value_and_holder &operator*() { return curr; }
         value_and_holder *operator->() { return &curr; }
     };
 
     iterator begin() { return iterator(inst, &tinfo); }
     iterator end() { return iterator(tinfo.size()); }
 
     iterator find(const type_info *find_type) {
         auto it = begin(), endit = end();
         while (it != endit && it->type != find_type) {
             ++it;
         }
         return it;
     }
 
     size_t size() { return tinfo.size(); }
 
     // Band-aid workaround to fix a subtle but serious bug in a minimalistic fashion. See PR #4762.
     bool is_redundant_value_and_holder(const value_and_holder &vh) {
         for (size_t i = 0; i < vh.index; i++) {
             if (PyType_IsSubtype(tinfo[i]->type, tinfo[vh.index]->type) != 0) {
                 return true;
             }
         }
         return false;
     }
 };
 
 /**
  * Extracts C++ value and holder pointer references from an instance (which may contain multiple
  * values/holders for python-side multiple inheritance) that match the given type.  Throws an error
  * if the given type (or ValueType, if omitted) is not a pybind11 base of the given instance.  If
  * `find_type` is omitted (or explicitly specified as nullptr) the first value/holder are returned,
  * regardless of type (and the resulting .type will be nullptr).
  *
  * The returned object should be short-lived: in particular, it must not outlive the called-upon
  * instance.
  */
 PYBIND11_NOINLINE value_and_holder
 instance::get_value_and_holder(const type_info *find_type /*= nullptr default in common.h*/,
                                bool throw_if_missing /*= true in common.h*/) {
     // Optimize common case:
     if (!find_type || Py_TYPE(this) == find_type->type) {
         return value_and_holder(this, find_type, 0, 0);
     }
 
     detail::values_and_holders vhs(this);
     auto it = vhs.find(find_type);
     if (it != vhs.end()) {
         return *it;
     }
 
     if (!throw_if_missing) {
         return value_and_holder();
     }
 
 #if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
     pybind11_fail("pybind11::detail::instance::get_value_and_holder: `"
                   + get_fully_qualified_tp_name(find_type->type)
                   + "' is not a pybind11 base of the given `"
                   + get_fully_qualified_tp_name(Py_TYPE(this)) + "' instance");
 #else
     pybind11_fail(
         "pybind11::detail::instance::get_value_and_holder: "
         "type is not a pybind11 base of the given instance "
         "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for type details)");
 #endif
 }
 
 PYBIND11_NOINLINE void instance::allocate_layout() {
     const auto &tinfo = all_type_info(Py_TYPE(this));
 
     const size_t n_types = tinfo.size();
 
     if (n_types == 0) {
         pybind11_fail(
             "instance allocation failed: new instance has no pybind11-registered base types");
     }
 
     simple_layout
         = n_types == 1 && tinfo.front()->holder_size_in_ptrs <= instance_simple_holder_in_ptrs();
 
     // Simple path: no python-side multiple inheritance, and a small-enough holder
     if (simple_layout) {
         simple_value_holder[0] = nullptr;
         simple_holder_constructed = false;
         simple_instance_registered = false;
     } else { // multiple base types or a too-large holder
         // Allocate space to hold: [v1*][h1][v2*][h2]...[bb...] where [vN*] is a value pointer,
         // [hN] is the (uninitialized) holder instance for value N, and [bb...] is a set of bool
         // values that tracks whether each associated holder has been initialized.  Each [block] is
         // padded, if necessary, to an integer multiple of sizeof(void *).
         size_t space = 0;
         for (auto *t : tinfo) {
             space += 1;                      // value pointer
             space += t->holder_size_in_ptrs; // holder instance
         }
         size_t flags_at = space;
         space += size_in_ptrs(n_types); // status bytes (holder_constructed and
                                         // instance_registered)
 
         // Allocate space for flags, values, and holders, and initialize it to 0 (flags and values,
         // in particular, need to be 0).  Use Python's memory allocation
         // functions: Python is using pymalloc, which is designed to be
         // efficient for small allocations like the one we're doing here;
         // for larger allocations they are just wrappers around malloc.
         // TODO: is this still true for pure Python 3.6?
         nonsimple.values_and_holders = (void **) PyMem_Calloc(space, sizeof(void *));
         if (!nonsimple.values_and_holders) {
             throw std::bad_alloc();
         }
         nonsimple.status
             = reinterpret_cast<std::uint8_t *>(&nonsimple.values_and_holders[flags_at]);
     }
     owned = true;
 }
 
 // NOLINTNEXTLINE(readability-make-member-function-const)
 PYBIND11_NOINLINE void instance::deallocate_layout() {
     if (!simple_layout) {
         PyMem_Free(reinterpret_cast<void *>(nonsimple.values_and_holders));
     }
 }
 
 PYBIND11_NOINLINE bool isinstance_generic(handle obj, const std::type_info &tp) {
     handle type = detail::get_type_handle(tp, false);
     if (!type) {
         return false;
     }
     return isinstance(obj, type);
 }
 
 PYBIND11_NOINLINE handle get_object_handle(const void *ptr, const detail::type_info *type) {
     return with_instance_map(ptr, [&](instance_map &instances) {
         auto range = instances.equal_range(ptr);
         for (auto it = range.first; it != range.second; ++it) {
             for (const auto &vh : values_and_holders(it->second)) {
                 if (vh.type == type) {
                     return handle((PyObject *) it->second);
                 }
             }
         }
         return handle();
     });
 }
 
 // Forward declarations
 void keep_alive_impl(handle nurse, handle patient);
 inline PyObject *make_new_instance(PyTypeObject *type);
 
 PYBIND11_WARNING_PUSH
 PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
 
 // PYBIND11:REMINDER: Needs refactoring of existing pybind11 code.
 inline bool deregister_instance(instance *self, void *valptr, const type_info *tinfo);
 
 PYBIND11_WARNING_POP
 
 PYBIND11_NAMESPACE_BEGIN(smart_holder_type_caster_support)
 
 struct value_and_holder_helper {
     value_and_holder loaded_v_h;
 
     bool have_holder() const {
         return loaded_v_h.vh != nullptr && loaded_v_h.holder_constructed();
     }
 
     smart_holder &holder() const { return loaded_v_h.holder<smart_holder>(); }
 
     void throw_if_uninitialized_or_disowned_holder(const char *typeid_name) const {
         static const std::string missing_value_msg = "Missing value for wrapped C++ type `";
         if (!holder().is_populated) {
             throw value_error(missing_value_msg + clean_type_id(typeid_name)
                               + "`: Python instance is uninitialized.");
         }
         if (!holder().has_pointee()) {
             throw value_error(missing_value_msg + clean_type_id(typeid_name)
                               + "`: Python instance was disowned.");
         }
     }
 
     void throw_if_uninitialized_or_disowned_holder(const std::type_info &type_info) const {
         throw_if_uninitialized_or_disowned_holder(type_info.name());
     }
 
     // have_holder() must be true or this function will fail.
     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.");
         }
     }
 
     void *get_void_ptr_or_nullptr() const {
         if (have_holder()) {
             auto &hld = holder();
             if (hld.is_populated && hld.has_pointee()) {
                 return hld.template as_raw_ptr_unowned<void>();
             }
         }
         return nullptr;
     }
 };
 
 template <typename T, typename D>
 handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
                                     return_value_policy policy,
                                     handle parent,
                                     const std::pair<const void *, const type_info *> &st) {
     if (policy == return_value_policy::copy) {
         throw cast_error("return_value_policy::copy is invalid for unique_ptr.");
     }
     if (!src) {
         return none().release();
     }
     void *src_raw_void_ptr = const_cast<void *>(st.first);
     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 = 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) {
                 auto &holder = v_h.holder<smart_holder>();
                 if (!holder.is_disowned) {
                     pybind11_fail("smart_holder_from_unique_ptr: unexpected "
                                   "smart_holder.is_disowned failure.");
                 }
                 // Critical transfer-of-ownership section. This must stay together.
                 self_life_support->deactivate_life_support();
                 holder.reclaim_disowned(tinfo->get_memory_guarded_delete);
                 (void) src.release();
                 // Critical section end.
                 return existing_inst;
             }
         }
         throw cast_error("Invalid unique_ptr: another instance owns this pointer already.");
     }
 
     auto inst = reinterpret_steal<object>(make_new_instance(tinfo->type));
     auto *inst_raw_ptr = reinterpret_cast<instance *>(inst.ptr());
     inst_raw_ptr->owned = true;
     void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
     valueptr = src_raw_void_ptr;
 
     if (static_cast<void *>(src.get()) == src_raw_void_ptr) {
         // This is a multiple-inheritance situation that is incompatible with the current
         // shared_from_this handling (see PR #3023). Is there a better solution?
         src_raw_void_ptr = nullptr;
     }
     auto smhldr = smart_holder::from_unique_ptr(std::move(src), src_raw_void_ptr);
     tinfo->init_instance(inst_raw_ptr, static_cast<const void *>(&smhldr));
 
     if (policy == return_value_policy::reference_internal) {
         keep_alive_impl(inst, parent);
     }
 
     return inst.release();
 }
 
 template <typename T, typename D>
 handle smart_holder_from_unique_ptr(std::unique_ptr<T const, D> &&src,
                                     return_value_policy policy,
                                     handle parent,
                                     const std::pair<const void *, const type_info *> &st) {
     return smart_holder_from_unique_ptr(
         std::unique_ptr<T, D>(const_cast<T *>(src.release()),
                               std::move(src.get_deleter())), // Const2Mutbl
         policy,
         parent,
         st);
 }
 
 template <typename T>
 handle smart_holder_from_shared_ptr(const std::shared_ptr<T> &src,
                                     return_value_policy policy,
                                     handle parent,
                                     const std::pair<const void *, const type_info *> &st) {
     switch (policy) {
         case return_value_policy::automatic:
         case return_value_policy::automatic_reference:
             break;
         case return_value_policy::take_ownership:
             throw cast_error("Invalid return_value_policy for shared_ptr (take_ownership).");
         case return_value_policy::copy:
         case return_value_policy::move:
             break;
         case return_value_policy::reference:
             throw cast_error("Invalid return_value_policy for shared_ptr (reference).");
         case return_value_policy::reference_internal:
             break;
     }
     if (!src) {
         return none().release();
     }
 
     auto src_raw_ptr = src.get();
     assert(st.second != nullptr);
     void *src_raw_void_ptr = static_cast<void *>(src_raw_ptr);
     const detail::type_info *tinfo = st.second;
     if (handle existing_inst = find_registered_python_instance(src_raw_void_ptr, tinfo)) {
         // PYBIND11:REMINDER: MISSING: Enforcement of consistency with existing smart_holder.
         // PYBIND11:REMINDER: MISSING: keep_alive.
         return existing_inst;
     }
 
     auto inst = reinterpret_steal<object>(make_new_instance(tinfo->type));
     auto *inst_raw_ptr = reinterpret_cast<instance *>(inst.ptr());
     inst_raw_ptr->owned = true;
     void *&valueptr = values_and_holders(inst_raw_ptr).begin()->value_ptr();
     valueptr = src_raw_void_ptr;
 
     auto smhldr
         = 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) {
         keep_alive_impl(inst, parent);
     }
 
     return inst.release();
 }
 
 template <typename T>
 handle smart_holder_from_shared_ptr(const std::shared_ptr<T const> &src,
                                     return_value_policy policy,
                                     handle parent,
                                     const std::pair<const void *, const type_info *> &st) {
     return smart_holder_from_shared_ptr(std::const_pointer_cast<T>(src), // Const2Mutbl
                                         policy,
                                         parent,
                                         st);
 }
 
 struct shared_ptr_parent_life_support {
     PyObject *parent;
     explicit shared_ptr_parent_life_support(PyObject *parent) : parent{parent} {
         Py_INCREF(parent);
     }
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
         Py_DECREF(parent);
     }
 };
 
 struct shared_ptr_trampoline_self_life_support {
     PyObject *self;
     explicit shared_ptr_trampoline_self_life_support(instance *inst)
         : self{reinterpret_cast<PyObject *>(inst)} {
         gil_scoped_acquire gil;
         Py_INCREF(self);
     }
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
         Py_DECREF(self);
     }
 };
 
 template <typename T,
           typename D,
           typename std::enable_if<std::is_default_constructible<D>::value, int>::type = 0>
 inline std::unique_ptr<T, D> unique_with_deleter(T *raw_ptr, std::unique_ptr<D> &&deleter) {
     if (deleter == nullptr) {
         return std::unique_ptr<T, D>(raw_ptr);
     }
     return std::unique_ptr<T, D>(raw_ptr, std::move(*deleter));
 }
 
 template <typename T,
           typename D,
           typename std::enable_if<!std::is_default_constructible<D>::value, int>::type = 0>
 inline std::unique_ptr<T, D> unique_with_deleter(T *raw_ptr, std::unique_ptr<D> &&deleter) {
     if (deleter == nullptr) {
         pybind11_fail("smart_holder_type_casters: deleter is not default constructible and no"
                       " instance available to return.");
     }
     return std::unique_ptr<T, D>(raw_ptr, std::move(*deleter));
 }
 
 template <typename T>
 struct load_helper : value_and_holder_helper {
     bool was_populated = false;
     bool python_instance_is_alias = false;
 
     void maybe_set_python_instance_is_alias(handle src) {
         if (was_populated) {
             python_instance_is_alias = reinterpret_cast<instance *>(src.ptr())->is_alias;
         }
     }
 
     static std::shared_ptr<T> make_shared_ptr_with_responsible_parent(T *raw_ptr, handle parent) {
         return std::shared_ptr<T>(raw_ptr, shared_ptr_parent_life_support(parent.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:
                                           bool force_potentially_slicing_shared_ptr
                                           = false) const {
         if (!have_holder()) {
             return nullptr;
         }
         throw_if_uninitialized_or_disowned_holder(typeid(T));
         smart_holder &hld = holder();
         hld.ensure_is_not_disowned("load_as_shared_ptr");
         if (hld.vptr_is_using_noop_deleter) {
             if (responsible_parent) {
                 return make_shared_ptr_with_responsible_parent(static_cast<T *>(void_raw_ptr),
                                                                responsible_parent);
             }
             throw std::runtime_error("Non-owning holder (load_as_shared_ptr).");
         }
         auto *type_raw_ptr = static_cast<T *>(void_raw_ptr);
         if (python_instance_is_alias && !force_potentially_slicing_shared_ptr) {
             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) {
                     return std::shared_ptr<T>(released_ptr, type_raw_ptr);
                 }
                 std::shared_ptr<T> to_be_released(
                     type_raw_ptr, shared_ptr_trampoline_self_life_support(loaded_v_h.inst));
                 vptr_gd_ptr->released_ptr = to_be_released;
                 return to_be_released;
             }
             auto *sptsls_ptr = std::get_deleter<shared_ptr_trampoline_self_life_support>(hld.vptr);
             if (sptsls_ptr != nullptr) {
                 // This code is reachable only if there are multiple registered_instances for the
                 // same pointee.
                 if (reinterpret_cast<PyObject *>(loaded_v_h.inst) == sptsls_ptr->self) {
                     pybind11_fail("smart_holder_type_caster_support load_as_shared_ptr failure: "
                                   "loaded_v_h.inst == sptsls_ptr->self");
                 }
             }
             if (sptsls_ptr != nullptr || !memory::type_has_shared_from_this(type_raw_ptr)) {
                 return std::shared_ptr<T>(
                     type_raw_ptr, shared_ptr_trampoline_self_life_support(loaded_v_h.inst));
             }
             if (hld.vptr_is_external_shared_ptr) {
                 pybind11_fail("smart_holder_type_casters load_as_shared_ptr failure: not "
                               "implemented: trampoline-self-life-support for external shared_ptr "
                               "to type inheriting from std::enable_shared_from_this.");
             }
             pybind11_fail(
                 "smart_holder_type_casters: load_as_shared_ptr failure: internal inconsistency.");
         }
         std::shared_ptr<void> void_shd_ptr = hld.template as_shared_ptr<void>();
         return std::shared_ptr<T>(void_shd_ptr, type_raw_ptr);
     }
 
     template <typename D>
     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(tinfo);
         holder().ensure_is_not_disowned(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 = 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, tinfo->get_memory_guarded_delete);
 
         // Critical transfer-of-ownership section. This must stay together.
         if (self_life_support != nullptr) {
             holder().disown(tinfo->get_memory_guarded_delete);
         } else {
             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) {
             self_life_support->activate_life_support(loaded_v_h);
         } else {
             void *value_void_ptr = loaded_v_h.value_ptr();
             loaded_v_h.value_ptr() = nullptr;
             deregister_instance(loaded_v_h.inst, value_void_ptr, loaded_v_h.type);
         }
         // Critical section end.
 
         return result;
     }
 
     // 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(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_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)));
     }
 };
 
 PYBIND11_NAMESPACE_END(smart_holder_type_caster_support)
 
 class type_caster_generic {
 public:
     PYBIND11_NOINLINE explicit type_caster_generic(const std::type_info &type_info)
         : typeinfo(get_type_info(type_info)), cpptype(&type_info) {}
 
     explicit type_caster_generic(const type_info *typeinfo)
         : typeinfo(typeinfo), cpptype(typeinfo ? typeinfo->cpptype : nullptr) {}
 
     bool load(handle src, bool convert) { return load_impl<type_caster_generic>(src, convert); }
 
     PYBIND11_NOINLINE static handle cast(const void *_src,
                                          return_value_policy policy,
                                          handle parent,
                                          const detail::type_info *tinfo,
                                          void *(*copy_constructor)(const void *),
                                          void *(*move_constructor)(const void *),
                                          const void *existing_holder = nullptr) {
         if (!tinfo) { // no type info: error will be set already
             return handle();
         }
 
         void *src = const_cast<void *>(_src);
         if (src == nullptr) {
             return none().release();
         }
 
         if (handle registered_inst = find_registered_python_instance(src, tinfo)) {
             return registered_inst;
         }
 
         auto inst = reinterpret_steal<object>(make_new_instance(tinfo->type));
         auto *wrapper = reinterpret_cast<instance *>(inst.ptr());
         wrapper->owned = false;
         void *&valueptr = values_and_holders(wrapper).begin()->value_ptr();
 
         switch (policy) {
             case return_value_policy::automatic:
             case return_value_policy::take_ownership:
                 valueptr = src;
                 wrapper->owned = true;
                 break;
 
             case return_value_policy::automatic_reference:
             case return_value_policy::reference:
                 valueptr = src;
                 wrapper->owned = false;
                 break;
 
             case return_value_policy::copy:
                 if (copy_constructor) {
                     valueptr = copy_constructor(src);
                 } else {
 #if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
                     std::string type_name(tinfo->cpptype->name());
                     detail::clean_type_id(type_name);
                     throw cast_error("return_value_policy = copy, but type " + type_name
                                      + " is non-copyable!");
 #else
                     throw cast_error("return_value_policy = copy, but type is "
                                      "non-copyable! (#define PYBIND11_DETAILED_ERROR_MESSAGES or "
                                      "compile in debug mode for details)");
 #endif
                 }
                 wrapper->owned = true;
                 break;
 
             case return_value_policy::move:
                 if (move_constructor) {
                     valueptr = move_constructor(src);
                 } else if (copy_constructor) {
                     valueptr = copy_constructor(src);
                 } else {
 #if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
                     std::string type_name(tinfo->cpptype->name());
                     detail::clean_type_id(type_name);
                     throw cast_error("return_value_policy = move, but type " + type_name
                                      + " is neither movable nor copyable!");
 #else
                     throw cast_error("return_value_policy = move, but type is neither "
                                      "movable nor copyable! "
                                      "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in "
                                      "debug mode for details)");
 #endif
                 }
                 wrapper->owned = true;
                 break;
 
             case return_value_policy::reference_internal:
                 valueptr = src;
                 wrapper->owned = false;
                 keep_alive_impl(inst, parent);
                 break;
 
             default:
                 throw cast_error("unhandled return_value_policy: should not happen!");
         }
 
         tinfo->init_instance(wrapper, existing_holder);
 
         return inst.release();
     }
 
     // Base methods for generic caster; there are overridden in copyable_holder_caster
     void load_value(value_and_holder &&v_h) {
         if (typeinfo->holder_enum_v == detail::holder_enum_t::smart_holder) {
             smart_holder_type_caster_support::value_and_holder_helper v_h_helper;
             v_h_helper.loaded_v_h = v_h;
             if (v_h_helper.have_holder()) {
                 v_h_helper.throw_if_uninitialized_or_disowned_holder(cpptype->name());
                 value = v_h_helper.holder().template as_raw_ptr_unowned<void>();
                 return;
             }
         }
         auto *&vptr = v_h.value_ptr();
         // Lazy allocation for unallocated values:
         if (vptr == nullptr) {
             const auto *type = v_h.type ? v_h.type : typeinfo;
             if (type->operator_new) {
                 vptr = type->operator_new(type->type_size);
             } else {
 #if defined(__cpp_aligned_new) && (!defined(_MSC_VER) || _MSC_VER >= 1912)
                 if (type->type_align > __STDCPP_DEFAULT_NEW_ALIGNMENT__) {
                     vptr = ::operator new(type->type_size, std::align_val_t(type->type_align));
                 } else {
                     vptr = ::operator new(type->type_size);
                 }
 #else
                 vptr = ::operator new(type->type_size);
 #endif
             }
         }
         value = vptr;
     }
     bool try_implicit_casts(handle src, bool convert) {
         for (const auto &cast : typeinfo->implicit_casts) {
             type_caster_generic sub_caster(*cast.first);
             if (sub_caster.load(src, convert)) {
                 value = cast.second(sub_caster.value);
                 return true;
             }
         }
         return false;
     }
     bool try_direct_conversions(handle src) {
         for (auto &converter : *typeinfo->direct_conversions) {
             if (converter(src.ptr(), value)) {
                 return true;
             }
         }
         return false;
     }
     bool try_cpp_conduit(handle src) {
         value = try_raw_pointer_ephemeral_from_cpp_conduit(src, cpptype);
         if (value != nullptr) {
             return true;
         }
         return false;
     }
     void check_holder_compat() {}
 
     PYBIND11_NOINLINE static void *local_load(PyObject *src, const type_info *ti) {
         auto caster = type_caster_generic(ti);
         if (caster.load(src, false)) {
             return caster.value;
         }
         return nullptr;
     }
 
     /// Try to load with foreign typeinfo, if available. Used when there is no
     /// native typeinfo, or when the native one wasn't able to produce a value.
     PYBIND11_NOINLINE bool try_load_foreign_module_local(handle src) {
         constexpr auto *local_key = PYBIND11_MODULE_LOCAL_ID;
         const auto pytype = type::handle_of(src);
         if (!hasattr(pytype, local_key)) {
             return false;
         }
 
         type_info *foreign_typeinfo = reinterpret_borrow<capsule>(getattr(pytype, local_key));
         // Only consider this foreign loader if actually foreign and is a loader of the correct cpp
         // type
         if (foreign_typeinfo->module_local_load == &local_load
             || (cpptype && !same_type(*cpptype, *foreign_typeinfo->cpptype))) {
             return false;
         }
 
         if (auto *result = foreign_typeinfo->module_local_load(src.ptr(), foreign_typeinfo)) {
             value = result;
             return true;
         }
         return false;
     }
 
     // Implementation of `load`; this takes the type of `this` so that it can dispatch the relevant
     // bits of code between here and copyable_holder_caster where the two classes need different
     // logic (without having to resort to virtual inheritance).
     template <typename ThisT>
     PYBIND11_NOINLINE bool load_impl(handle src, bool convert) {
         if (!src) {
             return false;
         }
         if (!typeinfo) {
             return try_load_foreign_module_local(src);
         }
 
         auto &this_ = static_cast<ThisT &>(*this);
         this_.check_holder_compat();
 
         PyTypeObject *srctype = Py_TYPE(src.ptr());
 
         // Case 1: If src is an exact type match for the target type then we can reinterpret_cast
         // the instance's value pointer to the target type:
         if (srctype == typeinfo->type) {
             this_.load_value(reinterpret_cast<instance *>(src.ptr())->get_value_and_holder());
             return true;
         }
         // Case 2: We have a derived class
         if (PyType_IsSubtype(srctype, typeinfo->type)) {
             const auto &bases = all_type_info(srctype);
             bool no_cpp_mi = typeinfo->simple_type;
 
             // Case 2a: the python type is a Python-inherited derived class that inherits from just
             // one simple (no MI) pybind11 class, or is an exact match, so the C++ instance is of
             // the right type and we can use reinterpret_cast.
             // (This is essentially the same as case 2b, but because not using multiple inheritance
             // is extremely common, we handle it specially to avoid the loop iterator and type
             // pointer lookup overhead)
             if (bases.size() == 1 && (no_cpp_mi || bases.front()->type == typeinfo->type)) {
                 this_.load_value(reinterpret_cast<instance *>(src.ptr())->get_value_and_holder());
                 return true;
             }
             // Case 2b: the python type inherits from multiple C++ bases.  Check the bases to see
             // if we can find an exact match (or, for a simple C++ type, an inherited match); if
             // so, we can safely reinterpret_cast to the relevant pointer.
             if (bases.size() > 1) {
                 for (auto *base : bases) {
                     if (no_cpp_mi ? PyType_IsSubtype(base->type, typeinfo->type)
                                   : base->type == typeinfo->type) {
                         this_.load_value(
                             reinterpret_cast<instance *>(src.ptr())->get_value_and_holder(base));
                         return true;
                     }
                 }
             }
 
             // Case 2c: C++ multiple inheritance is involved and we couldn't find an exact type
             // match in the registered bases, above, so try implicit casting (needed for proper C++
             // casting when MI is involved).
             if (this_.try_implicit_casts(src, convert)) {
                 return true;
             }
         }
 
         // Perform an implicit conversion
         if (convert) {
             for (const auto &converter : typeinfo->implicit_conversions) {
                 auto temp = reinterpret_steal<object>(converter(src.ptr(), typeinfo->type));
                 if (load_impl<ThisT>(temp, false)) {
                     loader_life_support::add_patient(temp);
                     return true;
                 }
             }
             if (this_.try_direct_conversions(src)) {
                 return true;
             }
         }
 
         // Failed to match local typeinfo. Try again with global.
         if (typeinfo->module_local) {
             if (auto *gtype = get_global_type_info(*typeinfo->cpptype)) {
                 typeinfo = gtype;
                 return load(src, false);
             }
         }
 
         // Global typeinfo has precedence over foreign module_local
         if (try_load_foreign_module_local(src)) {
             return true;
         }
 
         // Custom converters didn't take None, now we convert None to nullptr.
         if (src.is_none()) {
             // Defer accepting None to other overloads (if we aren't in convert mode):
             if (!convert) {
                 return false;
             }
             value = nullptr;
             return true;
         }
 
         if (convert && cpptype && this_.try_cpp_conduit(src)) {
             return true;
         }
 
         return false;
     }
 
     // Called to do type lookup and wrap the pointer and type in a pair when a dynamic_cast
     // isn't needed or can't be used.  If the type is unknown, sets the error and returns a pair
     // with .second = nullptr.  (p.first = nullptr is not an error: it becomes None).
     PYBIND11_NOINLINE static std::pair<const void *, const type_info *>
     src_and_type(const void *src,
                  const std::type_info &cast_type,
                  const std::type_info *rtti_type = nullptr) {
         if (auto *tpi = get_type_info(cast_type)) {
             return {src, const_cast<const type_info *>(tpi)};
         }
 
         // Not found, set error:
         std::string tname = rtti_type ? rtti_type->name() : cast_type.name();
         detail::clean_type_id(tname);
         std::string msg = "Unregistered type : " + tname;
         set_error(PyExc_TypeError, msg.c_str());
         return {nullptr, nullptr};
     }
 
     const type_info *typeinfo = nullptr;
     const std::type_info *cpptype = nullptr;
     void *value = nullptr;
 };
 
 inline object cpp_conduit_method(handle self,
                                  const bytes &pybind11_platform_abi_id,
                                  const capsule &cpp_type_info_capsule,
                                  const bytes &pointer_kind) {
 #ifdef PYBIND11_HAS_STRING_VIEW
     using cpp_str = std::string_view;
 #else
     using cpp_str = std::string;
 #endif
     if (cpp_str(pybind11_platform_abi_id) != PYBIND11_PLATFORM_ABI_ID) {
         return none();
     }
     if (std::strcmp(cpp_type_info_capsule.name(), typeid(std::type_info).name()) != 0) {
         return none();
     }
     if (cpp_str(pointer_kind) != "raw_pointer_ephemeral") {
         throw std::runtime_error("Invalid pointer_kind: \"" + std::string(pointer_kind) + "\"");
     }
     const auto *cpp_type_info = cpp_type_info_capsule.get_pointer<const std::type_info>();
     type_caster_generic caster(*cpp_type_info);
     if (!caster.load(self, false)) {
         return none();
     }
     return capsule(caster.value, cpp_type_info->name());
 }
 
 /**
  * Determine suitable casting operator for pointer-or-lvalue-casting type casters.  The type caster
  * needs to provide `operator T*()` and `operator T&()` operators.
  *
  * If the type supports moving the value away via an `operator T&&() &&` method, it should use
  * `movable_cast_op_type` instead.
  */
 template <typename T>
 using cast_op_type = conditional_t<std::is_pointer<remove_reference_t<T>>::value,
                                    typename std::add_pointer<intrinsic_t<T>>::type,
                                    typename std::add_lvalue_reference<intrinsic_t<T>>::type>;
 
 /**
  * Determine suitable casting operator for a type caster with a movable value.  Such a type caster
  * needs to provide `operator T*()`, `operator T&()`, and `operator T&&() &&`.  The latter will be
  * called in appropriate contexts where the value can be moved rather than copied.
  *
  * These operator are automatically provided when using the PYBIND11_TYPE_CASTER macro.
  */
 template <typename T>
 using movable_cast_op_type
     = conditional_t<std::is_pointer<typename std::remove_reference<T>::type>::value,
                     typename std::add_pointer<intrinsic_t<T>>::type,
                     conditional_t<std::is_rvalue_reference<T>::value,
                                   typename std::add_rvalue_reference<intrinsic_t<T>>::type,
                                   typename std::add_lvalue_reference<intrinsic_t<T>>::type>>;
 
 // Does the container have a mapped type and is it recursive?
 // Implemented by specializations below.
 template <typename Container, typename SFINAE = void>
 struct container_mapped_type_traits {
     static constexpr bool has_mapped_type = false;
     static constexpr bool has_recursive_mapped_type = false;
 };
 
 template <typename Container>
 struct container_mapped_type_traits<
     Container,
     typename std::enable_if<
         std::is_same<typename Container::mapped_type, Container>::value>::type> {
     static constexpr bool has_mapped_type = true;
     static constexpr bool has_recursive_mapped_type = true;
 };
 
 template <typename Container>
 struct container_mapped_type_traits<
     Container,
     typename std::enable_if<
         negation<std::is_same<typename Container::mapped_type, Container>>::value>::type> {
     static constexpr bool has_mapped_type = true;
     static constexpr bool has_recursive_mapped_type = false;
 };
 
 // Does the container have a value type and is it recursive?
 // Implemented by specializations below.
 template <typename Container, typename SFINAE = void>
 struct container_value_type_traits : std::false_type {
     static constexpr bool has_value_type = false;
     static constexpr bool has_recursive_value_type = false;
 };
 
 template <typename Container>
 struct container_value_type_traits<
     Container,
     typename std::enable_if<
         std::is_same<typename Container::value_type, Container>::value>::type> {
     static constexpr bool has_value_type = true;
     static constexpr bool has_recursive_value_type = true;
 };
 
 template <typename Container>
 struct container_value_type_traits<
     Container,
     typename std::enable_if<
         negation<std::is_same<typename Container::value_type, Container>>::value>::type> {
     static constexpr bool has_value_type = true;
     static constexpr bool has_recursive_value_type = false;
 };
 
 /*
  * Tag to be used for representing the bottom of recursively defined types.
  * Define this tag so we don't have to use void.
  */
 struct recursive_bottom {};
 
 /*
  * Implementation detail of `recursive_container_traits` below.
  * `T` is the `value_type` of the container, which might need to be modified to
  * avoid recursive types and const types.
  */
 template <typename T, bool is_this_a_map>
 struct impl_type_to_check_recursively {
     /*
      * If the container is recursive, then no further recursion should be done.
      */
     using if_recursive = recursive_bottom;
     /*
      * Otherwise yield `T` unchanged.
      */
     using if_not_recursive = T;
 };
 
 /*
  * For pairs - only as value type of a map -, the first type should remove the `const`.
  * Also, if the map is recursive, then the recursive checking should consider
  * the first type only.
  */
 template <typename A, typename B>
 struct impl_type_to_check_recursively<std::pair<A, B>, /* is_this_a_map = */ true> {
     using if_recursive = typename std::remove_const<A>::type;
     using if_not_recursive = std::pair<typename std::remove_const<A>::type, B>;
 };
 
 /*
  * Implementation of `recursive_container_traits` below.
  */
 template <typename Container, typename SFINAE = void>
 struct impl_recursive_container_traits {
     using type_to_check_recursively = recursive_bottom;
 };
 
 template <typename Container>
 struct impl_recursive_container_traits<
     Container,
     typename std::enable_if<container_value_type_traits<Container>::has_value_type>::type> {
     static constexpr bool is_recursive
         = container_mapped_type_traits<Container>::has_recursive_mapped_type
           || container_value_type_traits<Container>::has_recursive_value_type;
     /*
      * This member dictates which type Pybind11 should check recursively in traits
      * such as `is_move_constructible`, `is_copy_constructible`, `is_move_assignable`, ...
      * Direct access to `value_type` should be avoided:
      * 1. `value_type` might recursively contain the type again
      * 2. `value_type` of STL map types is `std::pair<A const, B>`, the `const`
      *    should be removed.
      *
      */
     using type_to_check_recursively = typename std::conditional<
         is_recursive,
         typename impl_type_to_check_recursively<
             typename Container::value_type,
             container_mapped_type_traits<Container>::has_mapped_type>::if_recursive,
         typename impl_type_to_check_recursively<
             typename Container::value_type,
             container_mapped_type_traits<Container>::has_mapped_type>::if_not_recursive>::type;
 };
 
 /*
  * This trait defines the `type_to_check_recursively` which is needed to properly
  * handle recursively defined traits such as `is_move_constructible` without going
  * into an infinite recursion.
  * Should be used instead of directly accessing the `value_type`.
  * It cancels the recursion by returning the `recursive_bottom` tag.
  *
  * The default definition of `type_to_check_recursively` is as follows:
  *
  * 1. By default, it is `recursive_bottom`, so that the recursion is canceled.
  * 2. If the type is non-recursive and defines a `value_type`, then the `value_type` is used.
  *    If the `value_type` is a pair and a `mapped_type` is defined,
  *    then the `const` is removed from the first type.
  * 3. If the type is recursive and `value_type` is not a pair, then `recursive_bottom` is returned.
  * 4. If the type is recursive and `value_type` is a pair and a `mapped_type` is defined,
  *    then `const` is removed from the first type and the first type is returned.
  *
  * This behavior can be extended by the user as seen in test_stl_binders.cpp.
  *
  * This struct is exactly the same as impl_recursive_container_traits.
  * The duplication achieves that user-defined specializations don't compete
  * with internal specializations, but take precedence.
  */
 template <typename Container, typename SFINAE = void>
 struct recursive_container_traits : impl_recursive_container_traits<Container> {};
 
 template <typename T>
 struct is_move_constructible
     : all_of<std::is_move_constructible<T>,
              is_move_constructible<
                  typename recursive_container_traits<T>::type_to_check_recursively>> {};
 
 template <>
 struct is_move_constructible<recursive_bottom> : std::true_type {};
 
 // Likewise for std::pair
 // (after C++17 it is mandatory that the move constructor not exist when the two types aren't
 // themselves move constructible, but this can not be relied upon when T1 or T2 are themselves
 // containers).
 template <typename T1, typename T2>
 struct is_move_constructible<std::pair<T1, T2>>
     : all_of<is_move_constructible<T1>, is_move_constructible<T2>> {};
 
 // std::is_copy_constructible isn't quite enough: it lets std::vector<T> (and similar) through when
 // T is non-copyable, but code containing such a copy constructor fails to actually compile.
 template <typename T>
 struct is_copy_constructible
     : all_of<std::is_copy_constructible<T>,
              is_copy_constructible<
                  typename recursive_container_traits<T>::type_to_check_recursively>> {};
 
 template <>
 struct is_copy_constructible<recursive_bottom> : std::true_type {};
 
 // Likewise for std::pair
 // (after C++17 it is mandatory that the copy constructor not exist when the two types aren't
 // themselves copy constructible, but this can not be relied upon when T1 or T2 are themselves
 // containers).
 template <typename T1, typename T2>
 struct is_copy_constructible<std::pair<T1, T2>>
     : all_of<is_copy_constructible<T1>, is_copy_constructible<T2>> {};
 
 // The same problems arise with std::is_copy_assignable, so we use the same workaround.
 template <typename T>
 struct is_copy_assignable
     : all_of<
           std::is_copy_assignable<T>,
           is_copy_assignable<typename recursive_container_traits<T>::type_to_check_recursively>> {
 };
 
 template <>
 struct is_copy_assignable<recursive_bottom> : std::true_type {};
 
 template <typename T1, typename T2>
 struct is_copy_assignable<std::pair<T1, T2>>
     : all_of<is_copy_assignable<T1>, is_copy_assignable<T2>> {};
 
 PYBIND11_NAMESPACE_END(detail)
 
 // polymorphic_type_hook<itype>::get(src, tinfo) determines whether the object pointed
 // to by `src` actually is an instance of some class derived from `itype`.
 // If so, it sets `tinfo` to point to the std::type_info representing that derived
 // type, and returns a pointer to the start of the most-derived object of that type
 // (in which `src` is a subobject; this will be the same address as `src` in most
 // single inheritance cases). If not, or if `src` is nullptr, it simply returns `src`
 // and leaves `tinfo` at its default value of nullptr.
 //
 // The default polymorphic_type_hook just returns src. A specialization for polymorphic
 // types determines the runtime type of the passed object and adjusts the this-pointer
 // appropriately via dynamic_cast<void*>. This is what enables a C++ Animal* to appear
 // to Python as a Dog (if Dog inherits from Animal, Animal is polymorphic, Dog is
 // registered with pybind11, and this Animal is in fact a Dog).
 //
 // You may specialize polymorphic_type_hook yourself for types that want to appear
 // polymorphic to Python but do not use C++ RTTI. (This is a not uncommon pattern
 // in performance-sensitive applications, used most notably in LLVM.)
 //
 // polymorphic_type_hook_base allows users to specialize polymorphic_type_hook with
 // std::enable_if. User provided specializations will always have higher priority than
 // the default implementation and specialization provided in polymorphic_type_hook_base.
 template <typename itype, typename SFINAE = void>
 struct polymorphic_type_hook_base {
     static const void *get(const itype *src, const std::type_info *&) { return src; }
 };
 template <typename itype>
 struct polymorphic_type_hook_base<itype, detail::enable_if_t<std::is_polymorphic<itype>::value>> {
     static const void *get(const itype *src, const std::type_info *&type) {
         type = src ? &typeid(*src) : nullptr;
         return dynamic_cast<const void *>(src);
     }
 };
 template <typename itype, typename SFINAE = void>
 struct polymorphic_type_hook : public polymorphic_type_hook_base<itype> {};
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 /// Generic type caster for objects stored on the heap
 template <typename type>
 class type_caster_base : public type_caster_generic {
     using itype = intrinsic_t<type>;
 
 public:
     static constexpr auto name = const_name<type>();
 
     type_caster_base() : type_caster_base(typeid(type)) {}
     explicit type_caster_base(const std::type_info &info) : type_caster_generic(info) {}
 
     static handle cast(const itype &src, return_value_policy policy, handle parent) {
         if (policy == return_value_policy::automatic
             || policy == return_value_policy::automatic_reference) {
             policy = return_value_policy::copy;
         }
         return cast(std::addressof(src), policy, parent);
     }
 
     static handle cast(itype &&src, return_value_policy, handle parent) {
         return cast(std::addressof(src), return_value_policy::move, parent);
     }
 
     // Returns a (pointer, type_info) pair taking care of necessary type lookup for a
     // polymorphic type (using RTTI by default, but can be overridden by specializing
     // polymorphic_type_hook). If the instance isn't derived, returns the base version.
     static std::pair<const void *, const type_info *> src_and_type(const itype *src) {
         const auto &cast_type = typeid(itype);
         const std::type_info *instance_type = nullptr;
         const void *vsrc = polymorphic_type_hook<itype>::get(src, instance_type);
         if (instance_type && !same_type(cast_type, *instance_type)) {
             // This is a base pointer to a derived type. If the derived type is registered
             // with pybind11, we want to make the full derived object available.
             // In the typical case where itype is polymorphic, we get the correct
             // derived pointer (which may be != base pointer) by a dynamic_cast to
             // most derived type. If itype is not polymorphic, we won't get here
             // except via a user-provided specialization of polymorphic_type_hook,
             // and the user has promised that no this-pointer adjustment is
             // required in that case, so it's OK to use static_cast.
             if (const auto *tpi = get_type_info(*instance_type)) {
                 return {vsrc, tpi};
             }
         }
         // Otherwise we have either a nullptr, an `itype` pointer, or an unknown derived pointer,
         // so don't do a cast
         return type_caster_generic::src_and_type(src, cast_type, instance_type);
     }
 
     static handle cast(const itype *src, return_value_policy policy, handle parent) {
         auto st = src_and_type(src);
         return type_caster_generic::cast(st.first,
                                          policy,
                                          parent,
                                          st.second,
                                          make_copy_constructor(src),
                                          make_move_constructor(src));
     }
 
     static handle cast_holder(const itype *src, const void *holder) {
         auto st = src_and_type(src);
         return type_caster_generic::cast(st.first,
                                          return_value_policy::take_ownership,
                                          {},
                                          st.second,
                                          nullptr,
                                          nullptr,
                                          holder);
     }
 
     template <typename T>
     using cast_op_type = detail::cast_op_type<T>;
 
     // NOLINTNEXTLINE(google-explicit-constructor)
     operator itype *() { return (type *) value; }
     // NOLINTNEXTLINE(google-explicit-constructor)
     operator itype &() {
         if (!value) {
             throw reference_cast_error();
         }
         return *((itype *) value);
     }
 
 protected:
     using Constructor = void *(*) (const void *);
 
     /* Only enabled when the types are {copy,move}-constructible *and* when the type
        does not have a private operator new implementation. A comma operator is used in the
        decltype argument to apply SFINAE to the public copy/move constructors.*/
     template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
     static auto make_copy_constructor(const T *)
         -> decltype(new T(std::declval<const T>()), Constructor{}) {
         return [](const void *arg) -> void * { return new T(*reinterpret_cast<const T *>(arg)); };
     }
 
     template <typename T, typename = enable_if_t<is_move_constructible<T>::value>>
     static auto make_move_constructor(const T *)
         -> decltype(new T(std::declval<T &&>()), Constructor{}) {
         return [](const void *arg) -> void * {
             return new T(std::move(*const_cast<T *>(reinterpret_cast<const T *>(arg))));
         };
     }
 
     static Constructor make_copy_constructor(...) { return nullptr; }
     static Constructor make_move_constructor(...) { return nullptr; }
 };
 
 inline std::string quote_cpp_type_name(const std::string &cpp_type_name) {
     return cpp_type_name; // No-op for now. See PR #4888
 }
 
 PYBIND11_NOINLINE std::string type_info_description(const std::type_info &ti) {
     if (auto *type_data = get_type_info(ti)) {
         handle th((PyObject *) type_data->type);
         return th.attr("__module__").cast<std::string>() + '.'
                + th.attr("__qualname__").cast<std::string>();
     }
     return quote_cpp_type_name(clean_type_id(ti.name()));
 }
 
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

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