Using factored-out make_constructor (PR #2798), removing duplicate code.

include/pybind11/cast.h

@@ -47,6 +47,921 @@
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 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 {
+public:
+    /// A new patient frame is created when a function is entered
+    loader_life_support() {
+        get_internals().loader_patient_stack.push_back(nullptr);
+    }
+
+    /// ... and destroyed after it returns
+    ~loader_life_support() {
+        auto &stack = get_internals().loader_patient_stack;
+        if (stack.empty())
+            pybind11_fail("loader_life_support: internal error");
+
+        auto ptr = stack.back();
+        stack.pop_back();
+        Py_CLEAR(ptr);
+
+        // A heuristic to reduce the stack's capacity (e.g. after long recursive calls)
+        if (stack.capacity() > 16 && !stack.empty() && stack.capacity() / stack.size() > 2)
+            stack.shrink_to_fit();
+    }
+
+    /// 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) {
+        auto &stack = get_internals().loader_patient_stack;
+        if (stack.empty())
+            throw cast_error("When called outside a bound function, py::cast() cannot "
+                             "do Python -> C++ conversions which require the creation "
+                             "of temporary values");
+
+        auto &list_ptr = stack.back();
+        if (list_ptr == nullptr) {
+            list_ptr = PyList_New(1);
+            if (!list_ptr)
+                pybind11_fail("loader_life_support: error allocating list");
+            PyList_SET_ITEM(list_ptr, 0, h.inc_ref().ptr());
+        } else {
+            auto result = PyList_Append(list_ptr, h.ptr());
+            if (result == -1)
+                pybind11_fail("loader_life_support: error adding patient");
+        }
+    }
+};
+
+// 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);
+
+// Populates a just-created cache entry.
+PYBIND11_NOINLINE inline void all_type_info_populate(PyTypeObject *t, std::vector<type_info *> &bases) {
+    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) bases.push_back(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) {
+    auto ins = all_type_info_get_cache(type);
+    if (ins.second)
+        // New cache entry: populate it
+        all_type_info_populate(type, ins.first->second);
+
+    return ins.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 inline detail::type_info* get_type_info(PyTypeObject *type) {
+    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 = registered_local_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) {
+    auto &types = get_internals().registered_types_cpp;
+    auto it = types.find(tp);
+    if (it != types.end())
+        return it->second;
+    return nullptr;
+}
+
+/// Return the type info for a given C++ type; on lookup failure can either throw or return nullptr.
+PYBIND11_NOINLINE inline 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 \"" + tname + "\"");
+    }
+    return nullptr;
+}
+
+PYBIND11_NOINLINE inline 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);
+}
+
+// Searches the inheritance graph for a registered Python instance, using all_type_info().
+PYBIND11_NOINLINE inline handle find_registered_python_instance(void *src,
+                                                                const detail::type_info *tinfo) {
+    auto it_instances = get_internals().registered_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))
+                return handle((PyObject *) it_i->second).inc_ref();
+        }
+    }
+    return handle();
+}
+
+struct value_and_holder {
+    instance *inst = nullptr;
+    size_t index = 0u;
+    const detail::type_info *type = nullptr;
+    void **vh = nullptr;
+
+    // Main constructor for a found value/holder:
+    value_and_holder(instance *i, const detail::type_info *type, size_t vpos, size_t index) :
+        inst{i}, index{index}, type{type},
+        vh{inst->simple_layout ? inst->simple_value_holder : &inst->nonsimple.values_and_holders[vpos]}
+    {}
+
+    // Default constructor (used to signal a value-and-holder not found by get_value_and_holder())
+    value_and_holder() = default;
+
+    // Used for past-the-end iterator
+    value_and_holder(size_t index) : index{index} {}
+
+    template <typename V = void> V *&value_ptr() const {
+        return reinterpret_cast<V *&>(vh[0]);
+    }
+    // True if this `value_and_holder` has a non-null value pointer
+    explicit operator bool() const { return value_ptr(); }
+
+    template <typename H> H &holder() const {
+        return reinterpret_cast<H &>(vh[1]);
+    }
+    bool holder_constructed() const {
+        return inst->simple_layout
+            ? inst->simple_holder_constructed
+            : inst->nonsimple.status[index] & instance::status_holder_constructed;
+    }
+    void set_holder_constructed(bool v = true) {
+        if (inst->simple_layout)
+            inst->simple_holder_constructed = v;
+        else if (v)
+            inst->nonsimple.status[index] |= instance::status_holder_constructed;
+        else
+            inst->nonsimple.status[index] &= (uint8_t) ~instance::status_holder_constructed;
+    }
+    bool instance_registered() const {
+        return inst->simple_layout
+            ? inst->simple_instance_registered
+            : inst->nonsimple.status[index] & instance::status_instance_registered;
+    }
+    void set_instance_registered(bool v = true) {
+        if (inst->simple_layout)
+            inst->simple_instance_registered = v;
+        else if (v)
+            inst->nonsimple.status[index] |= instance::status_instance_registered;
+        else
+            inst->nonsimple.status[index] &= (uint8_t) ~instance::status_instance_registered;
+    }
+};
+
+// 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:
+    values_and_holders(instance *inst) : inst{inst}, tinfo(all_type_info(Py_TYPE(inst))) {}
+
+    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},
+            curr(inst /* instance */,
+                 types->empty() ? nullptr : (*types)[0] /* type info */,
+                 0, /* vpos: (non-simple types only): the first vptr comes first */
+                 0 /* index */)
+        {}
+        // Past-the-end iterator:
+        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(); }
+};
+
+/**
+ * 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 inline 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(NDEBUG)
+    pybind11_fail("pybind11::detail::instance::get_value_and_holder: "
+            "type is not a pybind11 base of the given instance "
+            "(compile in debug mode for type details)");
+#else
+    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");
+#endif
+}
+
+PYBIND11_NOINLINE inline void instance::allocate_layout() {
+    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: in Python 3.6
+        // they default to using pymalloc, which is designed to be efficient for small allocations
+        // like the one we're doing here; in earlier versions (and for larger allocations) they are
+        // just wrappers around malloc.
+#if PY_VERSION_HEX >= 0x03050000
+        nonsimple.values_and_holders = (void **) PyMem_Calloc(space, sizeof(void *));
+        if (!nonsimple.values_and_holders) throw std::bad_alloc();
+#else
+        nonsimple.values_and_holders = (void **) PyMem_New(void *, space);
+        if (!nonsimple.values_and_holders) throw std::bad_alloc();
+        std::memset(nonsimple.values_and_holders, 0, space * sizeof(void *));
+#endif
+        nonsimple.status = reinterpret_cast<uint8_t *>(&nonsimple.values_and_holders[flags_at]);
+    }
+    owned = true;
+}
+
+PYBIND11_NOINLINE inline void instance::deallocate_layout() {
+    if (!simple_layout)
+        PyMem_Free(nonsimple.values_and_holders);
+}
+
+PYBIND11_NOINLINE inline 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 inline std::string error_string() {
+    if (!PyErr_Occurred()) {
+        PyErr_SetString(PyExc_RuntimeError, "Unknown internal error occurred");
+        return "Unknown internal error occurred";
+    }
+
+    error_scope scope; // Preserve error state
+
+    std::string errorString;
+    if (scope.type) {
+        errorString += handle(scope.type).attr("__name__").cast<std::string>();
+        errorString += ": ";
+    }
+    if (scope.value)
+        errorString += (std::string) str(scope.value);
+
+    PyErr_NormalizeException(&scope.type, &scope.value, &scope.trace);
+
+#if PY_MAJOR_VERSION >= 3
+    if (scope.trace != nullptr)
+        PyException_SetTraceback(scope.value, scope.trace);
+#endif
+
+#if !defined(PYPY_VERSION)
+    if (scope.trace) {
+        auto *trace = (PyTracebackObject *) scope.trace;
+
+        /* Get the deepest trace possible */
+        while (trace->tb_next)
+            trace = trace->tb_next;
+
+        PyFrameObject *frame = trace->tb_frame;
+        errorString += "\n\nAt:\n";
+        while (frame) {
+            int lineno = PyFrame_GetLineNumber(frame);
+            errorString +=
+                "  " + handle(frame->f_code->co_filename).cast<std::string>() +
+                "(" + std::to_string(lineno) + "): " +
+                handle(frame->f_code->co_name).cast<std::string>() + "\n";
+            frame = frame->f_back;
+        }
+    }
+#endif
+
+    return errorString;
+}
+
+PYBIND11_NOINLINE inline handle get_object_handle(const void *ptr, const detail::type_info *type ) {
+    auto &instances = get_internals().registered_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();
+}
+
+inline PyThreadState *get_thread_state_unchecked() {
+#if defined(PYPY_VERSION)
+    return PyThreadState_GET();
+#elif PY_VERSION_HEX < 0x03000000
+    return _PyThreadState_Current;
+#elif PY_VERSION_HEX < 0x03050000
+    return (PyThreadState*) _Py_atomic_load_relaxed(&_PyThreadState_Current);
+#elif PY_VERSION_HEX < 0x03050200
+    return (PyThreadState*) _PyThreadState_Current.value;
+#else
+    return _PyThreadState_UncheckedGet();
+#endif
+}
+
+// Forward declarations
+inline void keep_alive_impl(handle nurse, handle patient);
+inline PyObject *make_new_instance(PyTypeObject *type);
+
+class type_caster_generic {
+public:
+    PYBIND11_NOINLINE type_caster_generic(const std::type_info &type_info)
+        : typeinfo(get_type_info(type_info)), cpptype(&type_info) { }
+
+    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(NDEBUG)
+                    throw cast_error("return_value_policy = copy, but type is "
+                                     "non-copyable! (compile in debug mode for details)");
+#else
+                    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!");
+#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(NDEBUG)
+                    throw cast_error("return_value_policy = move, but type is neither "
+                                     "movable nor copyable! "
+                                     "(compile in debug mode for details)");
+#else
+                    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!");
+#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) {
+        auto *&vptr = v_h.value_ptr();
+        // Lazy allocation for unallocated values:
+        if (vptr == nullptr) {
+            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
+                #endif
+                vptr = ::operator new(type->type_size);
+            }
+        }
+        value = vptr;
+    }
+    bool try_implicit_casts(handle src, bool convert) {
+        for (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;
+    }
+    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);
+        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;
+        }
+
+        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
+        else if (PyType_IsSubtype(srctype, typeinfo->type)) {
+            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.
+            else 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 (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
+        return try_load_foreign_module_local(src);
+    }
+
+
+    // 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;
+        PyErr_SetString(PyExc_TypeError, msg.c_str());
+        return {nullptr, nullptr};
+    }
+
+    const type_info *typeinfo = nullptr;
+    const std::type_info *cpptype = nullptr;
+    void *value = nullptr;
+};
+
+/**
+ * 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>>;
+
+// 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, typename SFINAE = void> struct is_copy_constructible : std::is_copy_constructible<T> {};
+
+// Specialization for types that appear to be copy constructible but also look like stl containers
+// (we specifically check for: has `value_type` and `reference` with `reference = value_type&`): if
+// so, copy constructability depends on whether the value_type is copy constructible.
+template <typename Container> struct is_copy_constructible<Container, enable_if_t<all_of<
+        std::is_copy_constructible<Container>,
+        std::is_same<typename Container::value_type &, typename Container::reference>,
+        // Avoid infinite recursion
+        negation<std::is_same<Container, typename Container::value_type>>
+    >::value>> : is_copy_constructible<typename Container::value_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, typename SFINAE = void> struct is_copy_assignable : std::is_copy_assignable<T> {};
+template <typename Container> struct is_copy_assignable<Container, enable_if_t<all_of<
+        std::is_copy_assignable<Container>,
+        std::is_same<typename Container::value_type &, typename Container::reference>
+    >::value>> : is_copy_assignable<typename Container::value_type> {};
+template <typename T1, typename T2> struct is_copy_assignable<std::pair<T1, T2>>
+    : all_of<is_copy_assignable<T1>, is_copy_assignable<T2>> {};
+
+// Helper for type_caster_base.
+struct make_constructor {
+    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. */
+    template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
+    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
+        return [](const void *arg) -> void * {
+            return new T(*reinterpret_cast<const T *>(arg));
+        };
+    }
+
+    template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
+    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), 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; }
+};
+
+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,
+                                                  protected make_constructor {
+    using itype = intrinsic_t<type>;
+
+public:
+    static constexpr auto 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(&src, policy, parent);
+    }
+
+    static handle cast(itype &&src, return_value_policy, handle parent) {
+        return cast(&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) {
+        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>;
+
+    operator itype*() { return (type *) value; }
+    operator itype&() { if (!value) throw reference_cast_error(); return *((itype *) value); }
+};
+
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 
 

tests/test_classh_wip.cpp

@@ -123,8 +123,8 @@ struct type_caster<mpty> : smart_holder_type_caster_load<mpty> {
             policy,
             policy,
             parent,
             parent,
             st.second,
             st.second,
-            make_copy_constructor(src),
-            make_move_constructor(src));
+            make_constructor::make_copy_constructor(src),
+            make_constructor::make_move_constructor(src));
     }
     }
 
 
     static handle cast(mpty *src, return_value_policy policy, handle parent) {
     static handle cast(mpty *src, return_value_policy policy, handle parent) {
@@ -156,33 +156,6 @@ struct type_caster<mpty> : smart_holder_type_caster_load<mpty> {
 
 
     // clang-format on
     // clang-format on
 
 
-    // type_caster_base BEGIN
-    // clang-format off
-
-    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. */
-    template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
-    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
-        return [](const void *arg) -> void * {
-            return new T(*reinterpret_cast<const T *>(arg));
-        };
-    }
-
-    template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
-    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), 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; }
-
-    // clang-format on
-    // type_caster_base END
-
     // Originally type_caster_generic::cast.
     // Originally type_caster_generic::cast.
     PYBIND11_NOINLINE static handle cast_const_raw_ptr(const void *_src,
     PYBIND11_NOINLINE static handle cast_const_raw_ptr(const void *_src,
                                                        return_value_policy policy,
                                                        return_value_policy policy,