cast.h

/*
    pybind11/cast.h: Partial template specializations to cast between
    C++ and Python types

    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 "pytypes.h"
#include "typeid.h"
#include "descr.h"
#include <array>
#include <limits>

NAMESPACE_BEGIN(pybind11)
NAMESPACE_BEGIN(detail)

/// Additional type information which does not fit into the PyTypeObject
struct type_info {
    PyTypeObject *type;
    size_t type_size;
    void (*init_holder)(PyObject *, const void *);
    std::vector<PyObject *(*)(PyObject *, PyTypeObject *)> implicit_conversions;
    std::vector<std::pair<const std::type_info *, void *(*)(void *)>> implicit_casts;
    std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;
    buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
    void *get_buffer_data = nullptr;
    /** A simple type never occurs as a (direct or indirect) parent
     * of a class that makes use of multiple inheritance */
    bool simple_type = true;
};

PYBIND11_NOINLINE inline internals &get_internals() {
    static internals *internals_ptr = nullptr;
    if (internals_ptr)
        return *internals_ptr;
    handle builtins(PyEval_GetBuiltins());
    const char *id = PYBIND11_INTERNALS_ID;
    if (builtins.contains(id) && isinstance<capsule>(builtins[id])) {
        internals_ptr = capsule(builtins[id]);
    } else {
        internals_ptr = new internals();
        #if defined(WITH_THREAD)
            PyEval_InitThreads();
            PyThreadState *tstate = PyThreadState_Get();
            internals_ptr->tstate = PyThread_create_key();
            PyThread_set_key_value(internals_ptr->tstate, tstate);
            internals_ptr->istate = tstate->interp;
        #endif
        builtins[id] = capsule(internals_ptr);
        internals_ptr->registered_exception_translators.push_front(
            [](std::exception_ptr p) -> void {
                try {
                    if (p) std::rethrow_exception(p);
                } catch (error_already_set &e)           { e.restore();                                    return;
                } catch (const builtin_exception &e)     { e.set_error();                                  return;
                } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return;
                } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
                } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return;
                } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
                } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return;
                } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return;
                } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return;
                } catch (...) {
                    PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
                    return;
                }
            }
        );
    }
    return *internals_ptr;
}

PYBIND11_NOINLINE inline detail::type_info* get_type_info(PyTypeObject *type) {
    auto const &type_dict = get_internals().registered_types_py;
    do {
        auto it = type_dict.find(type);
        if (it != type_dict.end())
            return (detail::type_info *) it->second;
        type = type->tp_base;
        if (!type)
            return nullptr;
    } while (true);
}

PYBIND11_NOINLINE inline detail::type_info *get_type_info(const std::type_info &tp,
                                                          bool throw_if_missing = false) {
    auto &types = get_internals().registered_types_cpp;

    auto it = types.find(std::type_index(tp));
    if (it != types.end())
        return (detail::type_info *) it->second;
    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);
}

PYBIND11_NOINLINE inline bool isinstance_generic(handle obj, const std::type_info &tp) {
    const auto type = detail::get_type_handle(tp, false);
    if (!type)
        return false;

    const auto result = PyObject_IsInstance(obj.ptr(), type.ptr());
    if (result == -1)
        throw error_already_set();
    return result != 0;
}

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 (scope.trace) {
        PyTracebackObject *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;
        }
        trace = trace->tb_next;
    }

    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) {
        auto instance_type = detail::get_type_info(Py_TYPE(it->second));
        if (instance_type && instance_type == type)
            return handle((PyObject *) it->second);
    }
    return handle();
}

inline PyThreadState *get_thread_state_unchecked() {
#if   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 declaration
inline void keep_alive_impl(handle nurse, handle patient);

class type_caster_generic {
public:
    PYBIND11_NOINLINE type_caster_generic(const std::type_info &type_info)
     : typeinfo(get_type_info(type_info)) { }

    PYBIND11_NOINLINE bool load(handle src, bool convert) {
        if (!src)
            return false;
        return load(src, convert, Py_TYPE(src.ptr()));
    }

    bool load(handle src, bool convert, PyTypeObject *tobj) {
        if (!src || !typeinfo)
            return false;
        if (src.is_none()) {
            value = nullptr;
            return true;
        }

        if (typeinfo->simple_type) { /* Case 1: no multiple inheritance etc. involved */
            /* Check if we can safely perform a reinterpret-style cast */
            if (PyType_IsSubtype(tobj, typeinfo->type)) {
                value = reinterpret_cast<instance<void> *>(src.ptr())->value;
                return true;
            }
        } else { /* Case 2: multiple inheritance */
            /* Check if we can safely perform a reinterpret-style cast */
            if (tobj == typeinfo->type) {
                value = reinterpret_cast<instance<void> *>(src.ptr())->value;
                return true;
            }

            /* If this is a python class, also check the parents recursively */
            auto const &type_dict = get_internals().registered_types_py;
            bool new_style_class = PyType_Check(tobj);
            if (type_dict.find(tobj) == type_dict.end() && new_style_class && tobj->tp_bases) {
                auto parents = reinterpret_borrow<tuple>(tobj->tp_bases);
                for (handle parent : parents) {
                    bool result = load(src, convert, (PyTypeObject *) parent.ptr());
                    if (result)
                        return true;
                }
            }

            /* Try implicit casts */
            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;
                }
            }
        }

        /* Perform an implicit conversion */
        if (convert) {
            for (auto &converter : typeinfo->implicit_conversions) {
                temp = reinterpret_steal<object>(converter(src.ptr(), typeinfo->type));
                if (load(temp, false))
                    return true;
            }
            for (auto &converter : *typeinfo->direct_conversions) {
                if (converter(src.ptr(), value))
                    return true;
            }
        }
        return false;
    }

    PYBIND11_NOINLINE static handle cast(const void *_src, return_value_policy policy, handle parent,
                                         const std::type_info *type_info,
                                         const std::type_info *type_info_backup,
                                         void *(*copy_constructor)(const void *),
                                         void *(*move_constructor)(const void *),
                                         const void *existing_holder = nullptr) {
        void *src = const_cast<void *>(_src);
        if (src == nullptr)
            return none().inc_ref();

        auto &internals = get_internals();

        auto it = internals.registered_types_cpp.find(std::type_index(*type_info));
        if (it == internals.registered_types_cpp.end()) {
            type_info = type_info_backup;
            it = internals.registered_types_cpp.find(std::type_index(*type_info));
        }

        if (it == internals.registered_types_cpp.end()) {
            std::string tname = type_info->name();
            detail::clean_type_id(tname);
            std::string msg = "Unregistered type : " + tname;
            PyErr_SetString(PyExc_TypeError, msg.c_str());
            return handle();
        }

        auto tinfo = (const detail::type_info *) it->second;

        auto it_instances = internals.registered_instances.equal_range(src);
        for (auto it_i = it_instances.first; it_i != it_instances.second; ++it_i) {
            auto instance_type = detail::get_type_info(Py_TYPE(it_i->second));
            if (instance_type && instance_type == tinfo)
                return handle((PyObject *) it_i->second).inc_ref();
        }

        auto inst = reinterpret_steal<object>(PyType_GenericAlloc(tinfo->type, 0));

        auto wrapper = (instance<void> *) inst.ptr();

        wrapper->value = nullptr;
        wrapper->owned = false;

        switch (policy) {
            case return_value_policy::automatic:
            case return_value_policy::take_ownership:
                wrapper->value = src;
                wrapper->owned = true;
                break;

            case return_value_policy::automatic_reference:
            case return_value_policy::reference:
                wrapper->value = src;
                wrapper->owned = false;
                break;

            case return_value_policy::copy:
                if (copy_constructor)
                    wrapper->value = copy_constructor(src);
                else
                    throw cast_error("return_value_policy = copy, but the "
                                     "object is non-copyable!");
                wrapper->owned = true;
                break;

            case return_value_policy::move:
                if (move_constructor)
                    wrapper->value = move_constructor(src);
                else if (copy_constructor)
                    wrapper->value = copy_constructor(src);
                else
                    throw cast_error("return_value_policy = move, but the "
                                     "object is neither movable nor copyable!");
                wrapper->owned = true;
                break;

            case return_value_policy::reference_internal:
                wrapper->value = src;
                wrapper->owned = false;
                detail::keep_alive_impl(inst, parent);
                break;

            default:
                throw cast_error("unhandled return_value_policy: should not happen!");
        }

        tinfo->init_holder(inst.ptr(), existing_holder);

        internals.registered_instances.emplace(wrapper->value, inst.ptr());

        return inst.release();
    }

protected:
    const type_info *typeinfo = nullptr;
    void *value = nullptr;
    object temp;
};

/* Determine suitable casting operator */
template <typename T>
using cast_op_type = typename std::conditional<std::is_pointer<typename std::remove_reference<T>::type>::value,
    typename std::add_pointer<intrinsic_t<T>>::type,
    typename std::add_lvalue_reference<intrinsic_t<T>>::type>::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<
        std::is_copy_constructible<Container>::value &&
        std::is_same<typename Container::value_type &, typename Container::reference>::value
    >> : std::is_copy_constructible<typename Container::value_type> {};

/// 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 PYBIND11_DESCR name() { return type_descr(_<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);
    }

    static handle cast(const itype *src, return_value_policy policy, handle parent) {
        return type_caster_generic::cast(
            src, policy, parent, src ? &typeid(*src) : nullptr, &typeid(type),
            make_copy_constructor(src), make_move_constructor(src));
    }

    template <typename T> using cast_op_type = pybind11::detail::cast_op_type<T>;

    operator itype*() { return (type *) value; }
    operator itype&() { if (!value) throw reference_cast_error(); return *((itype *) value); }

protected:
    typedef void *(*Constructor)(const void *stream);
#if !defined(_MSC_VER)
    /* Only enabled when the types are {copy,move}-constructible *and* when the type
       does not have a private operator new implementaton. */
    template <typename T = type, typename = enable_if_t<is_copy_constructible<T>::value>> static auto make_copy_constructor(const T *value) -> decltype(new T(*value), Constructor(nullptr)) {
        return [](const void *arg) -> void * { return new T(*((const T *) arg)); }; }
    template <typename T = type> static auto make_move_constructor(const T *value) -> decltype(new T(std::move(*((T *) value))), Constructor(nullptr)) {
        return [](const void *arg) -> void * { return (void *) new T(std::move(*((T *) arg))); }; }
#else
    /* Visual Studio 2015's SFINAE implementation doesn't yet handle the above robustly in all situations.
       Use a workaround that only tests for constructibility for now. */
    template <typename T = type, typename = enable_if_t<is_copy_constructible<T>::value>>
    static Constructor make_copy_constructor(const T *value) {
        return [](const void *arg) -> void * { return new T(*((const T *)arg)); }; }
    template <typename T = type, typename = enable_if_t<std::is_move_constructible<T>::value>>
    static Constructor make_move_constructor(const T *value) {
        return [](const void *arg) -> void * { return (void *) new T(std::move(*((T *)arg))); }; }
#endif

    static Constructor make_copy_constructor(...) { return nullptr; }
    static Constructor make_move_constructor(...) { return nullptr; }
};

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>>;

// Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
template <typename T> typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &caster) {
    return caster.operator typename make_caster<T>::template cast_op_type<T>();
}
template <typename T> typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &&caster) {
    return cast_op<T>(caster);
}

template <typename type> class type_caster<std::reference_wrapper<type>> : public type_caster_base<type> {
public:
    static handle cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
        return type_caster_base<type>::cast(&src.get(), policy, parent);
    }
    template <typename T> using cast_op_type = std::reference_wrapper<type>;
    operator std::reference_wrapper<type>() { return std::ref(*((type *) this->value)); }
};

#define PYBIND11_TYPE_CASTER(type, py_name) \
    protected: \
        type value; \
    public: \
        static PYBIND11_DESCR name() { return type_descr(py_name); } \
        static handle cast(const type *src, return_value_policy policy, handle parent) { \
            return cast(*src, policy, parent); \
        } \
        operator type*() { return &value; } \
        operator type&() { return value; } \
        template <typename _T> using cast_op_type = pybind11::detail::cast_op_type<_T>


template <typename T>
struct type_caster<T, enable_if_t<std::is_arithmetic<T>::value>> {
    typedef typename std::conditional<sizeof(T) <= sizeof(long), long, long long>::type _py_type_0;
    typedef typename std::conditional<std::is_signed<T>::value, _py_type_0, typename std::make_unsigned<_py_type_0>::type>::type _py_type_1;
    typedef typename std::conditional<std::is_floating_point<T>::value, double, _py_type_1>::type py_type;
public:

    bool load(handle src, bool) {
        py_type py_value;

        if (!src) {
            return false;
        } if (std::is_floating_point<T>::value) {
            py_value = (py_type) PyFloat_AsDouble(src.ptr());
        } else if (sizeof(T) <= sizeof(long)) {
            if (PyFloat_Check(src.ptr()))
                return false;
            if (std::is_signed<T>::value)
                py_value = (py_type) PyLong_AsLong(src.ptr());
            else
                py_value = (py_type) PyLong_AsUnsignedLong(src.ptr());
        } else {
            if (PyFloat_Check(src.ptr()))
                return false;
            if (std::is_signed<T>::value)
                py_value = (py_type) PYBIND11_LONG_AS_LONGLONG(src.ptr());
            else
                py_value = (py_type) PYBIND11_LONG_AS_UNSIGNED_LONGLONG(src.ptr());
        }

        if ((py_value == (py_type) -1 && PyErr_Occurred()) ||
            (std::is_integral<T>::value && sizeof(py_type) != sizeof(T) &&
               (py_value < (py_type) std::numeric_limits<T>::min() ||
                py_value > (py_type) std::numeric_limits<T>::max()))) {
#if PY_VERSION_HEX < 0x03000000
            bool type_error = PyErr_ExceptionMatches(PyExc_SystemError);
#else
            bool type_error = PyErr_ExceptionMatches(PyExc_TypeError);
#endif
            PyErr_Clear();
            if (type_error && PyNumber_Check(src.ptr())) {
                auto tmp = reinterpret_borrow<object>(std::is_floating_point<T>::value
                                                      ? PyNumber_Float(src.ptr())
                                                      : PyNumber_Long(src.ptr()));
                PyErr_Clear();
                return load(tmp, false);
            }
            return false;
        }

        value = (T) py_value;
        return true;
    }

    static handle cast(T src, return_value_policy /* policy */, handle /* parent */) {
        if (std::is_floating_point<T>::value) {
            return PyFloat_FromDouble((double) src);
        } else if (sizeof(T) <= sizeof(long)) {
            if (std::is_signed<T>::value)
                return PyLong_FromLong((long) src);
            else
                return PyLong_FromUnsignedLong((unsigned long) src);
        } else {
            if (std::is_signed<T>::value)
                return PyLong_FromLongLong((long long) src);
            else
                return PyLong_FromUnsignedLongLong((unsigned long long) src);
        }
    }

    PYBIND11_TYPE_CASTER(T, _<std::is_integral<T>::value>("int", "float"));
};

template<typename T> struct void_caster {
public:
    bool load(handle, bool) { return false; }
    static handle cast(T, return_value_policy /* policy */, handle /* parent */) {
        return none().inc_ref();
    }
    PYBIND11_TYPE_CASTER(T, _("None"));
};

template <> class type_caster<void_type> : public void_caster<void_type> {};

template <> class type_caster<void> : public type_caster<void_type> {
public:
    using type_caster<void_type>::cast;

    bool load(handle h, bool) {
        if (!h) {
            return false;
        } else if (h.is_none()) {
            value = nullptr;
            return true;
        }

        /* Check if this is a capsule */
        if (isinstance<capsule>(h)) {
            value = reinterpret_borrow<capsule>(h);
            return true;
        }

        /* Check if this is a C++ type */
        if (get_type_info((PyTypeObject *) h.get_type().ptr())) {
            value = ((instance<void> *) h.ptr())->value;
            return true;
        }

        /* Fail */
        return false;
    }

    static handle cast(const void *ptr, return_value_policy /* policy */, handle /* parent */) {
        if (ptr)
            return capsule(ptr).release();
        else
            return none().inc_ref();
    }

    template <typename T> using cast_op_type = void*&;
    operator void *&() { return value; }
    static PYBIND11_DESCR name() { return type_descr(_("capsule")); }
private:
    void *value = nullptr;
};

template <> class type_caster<std::nullptr_t> : public type_caster<void_type> { };

template <> class type_caster<bool> {
public:
    bool load(handle src, bool) {
        if (!src) return false;
        else if (src.ptr() == Py_True) { value = true; return true; }
        else if (src.ptr() == Py_False) { value = false; return true; }
        else return false;
    }
    static handle cast(bool src, return_value_policy /* policy */, handle /* parent */) {
        return handle(src ? Py_True : Py_False).inc_ref();
    }
    PYBIND11_TYPE_CASTER(bool, _("bool"));
};

template <> class type_caster<std::string> {
public:
    bool load(handle src, bool) {
        object temp;
        handle load_src = src;
        if (!src) {
            return false;
        } else if (PyUnicode_Check(load_src.ptr())) {
            temp = reinterpret_steal<object>(PyUnicode_AsUTF8String(load_src.ptr()));
            if (!temp) { PyErr_Clear(); return false; }  // UnicodeEncodeError
            load_src = temp;
        }
        char *buffer;
        ssize_t length;
        int err = PYBIND11_BYTES_AS_STRING_AND_SIZE(load_src.ptr(), &buffer, &length);
        if (err == -1) { PyErr_Clear(); return false; }  // TypeError
        value = std::string(buffer, (size_t) length);
        success = true;
        return true;
    }

    static handle cast(const std::string &src, return_value_policy /* policy */, handle /* parent */) {
        return PyUnicode_FromStringAndSize(src.c_str(), (ssize_t) src.length());
    }

    PYBIND11_TYPE_CASTER(std::string, _(PYBIND11_STRING_NAME));
protected:
    bool success = false;
};

template <typename type, typename deleter> class type_caster<std::unique_ptr<type, deleter>> {
public:
    static handle cast(std::unique_ptr<type, deleter> &&src, return_value_policy policy, handle parent) {
        handle result = type_caster_base<type>::cast(src.get(), policy, parent);
        if (result)
            src.release();
        return result;
    }
    static PYBIND11_DESCR name() { return type_caster_base<type>::name(); }
};

template <> class type_caster<std::wstring> {
public:
    bool load(handle src, bool) {
        object temp;
        handle load_src = src;
        if (!src) {
            return false;
        } else if (!PyUnicode_Check(load_src.ptr())) {
            temp = reinterpret_steal<object>(PyUnicode_FromObject(load_src.ptr()));
            if (!temp) { PyErr_Clear(); return false; }
            load_src = temp;
        }
        wchar_t *buffer = nullptr;
        ssize_t length = -1;
#if PY_MAJOR_VERSION >= 3
        buffer = PyUnicode_AsWideCharString(load_src.ptr(), &length);
#else
        temp = reinterpret_steal<object>(
            sizeof(wchar_t) == sizeof(short)
                ? PyUnicode_AsUTF16String(load_src.ptr())
                : PyUnicode_AsUTF32String(load_src.ptr()));
        if (temp) {
            int err = PYBIND11_BYTES_AS_STRING_AND_SIZE(temp.ptr(), (char **) &buffer, &length);
            if (err == -1) { buffer = nullptr; }  // TypeError
            length = length / (ssize_t) sizeof(wchar_t) - 1; ++buffer; // Skip BOM
        }
#endif
        if (!buffer) { PyErr_Clear(); return false; }
        value = std::wstring(buffer, (size_t) length);
        success = true;
        return true;
    }

    static handle cast(const std::wstring &src, return_value_policy /* policy */, handle /* parent */) {
        return PyUnicode_FromWideChar(src.c_str(), (ssize_t) src.length());
    }

    PYBIND11_TYPE_CASTER(std::wstring, _(PYBIND11_STRING_NAME));
protected:
    bool success = false;
};

template <> class type_caster<char> : public type_caster<std::string> {
public:
    bool load(handle src, bool convert) {
        if (src.is_none()) return true;
        return type_caster<std::string>::load(src, convert);
    }

    static handle cast(const char *src, return_value_policy /* policy */, handle /* parent */) {
        if (src == nullptr) return none().inc_ref();
        return PyUnicode_FromString(src);
    }

    static handle cast(char src, return_value_policy /* policy */, handle /* parent */) {
        char str[2] = { src, '\0' };
        return PyUnicode_DecodeLatin1(str, 1, nullptr);
    }

    operator char*() { return success ? (char *) value.c_str() : nullptr; }
    operator char&() { return value[0]; }

    static PYBIND11_DESCR name() { return type_descr(_(PYBIND11_STRING_NAME)); }
};

template <> class type_caster<wchar_t> : public type_caster<std::wstring> {
public:
    bool load(handle src, bool convert) {
        if (src.is_none()) return true;
        return type_caster<std::wstring>::load(src, convert);
    }

    static handle cast(const wchar_t *src, return_value_policy /* policy */, handle /* parent */) {
        if (src == nullptr) return none().inc_ref();
        return PyUnicode_FromWideChar(src, (ssize_t) wcslen(src));
    }

    static handle cast(wchar_t src, return_value_policy /* policy */, handle /* parent */) {
        wchar_t wstr[2] = { src, L'\0' };
        return PyUnicode_FromWideChar(wstr, 1);
    }

    operator wchar_t*() { return success ? (wchar_t *) value.c_str() : nullptr; }
    operator wchar_t&() { return value[0]; }

    static PYBIND11_DESCR name() { return type_descr(_(PYBIND11_STRING_NAME)); }
};

template <typename T1, typename T2> class type_caster<std::pair<T1, T2>> {
    typedef std::pair<T1, T2> type;
public:
    bool load(handle src, bool convert) {
        if (!isinstance<sequence>(src))
            return false;
        const auto seq = reinterpret_borrow<sequence>(src);
        if (seq.size() != 2)
            return false;
        return first.load(seq[0], convert) && second.load(seq[1], convert);
    }

    static handle cast(const type &src, return_value_policy policy, handle parent) {
        auto o1 = reinterpret_steal<object>(make_caster<T1>::cast(src.first, policy, parent));
        auto o2 = reinterpret_steal<object>(make_caster<T2>::cast(src.second, policy, parent));
        if (!o1 || !o2)
            return handle();
        tuple result(2);
        PyTuple_SET_ITEM(result.ptr(), 0, o1.release().ptr());
        PyTuple_SET_ITEM(result.ptr(), 1, o2.release().ptr());
        return result.release();
    }

    static PYBIND11_DESCR name() {
        return type_descr(
            _("Tuple[") + make_caster<T1>::name() + _(", ") + make_caster<T2>::name() + _("]")
        );
    }

    template <typename T> using cast_op_type = type;

    operator type() {
        return type(cast_op<T1>(first), cast_op<T2>(second));
    }
protected:
    make_caster<T1> first;
    make_caster<T2> second;
};

template <typename... Tuple> class type_caster<std::tuple<Tuple...>> {
    using type = std::tuple<Tuple...>;
    using indices = make_index_sequence<sizeof...(Tuple)>;
    static constexpr auto size = sizeof...(Tuple);

public:
    bool load(handle src, bool convert) {
        if (!isinstance<sequence>(src))
            return false;
        const auto seq = reinterpret_borrow<sequence>(src);
        if (seq.size() != size)
            return false;
        return load_impl(seq, convert, indices{});
    }

    static handle cast(const type &src, return_value_policy policy, handle parent) {
        return cast_impl(src, policy, parent, indices{});
    }

    static PYBIND11_DESCR name() {
        return type_descr(_("Tuple[") + detail::concat(make_caster<Tuple>::name()...) + _("]"));
    }

    template <typename T> using cast_op_type = type;

    operator type() { return implicit_cast(indices{}); }

protected:
    template <size_t... Is>
    type implicit_cast(index_sequence<Is...>) { return type(cast_op<Tuple>(std::get<Is>(value))...); }

    static constexpr bool load_impl(const sequence &, bool, index_sequence<>) { return true; }

    template <size_t... Is>
    bool load_impl(const sequence &seq, bool convert, index_sequence<Is...>) {
        for (bool r : {std::get<Is>(value).load(seq[Is], convert)...})
            if (!r)
                return false;
        return true;
    }

    static handle cast_impl(const type &, return_value_policy, handle,
                            index_sequence<>) { return tuple().release(); }

    /* Implementation: Convert a C++ tuple into a Python tuple */
    template <size_t... Is>
    static handle cast_impl(const type &src, return_value_policy policy, handle parent, index_sequence<Is...>) {
        std::array<object, size> entries {{
            reinterpret_steal<object>(make_caster<Tuple>::cast(std::get<Is>(src), policy, parent))...
        }};
        for (const auto &entry: entries)
            if (!entry)
                return handle();
        tuple result(size);
        int counter = 0;
        for (auto & entry: entries)
            PyTuple_SET_ITEM(result.ptr(), counter++, entry.release().ptr());
        return result.release();
    }

    std::tuple<make_caster<Tuple>...> value;
};

/// Type caster for holder types like std::shared_ptr, etc.
template <typename type, typename holder_type> class type_caster_holder : public type_caster_base<type> {
public:
    using base = type_caster_base<type>;
    using base::base;
    using base::cast;
    using base::typeinfo;
    using base::value;
    using base::temp;

    PYBIND11_NOINLINE bool load(handle src, bool convert) {
        return load(src, convert, Py_TYPE(src.ptr()));
    }

    bool load(handle src, bool convert, PyTypeObject *tobj) {
        if (!src || !typeinfo)
            return false;
        if (src.is_none()) {
            value = nullptr;
            return true;
        }

        if (typeinfo->simple_type) { /* Case 1: no multiple inheritance etc. involved */
            /* Check if we can safely perform a reinterpret-style cast */
            if (PyType_IsSubtype(tobj, typeinfo->type))
                return load_value_and_holder(src);
        } else { /* Case 2: multiple inheritance */
            /* Check if we can safely perform a reinterpret-style cast */
            if (tobj == typeinfo->type)
                return load_value_and_holder(src);

            /* If this is a python class, also check the parents recursively */
            auto const &type_dict = get_internals().registered_types_py;
            bool new_style_class = PyType_Check(tobj);
            if (type_dict.find(tobj) == type_dict.end() && new_style_class && tobj->tp_bases) {
                auto parents = reinterpret_borrow<tuple>(tobj->tp_bases);
                for (handle parent : parents) {
                    bool result = load(src, convert, (PyTypeObject *) parent.ptr());
                    if (result)
                        return true;
                }
            }

            if (try_implicit_casts(src, convert))
                return true;
        }

        if (convert) {
            for (auto &converter : typeinfo->implicit_conversions) {
                temp = reinterpret_steal<object>(converter(src.ptr(), typeinfo->type));
                if (load(temp, false))
                    return true;
            }
        }

        return false;
    }

    bool load_value_and_holder(handle src) {
        auto inst = (instance<type, holder_type> *) src.ptr();
        value = (void *) inst->value;
        if (inst->holder_constructed) {
            holder = inst->holder;
            return true;
        } else {
            throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "
#if defined(NDEBUG)
                             "(compile in debug mode for type information)");
#else
                             "of type '" + type_id<holder_type>() + "''");
#endif
        }
    }

    template <typename T = holder_type, detail::enable_if_t<!std::is_constructible<T, const T &, type*>::value, int> = 0>
    bool try_implicit_casts(handle, bool) { return false; }

    template <typename T = holder_type, detail::enable_if_t<std::is_constructible<T, const T &, type*>::value, int> = 0>
    bool try_implicit_casts(handle src, bool convert) {
        for (auto &cast : typeinfo->implicit_casts) {
            type_caster_holder sub_caster(*cast.first);
            if (sub_caster.load(src, convert)) {
                value = cast.second(sub_caster.value);
                holder = holder_type(sub_caster.holder, (type *) value);
                return true;
            }
        }
        return false;
    }

    explicit operator type*() { return this->value; }
    explicit operator type&() { return *(this->value); }
    explicit operator holder_type*() { return &holder; }

    // Workaround for Intel compiler bug
    // see pybind11 issue 94
    #if defined(__ICC) || defined(__INTEL_COMPILER)
    operator holder_type&() { return holder; }
    #else
    explicit operator holder_type&() { return holder; }
    #endif

    static handle cast(const holder_type &src, return_value_policy, handle) {
        return type_caster_generic::cast(
            src.get(), return_value_policy::take_ownership, handle(),
            src.get() ? &typeid(*src.get()) : nullptr, &typeid(type),
            nullptr, nullptr, &src);
    }

protected:
    holder_type holder;
};

/// Specialize for the common std::shared_ptr, so users don't need to
template <typename T>
class type_caster<std::shared_ptr<T>> : public type_caster_holder<T, std::shared_ptr<T>> { };

/// Create a specialization for custom holder types (silently ignores std::shared_ptr)
#define PYBIND11_DECLARE_HOLDER_TYPE(type, holder_type) \
    namespace pybind11 { namespace detail { \
    template <typename type> \
    class type_caster<holder_type, enable_if_t<!is_shared_ptr<holder_type>::value>> \
        : public type_caster_holder<type, holder_type> { }; \
    }}

// PYBIND11_DECLARE_HOLDER_TYPE holder types:
template <typename base, typename holder> struct is_holder_type :
    std::is_base_of<detail::type_caster_holder<base, holder>, detail::type_caster<holder>> {};
// Specialization for always-supported unique_ptr holders:
template <typename base, typename deleter> struct is_holder_type<base, std::unique_ptr<base, deleter>> :
    std::true_type {};

template <typename T> struct handle_type_name { static PYBIND11_DESCR name() { return _<T>(); } };
template <> struct handle_type_name<bytes> { static PYBIND11_DESCR name() { return _(PYBIND11_BYTES_NAME); } };
template <> struct handle_type_name<args> { static PYBIND11_DESCR name() { return _("*args"); } };
template <> struct handle_type_name<kwargs> { static PYBIND11_DESCR name() { return _("**kwargs"); } };

template <typename type>
struct pyobject_caster {
    template <typename T = type, enable_if_t<std::is_same<T, handle>::value, int> = 0>
    bool load(handle src, bool /* convert */) { value = src; return static_cast<bool>(value); }

    template <typename T = type, enable_if_t<std::is_base_of<object, T>::value, int> = 0>
    bool load(handle src, bool /* convert */) {
        if (!isinstance<type>(src))
            return false;
        value = reinterpret_borrow<type>(src);
        return true;
    }

    static handle cast(const handle &src, return_value_policy /* policy */, handle /* parent */) {
        return src.inc_ref();
    }
    PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name());
};

template <typename T>
class type_caster<T, enable_if_t<is_pyobject<T>::value>> : public pyobject_caster<T> { };

// Our conditions for enabling moving are quite restrictive:
// At compile time:
// - T needs to be a non-const, non-pointer, non-reference type
// - type_caster<T>::operator T&() must exist
// - the type must be move constructible (obviously)
// At run-time:
// - if the type is non-copy-constructible, the object must be the sole owner of the type (i.e. it
//   must have ref_count() == 1)h
// If any of the above are not satisfied, we fall back to copying.
template <typename T, typename SFINAE = void> struct move_is_plain_type : std::false_type {};
template <typename T> struct move_is_plain_type<T, enable_if_t<
        !std::is_void<T>::value && !std::is_pointer<T>::value && !std::is_reference<T>::value && !std::is_const<T>::value
    >> : std::true_type { };
template <typename T, typename SFINAE = void> struct move_always : std::false_type {};
template <typename T> struct move_always<T, enable_if_t<
        move_is_plain_type<T>::value &&
        !std::is_copy_constructible<T>::value && std::is_move_constructible<T>::value &&
        std::is_same<decltype(std::declval<type_caster<T>>().operator T&()), T&>::value
    >> : std::true_type { };
template <typename T, typename SFINAE = void> struct move_if_unreferenced : std::false_type {};
template <typename T> struct move_if_unreferenced<T, enable_if_t<
        move_is_plain_type<T>::value &&
        !move_always<T>::value && std::is_move_constructible<T>::value &&
        std::is_same<decltype(std::declval<type_caster<T>>().operator T&()), T&>::value
    >> : std::true_type { };
template <typename T> using move_never = std::integral_constant<bool, !move_always<T>::value && !move_if_unreferenced<T>::value>;

// Detect whether returning a `type` from a cast on type's type_caster is going to result in a
// reference or pointer to a local variable of the type_caster.  Basically, only
// non-reference/pointer `type`s and reference/pointers from a type_caster_generic are safe;
// everything else returns a reference/pointer to a local variable.
template <typename type> using cast_is_temporary_value_reference = bool_constant<
    (std::is_reference<type>::value || std::is_pointer<type>::value) &&
    !std::is_base_of<type_caster_generic, make_caster<type>>::value
>;

// Basic python -> C++ casting; throws if casting fails
template <typename T, typename SFINAE> type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &handle) {
    if (!conv.load(handle, true)) {
#if defined(NDEBUG)
        throw cast_error("Unable to cast Python instance to C++ type (compile in debug mode for details)");
#else
        throw cast_error("Unable to cast Python instance of type " +
            (std::string) str(handle.get_type()) + " to C++ type '" + type_id<T>() + "''");
#endif
    }
    return conv;
}
// Wrapper around the above that also constructs and returns a type_caster
template <typename T> make_caster<T> load_type(const handle &handle) {
    make_caster<T> conv;
    load_type(conv, handle);
    return conv;
}

NAMESPACE_END(detail)

// pytype -> C++ type
template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>
T cast(const handle &handle) {
    using namespace detail;
    static_assert(!cast_is_temporary_value_reference<T>::value,
            "Unable to cast type to reference: value is local to type caster");
    return cast_op<T>(load_type<T>(handle));
}

// pytype -> pytype (calls converting constructor)
template <typename T, detail::enable_if_t<detail::is_pyobject<T>::value, int> = 0>
T cast(const handle &handle) { return T(reinterpret_borrow<object>(handle)); }

// C++ type -> py::object
template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> = 0>
object cast(const T &value, return_value_policy policy = return_value_policy::automatic_reference,
            handle parent = handle()) {
    if (policy == return_value_policy::automatic)
        policy = std::is_pointer<T>::value ? return_value_policy::take_ownership : return_value_policy::copy;
    else if (policy == return_value_policy::automatic_reference)
        policy = std::is_pointer<T>::value ? return_value_policy::reference : return_value_policy::copy;
    return reinterpret_steal<object>(detail::make_caster<T>::cast(value, policy, parent));
}

template <typename T> T handle::cast() const { return pybind11::cast<T>(*this); }
template <> inline void handle::cast() const { return; }

template <typename T>
detail::enable_if_t<detail::move_always<T>::value || detail::move_if_unreferenced<T>::value, T> move(object &&obj) {
    if (obj.ref_count() > 1)
#if defined(NDEBUG)
        throw cast_error("Unable to cast Python instance to C++ rvalue: instance has multiple references"
            " (compile in debug mode for details)");
#else
        throw cast_error("Unable to move from Python " + (std::string) str(obj.get_type()) +
                " instance to C++ " + type_id<T>() + " instance: instance has multiple references");
#endif

    // Move into a temporary and return that, because the reference may be a local value of `conv`
    T ret = std::move(detail::load_type<T>(obj).operator T&());
    return ret;
}

// Calling cast() on an rvalue calls pybind::cast with the object rvalue, which does:
// - If we have to move (because T has no copy constructor), do it.  This will fail if the moved
//   object has multiple references, but trying to copy will fail to compile.
// - If both movable and copyable, check ref count: if 1, move; otherwise copy
// - Otherwise (not movable), copy.
template <typename T> detail::enable_if_t<detail::move_always<T>::value, T> cast(object &&object) {
    return move<T>(std::move(object));
}
template <typename T> detail::enable_if_t<detail::move_if_unreferenced<T>::value, T> cast(object &&object) {
    if (object.ref_count() > 1)
        return cast<T>(object);
    else
        return move<T>(std::move(object));
}
template <typename T> detail::enable_if_t<detail::move_never<T>::value, T> cast(object &&object) {
    return cast<T>(object);
}

template <typename T> T object::cast() const & { return pybind11::cast<T>(*this); }
template <typename T> T object::cast() && { return pybind11::cast<T>(std::move(*this)); }
template <> inline void object::cast() const & { return; }
template <> inline void object::cast() && { return; }

NAMESPACE_BEGIN(detail)

// Declared in pytypes.h:
template <typename T, enable_if_t<!is_pyobject<T>::value, int>>
object object_or_cast(T &&o) { return pybind11::cast(std::forward<T>(o)); }

struct overload_unused {}; // Placeholder type for the unneeded (and dead code) static variable in the OVERLOAD_INT macro
template <typename ret_type> using overload_caster_t = conditional_t<
    cast_is_temporary_value_reference<ret_type>::value, make_caster<ret_type>, overload_unused>;

// Trampoline use: for reference/pointer types to value-converted values, we do a value cast, then
// store the result in the given variable.  For other types, this is a no-op.
template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&o, make_caster<T> &caster) {
    return cast_op<T>(load_type(caster, o));
}
template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&, overload_unused &) {
    pybind11_fail("Internal error: cast_ref fallback invoked"); }

// Trampoline use: Having a pybind11::cast with an invalid reference type is going to static_assert, even
// though if it's in dead code, so we provide a "trampoline" to pybind11::cast that only does anything in
// cases where pybind11::cast is valid.
template <typename T> enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&o) {
    return pybind11::cast<T>(std::move(o)); }
template <typename T> enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&) {
    pybind11_fail("Internal error: cast_safe fallback invoked"); }
template <> inline void cast_safe<void>(object &&) {}

NAMESPACE_END(detail)

template <return_value_policy policy = return_value_policy::automatic_reference,
          typename... Args> tuple make_tuple(Args&&... args_) {
    const size_t size = sizeof...(Args);
    std::array<object, size> args {
        { reinterpret_steal<object>(detail::make_caster<Args>::cast(
            std::forward<Args>(args_), policy, nullptr))... }
    };
    for (auto &arg_value : args) {
        if (!arg_value) {
#if defined(NDEBUG)
            throw cast_error("make_tuple(): unable to convert arguments to Python object (compile in debug mode for details)");
#else
            throw cast_error("make_tuple(): unable to convert arguments of types '" +
                (std::string) type_id<std::tuple<Args...>>() + "' to Python object");
#endif
        }
    }
    tuple result(size);
    int counter = 0;
    for (auto &arg_value : args)
        PyTuple_SET_ITEM(result.ptr(), counter++, arg_value.release().ptr());
    return result;
}

/// Annotation for keyword arguments
struct arg {
    constexpr explicit arg(const char *name) : name(name) { }
    template <typename T> arg_v operator=(T &&value) const;

    const char *name;
};

/// Annotation for keyword arguments with values
struct arg_v : arg {
    template <typename T>
    arg_v(const char *name, T &&x, const char *descr = nullptr)
        : arg(name),
          value(reinterpret_steal<object>(
              detail::make_caster<T>::cast(x, return_value_policy::automatic, {})
          )),
          descr(descr)
#if !defined(NDEBUG)
        , type(type_id<T>())
#endif
    { }

    object value;
    const char *descr;
#if !defined(NDEBUG)
    std::string type;
#endif
};

template <typename T>
arg_v arg::operator=(T &&value) const { return {name, std::forward<T>(value)}; }

/// Alias for backward compatibility -- to be removed in version 2.0
template <typename /*unused*/> using arg_t = arg_v;

inline namespace literals {
/// String literal version of arg
constexpr arg operator"" _a(const char *name, size_t) { return arg(name); }
}

NAMESPACE_BEGIN(detail)

/// Helper class which loads arguments for C++ functions called from Python
template <typename... Args>
class argument_loader {
    using itypes = type_list<intrinsic_t<Args>...>;
    using indices = make_index_sequence<sizeof...(Args)>;

public:
    argument_loader() : value() {} // Helps gcc-7 properly initialize value

    static constexpr auto has_kwargs = std::is_same<itypes, type_list<args, kwargs>>::value;
    static constexpr auto has_args = has_kwargs || std::is_same<itypes, type_list<args>>::value;

    static PYBIND11_DESCR arg_names() { return detail::concat(make_caster<Args>::name()...); }

    bool load_args(handle args, handle kwargs) {
        return load_impl(args, kwargs, itypes{});
    }

    template <typename Return, typename Func>
    enable_if_t<!std::is_void<Return>::value, Return> call(Func &&f) {
        return call_impl<Return>(std::forward<Func>(f), indices{});
    }

    template <typename Return, typename Func>
    enable_if_t<std::is_void<Return>::value, void_type> call(Func &&f) {
        call_impl<Return>(std::forward<Func>(f), indices{});
        return void_type();
    }

private:
    bool load_impl(handle args_, handle, type_list<args>) {
        std::get<0>(value).load(args_, true);
        return true;
    }

    bool load_impl(handle args_, handle kwargs_, type_list<args, kwargs>) {
        std::get<0>(value).load(args_, true);
        std::get<1>(value).load(kwargs_, true);
        return true;
    }

    bool load_impl(handle args, handle, ... /* anything else */) {
        return load_impl_sequence(args, indices{});
    }

    static constexpr bool load_impl_sequence(handle, index_sequence<>) { return true; }

    template <size_t... Is>
    bool load_impl_sequence(handle src, index_sequence<Is...>) {
        for (bool r : {std::get<Is>(value).load(PyTuple_GET_ITEM(src.ptr(), Is), true)...})
            if (!r)
                return false;
        return true;
    }

    template <typename Return, typename Func, size_t... Is>
    Return call_impl(Func &&f, index_sequence<Is...>) {
        return std::forward<Func>(f)(cast_op<Args>(std::get<Is>(value))...);
    }

    std::tuple<make_caster<Args>...> value;
};

NAMESPACE_BEGIN(constexpr_impl)
/// Implementation details for constexpr functions
constexpr int first(int i) { return i; }
template <typename T, typename... Ts>
constexpr int first(int i, T v, Ts... vs) { return v ? i : first(i + 1, vs...); }

constexpr int last(int /*i*/, int result) { return result; }
template <typename T, typename... Ts>
constexpr int last(int i, int result, T v, Ts... vs) { return last(i + 1, v ? i : result, vs...); }
NAMESPACE_END(constexpr_impl)

/// Return the index of the first type in Ts which satisfies Predicate<T>
template <template<typename> class Predicate, typename... Ts>
constexpr int constexpr_first() { return constexpr_impl::first(0, Predicate<Ts>::value...); }

/// Return the index of the last type in Ts which satisfies Predicate<T>
template <template<typename> class Predicate, typename... Ts>
constexpr int constexpr_last() { return constexpr_impl::last(0, -1, Predicate<Ts>::value...); }

/// Helper class which collects only positional arguments for a Python function call.
/// A fancier version below can collect any argument, but this one is optimal for simple calls.
template <return_value_policy policy>
class simple_collector {
public:
    template <typename... Ts>
    explicit simple_collector(Ts &&...values)
        : m_args(pybind11::make_tuple<policy>(std::forward<Ts>(values)...)) { }

    const tuple &args() const & { return m_args; }
    dict kwargs() const { return {}; }

    tuple args() && { return std::move(m_args); }

    /// Call a Python function and pass the collected arguments
    object call(PyObject *ptr) const {
        PyObject *result = PyObject_CallObject(ptr, m_args.ptr());
        if (!result)
            throw error_already_set();
        return reinterpret_steal<object>(result);
    }

private:
    tuple m_args;
};

/// Helper class which collects positional, keyword, * and ** arguments for a Python function call
template <return_value_policy policy>
class unpacking_collector {
public:
    template <typename... Ts>
    explicit unpacking_collector(Ts &&...values) {
        // Tuples aren't (easily) resizable so a list is needed for collection,
        // but the actual function call strictly requires a tuple.
        auto args_list = list();
        int _[] = { 0, (process(args_list, std::forward<Ts>(values)), 0)... };
        ignore_unused(_);

        m_args = std::move(args_list);
    }

    const tuple &args() const & { return m_args; }
    const dict &kwargs() const & { return m_kwargs; }

    tuple args() && { return std::move(m_args); }
    dict kwargs() && { return std::move(m_kwargs); }

    /// Call a Python function and pass the collected arguments
    object call(PyObject *ptr) const {
        PyObject *result = PyObject_Call(ptr, m_args.ptr(), m_kwargs.ptr());
        if (!result)
            throw error_already_set();
        return reinterpret_steal<object>(result);
    }

private:
    template <typename T>
    void process(list &args_list, T &&x) {
        auto o = reinterpret_steal<object>(detail::make_caster<T>::cast(std::forward<T>(x), policy, {}));
        if (!o) {
#if defined(NDEBUG)
            argument_cast_error();
#else
            argument_cast_error(std::to_string(args_list.size()), type_id<T>());
#endif
        }
        args_list.append(o);
    }

    void process(list &args_list, detail::args_proxy ap) {
        for (const auto &a : ap)
            args_list.append(a);
    }

    void process(list &/*args_list*/, arg_v a) {
        if (m_kwargs.contains(a.name)) {
#if defined(NDEBUG)
            multiple_values_error();
#else
            multiple_values_error(a.name);
#endif
        }
        if (!a.value) {
#if defined(NDEBUG)
            argument_cast_error();
#else
            argument_cast_error(a.name, a.type);
#endif
        }
        m_kwargs[a.name] = a.value;
    }

    void process(list &/*args_list*/, detail::kwargs_proxy kp) {
        if (!kp)
            return;
        for (const auto &k : reinterpret_borrow<dict>(kp)) {
            if (m_kwargs.contains(k.first)) {
#if defined(NDEBUG)
                multiple_values_error();
#else
                multiple_values_error(str(k.first));
#endif
            }
            m_kwargs[k.first] = k.second;
        }
    }

    [[noreturn]] static void multiple_values_error() {
        throw type_error("Got multiple values for keyword argument "
                         "(compile in debug mode for details)");
    }

    [[noreturn]] static void multiple_values_error(std::string name) {
        throw type_error("Got multiple values for keyword argument '" + name + "'");
    }

    [[noreturn]] static void argument_cast_error() {
        throw cast_error("Unable to convert call argument to Python object "
                         "(compile in debug mode for details)");
    }

    [[noreturn]] static void argument_cast_error(std::string name, std::string type) {
        throw cast_error("Unable to convert call argument '" + name
                         + "' of type '" + type + "' to Python object");
    }

private:
    tuple m_args;
    dict m_kwargs;
};

/// Collect only positional arguments for a Python function call
template <return_value_policy policy, typename... Args,
          typename = enable_if_t<all_of_t<is_positional, Args...>::value>>
simple_collector<policy> collect_arguments(Args &&...args) {
    return simple_collector<policy>(std::forward<Args>(args)...);
}

/// Collect all arguments, including keywords and unpacking (only instantiated when needed)
template <return_value_policy policy, typename... Args,
          typename = enable_if_t<!all_of_t<is_positional, Args...>::value>>
unpacking_collector<policy> collect_arguments(Args &&...args) {
    // Following argument order rules for generalized unpacking according to PEP 448
    static_assert(
        constexpr_last<is_positional, Args...>() < constexpr_first<is_keyword_or_ds, Args...>()
        && constexpr_last<is_s_unpacking, Args...>() < constexpr_first<is_ds_unpacking, Args...>(),
        "Invalid function call: positional args must precede keywords and ** unpacking; "
        "* unpacking must precede ** unpacking"
    );
    return unpacking_collector<policy>(std::forward<Args>(args)...);
}

template <typename Derived>
template <return_value_policy policy, typename... Args>
object object_api<Derived>::operator()(Args &&...args) const {
    return detail::collect_arguments<policy>(std::forward<Args>(args)...).call(derived().ptr());
}

template <typename Derived>
template <return_value_policy policy, typename... Args>
object object_api<Derived>::call(Args &&...args) const {
    return operator()<policy>(std::forward<Args>(args)...);
}

NAMESPACE_END(detail)

#define PYBIND11_MAKE_OPAQUE(Type) \
    namespace pybind11 { namespace detail { \
        template<> class type_caster<Type> : public type_caster_base<Type> { }; \
    }}

NAMESPACE_END(pybind11)