3rdparty: Bump vendored pybind11 version for py3.11 support

Signed-off-by: gatecat <gatecat@ds0.me>

1 files changed, 824 insertions, 1378 deletions

diff --git a/3rdparty/pybind11/include/pybind11/cast.h b/3rdparty/pybind11/include/pybind11/cast.h
index 11c61a44..a0e32281 100644
--- a/3rdparty/pybind11/include/pybind11/cast.h
+++ b/3rdparty/pybind11/include/pybind11/cast.h
@@ -10,1029 +10,168 @@
 
 #pragma once
 
-#include "pytypes.h"
-#include "detail/typeid.h"
+#include "detail/common.h"
 #include "detail/descr.h"
-#include "detail/internals.h"
+#include "detail/type_caster_base.h"
+#include "detail/typeid.h"
+#include "pytypes.h"
+
 #include <array>
-#include <limits>
+#include <cstring>
+#include <functional>
+#include <iosfwd>
+#include <iterator>
+#include <memory>
+#include <string>
 #include <tuple>
 #include <type_traits>
-
-#if defined(PYBIND11_CPP17)
-#  if defined(__has_include)
-#    if __has_include(<string_view>)
-#      define PYBIND11_HAS_STRING_VIEW
-#    endif
-#  elif defined(_MSC_VER)
-#    define PYBIND11_HAS_STRING_VIEW
-#  endif
-#endif
-#ifdef PYBIND11_HAS_STRING_VIEW
-#include <string_view>
-#endif
-
-#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
-#  define PYBIND11_HAS_U8STRING
-#endif
+#include <utility>
+#include <vector>
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
-/// A life support system for temporary objects created by `type_caster::load()`.
-/// Adding a patient will keep it alive up until the enclosing function returns.
-class loader_life_support {
-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);
-}
-
-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();
-
-        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();
-            }
-        }
-
-        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>> {};
-
-PYBIND11_NAMESPACE_END(detail)
-
-// polymorphic_type_hook<itype>::get(src, tinfo) determines whether the object pointed
-// to by `src` actually is an instance of some class derived from `itype`.
-// If so, it sets `tinfo` to point to the std::type_info representing that derived
-// type, and returns a pointer to the start of the most-derived object of that type
-// (in which `src` is a subobject; this will be the same address as `src` in most
-// single inheritance cases). If not, or if `src` is nullptr, it simply returns `src`
-// and leaves `tinfo` at its default value of nullptr.
-//
-// The default polymorphic_type_hook just returns src. A specialization for polymorphic
-// types determines the runtime type of the passed object and adjusts the this-pointer
-// appropriately via dynamic_cast<void*>. This is what enables a C++ Animal* to appear
-// to Python as a Dog (if Dog inherits from Animal, Animal is polymorphic, Dog is
-// registered with pybind11, and this Animal is in fact a Dog).
-//
-// You may specialize polymorphic_type_hook yourself for types that want to appear
-// polymorphic to Python but do not use C++ RTTI. (This is a not uncommon pattern
-// in performance-sensitive applications, used most notably in LLVM.)
-//
-// polymorphic_type_hook_base allows users to specialize polymorphic_type_hook with
-// std::enable_if. User provided specializations will always have higher priority than
-// the default implementation and specialization provided in polymorphic_type_hook_base.
-template <typename itype, typename SFINAE = void>
-struct polymorphic_type_hook_base
-{
-    static const void *get(const itype *src, const std::type_info*&) { return src; }
-};
-template <typename itype>
-struct polymorphic_type_hook_base<itype, detail::enable_if_t<std::is_polymorphic<itype>::value>>
-{
-    static const void *get(const itype *src, const std::type_info*& type) {
-        type = src ? &typeid(*src) : nullptr;
-        return dynamic_cast<const void*>(src);
-    }
-};
-template <typename itype, typename SFINAE = void>
-struct polymorphic_type_hook : public polymorphic_type_hook_base<itype> {};
-
-PYBIND11_NAMESPACE_BEGIN(detail)
-
-/// Generic type caster for objects stored on the heap
-template <typename type> class type_caster_base : public type_caster_generic {
-    using itype = intrinsic_t<type>;
-
-public:
-    static constexpr auto name = _<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); }
-
-protected:
-    using Constructor = void *(*)(const void *);
-
-    /* Only enabled when the types are {copy,move}-constructible *and* when the type
-       does not have a private operator new implementation. */
-    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; }
-};
-
-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, 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) {
+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<typename std::add_rvalue_reference<T>::type>
+template <typename T>
+typename make_caster<T>::template cast_op_type<typename std::add_rvalue_reference<T>::type>
 cast_op(make_caster<T> &&caster) {
-    return std::move(caster).operator
-        typename make_caster<T>::template cast_op_type<typename std::add_rvalue_reference<T>::type>();
+    return std::move(caster).operator typename make_caster<T>::
+        template cast_op_type<typename std::add_rvalue_reference<T>::type>();
 }
 
-template <typename type> class type_caster<std::reference_wrapper<type>> {
+template <typename type>
+class type_caster<std::reference_wrapper<type>> {
 private:
     using caster_t = make_caster<type>;
     caster_t subcaster;
-    using subcaster_cast_op_type = typename caster_t::template cast_op_type<type>;
-    static_assert(std::is_same<typename std::remove_const<type>::type &, subcaster_cast_op_type>::value,
-            "std::reference_wrapper<T> caster requires T to have a caster with an `T &` operator");
+    using reference_t = type &;
+    using subcaster_cast_op_type = typename caster_t::template cast_op_type<reference_t>;
+
+    static_assert(
+        std::is_same<typename std::remove_const<type>::type &, subcaster_cast_op_type>::value
+            || std::is_same<reference_t, subcaster_cast_op_type>::value,
+        "std::reference_wrapper<T> caster requires T to have a caster with an "
+        "`operator T &()` or `operator const T &()`");
+
 public:
     bool load(handle src, bool convert) { return subcaster.load(src, convert); }
     static constexpr auto name = caster_t::name;
-    static handle cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
+    static handle
+    cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
         // It is definitely wrong to take ownership of this pointer, so mask that rvp
-        if (policy == return_value_policy::take_ownership || policy == return_value_policy::automatic)
+        if (policy == return_value_policy::take_ownership
+            || policy == return_value_policy::automatic) {
             policy = return_value_policy::automatic_reference;
+        }
         return caster_t::cast(&src.get(), policy, parent);
     }
-    template <typename T> using cast_op_type = std::reference_wrapper<type>;
-    operator std::reference_wrapper<type>() { return subcaster.operator subcaster_cast_op_type&(); }
+    template <typename T>
+    using cast_op_type = std::reference_wrapper<type>;
+    explicit operator std::reference_wrapper<type>() { return cast_op<type &>(subcaster); }
 };
 
-#define PYBIND11_TYPE_CASTER(type, py_name) \
-    protected: \
-        type value; \
-    public: \
-        static constexpr auto name = py_name; \
-        template <typename T_, enable_if_t<std::is_same<type, remove_cv_t<T_>>::value, int> = 0> \
-        static handle cast(T_ *src, return_value_policy policy, handle parent) { \
-            if (!src) return none().release(); \
-            if (policy == return_value_policy::take_ownership) { \
-                auto h = cast(std::move(*src), policy, parent); delete src; return h; \
-            } else { \
-                return cast(*src, policy, parent); \
-            } \
-        } \
-        operator type*() { return &value; } \
-        operator type&() { return value; } \
-        operator type&&() && { return std::move(value); } \
-        template <typename T_> using cast_op_type = pybind11::detail::movable_cast_op_type<T_>
-
-
-template <typename CharT> using is_std_char_type = any_of<
-    std::is_same<CharT, char>, /* std::string */
+#define PYBIND11_TYPE_CASTER(type, py_name)                                                       \
+protected:                                                                                        \
+    type value;                                                                                   \
+                                                                                                  \
+public:                                                                                           \
+    static constexpr auto name = py_name;                                                         \
+    template <typename T_,                                                                        \
+              ::pybind11::detail::enable_if_t<                                                    \
+                  std::is_same<type, ::pybind11::detail::remove_cv_t<T_>>::value,                 \
+                  int> = 0>                                                                       \
+    static ::pybind11::handle cast(                                                               \
+        T_ *src, ::pybind11::return_value_policy policy, ::pybind11::handle parent) {             \
+        if (!src)                                                                                 \
+            return ::pybind11::none().release();                                                  \
+        if (policy == ::pybind11::return_value_policy::take_ownership) {                          \
+            auto h = cast(std::move(*src), policy, parent);                                       \
+            delete src;                                                                           \
+            return h;                                                                             \
+        }                                                                                         \
+        return cast(*src, policy, parent);                                                        \
+    }                                                                                             \
+    operator type *() { return &value; }               /* NOLINT(bugprone-macro-parentheses) */   \
+    operator type &() { return value; }                /* NOLINT(bugprone-macro-parentheses) */   \
+    operator type &&() && { return std::move(value); } /* NOLINT(bugprone-macro-parentheses) */   \
+    template <typename T_>                                                                        \
+    using cast_op_type = ::pybind11::detail::movable_cast_op_type<T_>
+
+template <typename CharT>
+using is_std_char_type = any_of<std::is_same<CharT, char>, /* std::string */
 #if defined(PYBIND11_HAS_U8STRING)
-    std::is_same<CharT, char8_t>, /* std::u8string */
+                                std::is_same<CharT, char8_t>, /* std::u8string */
 #endif
-    std::is_same<CharT, char16_t>, /* std::u16string */
-    std::is_same<CharT, char32_t>, /* std::u32string */
-    std::is_same<CharT, wchar_t> /* std::wstring */
->;
-
+                                std::is_same<CharT, char16_t>, /* std::u16string */
+                                std::is_same<CharT, char32_t>, /* std::u32string */
+                                std::is_same<CharT, wchar_t>   /* std::wstring */
+                                >;
 
 template <typename T>
 struct type_caster<T, enable_if_t<std::is_arithmetic<T>::value && !is_std_char_type<T>::value>> {
     using _py_type_0 = conditional_t<sizeof(T) <= sizeof(long), long, long long>;
-    using _py_type_1 = conditional_t<std::is_signed<T>::value, _py_type_0, typename std::make_unsigned<_py_type_0>::type>;
+    using _py_type_1 = conditional_t<std::is_signed<T>::value,
+                                     _py_type_0,
+                                     typename std::make_unsigned<_py_type_0>::type>;
     using py_type = conditional_t<std::is_floating_point<T>::value, double, _py_type_1>;
-public:
 
+public:
     bool load(handle src, bool convert) {
         py_type py_value;
 
-        if (!src)
+        if (!src) {
             return false;
+        }
+
+#if !defined(PYPY_VERSION)
+        auto index_check = [](PyObject *o) { return PyIndex_Check(o); };
+#else
+        // In PyPy 7.3.3, `PyIndex_Check` is implemented by calling `__index__`,
+        // while CPython only considers the existence of `nb_index`/`__index__`.
+        auto index_check = [](PyObject *o) { return hasattr(o, "__index__"); };
+#endif
 
         if (std::is_floating_point<T>::value) {
-            if (convert || PyFloat_Check(src.ptr()))
+            if (convert || PyFloat_Check(src.ptr())) {
                 py_value = (py_type) PyFloat_AsDouble(src.ptr());
-            else
+            } else {
                 return false;
-        } else if (PyFloat_Check(src.ptr())) {
+            }
+        } else if (PyFloat_Check(src.ptr())
+                   || (!convert && !PYBIND11_LONG_CHECK(src.ptr()) && !index_check(src.ptr()))) {
             return false;
-        } else if (std::is_unsigned<py_type>::value) {
-            py_value = as_unsigned<py_type>(src.ptr());
-        } else { // signed integer:
-            py_value = sizeof(T) <= sizeof(long)
-                ? (py_type) PyLong_AsLong(src.ptr())
-                : (py_type) PYBIND11_LONG_AS_LONGLONG(src.ptr());
+        } else {
+            handle src_or_index = src;
+            // PyPy: 7.3.7's 3.8 does not implement PyLong_*'s __index__ calls.
+#if PY_VERSION_HEX < 0x03080000 || defined(PYPY_VERSION)
+            object index;
+            if (!PYBIND11_LONG_CHECK(src.ptr())) { // So: index_check(src.ptr())
+                index = reinterpret_steal<object>(PyNumber_Index(src.ptr()));
+                if (!index) {
+                    PyErr_Clear();
+                    if (!convert)
+                        return false;
+                } else {
+                    src_or_index = index;
+                }
+            }
+#endif
+            if (std::is_unsigned<py_type>::value) {
+                py_value = as_unsigned<py_type>(src_or_index.ptr());
+            } else { // signed integer:
+                py_value = sizeof(T) <= sizeof(long)
+                               ? (py_type) PyLong_AsLong(src_or_index.ptr())
+                               : (py_type) PYBIND11_LONG_AS_LONGLONG(src_or_index.ptr());
+            }
         }
 
         // Python API reported an error
@@ -1040,19 +179,14 @@ public:
 
         // Check to see if the conversion is valid (integers should match exactly)
         // Signed/unsigned checks happen elsewhere
-        if (py_err || (std::is_integral<T>::value && sizeof(py_type) != sizeof(T) && py_value != (py_type) (T) py_value)) {
-            bool type_error = py_err && PyErr_ExceptionMatches(
-#if PY_VERSION_HEX < 0x03000000 && !defined(PYPY_VERSION)
-                PyExc_SystemError
-#else
-                PyExc_TypeError
-#endif
-            );
+        if (py_err
+            || (std::is_integral<T>::value && sizeof(py_type) != sizeof(T)
+                && py_value != (py_type) (T) py_value)) {
             PyErr_Clear();
-            if (type_error && convert && PyNumber_Check(src.ptr())) {
+            if (py_err && convert && (PyNumber_Check(src.ptr()) != 0)) {
                 auto tmp = reinterpret_steal<object>(std::is_floating_point<T>::value
-                                                     ? PyNumber_Float(src.ptr())
-                                                     : PyNumber_Long(src.ptr()));
+                                                         ? PyNumber_Float(src.ptr())
+                                                         : PyNumber_Long(src.ptr()));
                 PyErr_Clear();
                 return load(tmp, false);
             }
@@ -1063,62 +197,75 @@ public:
         return true;
     }
 
-    template<typename U = T>
+    template <typename U = T>
     static typename std::enable_if<std::is_floating_point<U>::value, handle>::type
     cast(U src, return_value_policy /* policy */, handle /* parent */) {
         return PyFloat_FromDouble((double) src);
     }
 
-    template<typename U = T>
-    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_signed<U>::value && (sizeof(U) <= sizeof(long)), handle>::type
+    template <typename U = T>
+    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_signed<U>::value
+                                       && (sizeof(U) <= sizeof(long)),
+                                   handle>::type
     cast(U src, return_value_policy /* policy */, handle /* parent */) {
         return PYBIND11_LONG_FROM_SIGNED((long) src);
     }
 
-    template<typename U = T>
-    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_unsigned<U>::value && (sizeof(U) <= sizeof(unsigned long)), handle>::type
+    template <typename U = T>
+    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_unsigned<U>::value
+                                       && (sizeof(U) <= sizeof(unsigned long)),
+                                   handle>::type
     cast(U src, return_value_policy /* policy */, handle /* parent */) {
         return PYBIND11_LONG_FROM_UNSIGNED((unsigned long) src);
     }
 
-    template<typename U = T>
-    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_signed<U>::value && (sizeof(U) > sizeof(long)), handle>::type
+    template <typename U = T>
+    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_signed<U>::value
+                                       && (sizeof(U) > sizeof(long)),
+                                   handle>::type
     cast(U src, return_value_policy /* policy */, handle /* parent */) {
         return PyLong_FromLongLong((long long) src);
     }
 
-    template<typename U = T>
-    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_unsigned<U>::value && (sizeof(U) > sizeof(unsigned long)), handle>::type
+    template <typename U = T>
+    static typename std::enable_if<!std::is_floating_point<U>::value && std::is_unsigned<U>::value
+                                       && (sizeof(U) > sizeof(unsigned long)),
+                                   handle>::type
     cast(U src, return_value_policy /* policy */, handle /* parent */) {
         return PyLong_FromUnsignedLongLong((unsigned long long) src);
     }
 
-    PYBIND11_TYPE_CASTER(T, _<std::is_integral<T>::value>("int", "float"));
+    PYBIND11_TYPE_CASTER(T, const_name<std::is_integral<T>::value>("int", "float"));
 };
 
-template<typename T> struct void_caster {
+template <typename T>
+struct void_caster {
 public:
     bool load(handle src, bool) {
-        if (src && src.is_none())
+        if (src && src.is_none()) {
             return true;
+        }
         return false;
     }
     static handle cast(T, return_value_policy /* policy */, handle /* parent */) {
         return none().inc_ref();
     }
-    PYBIND11_TYPE_CASTER(T, _("None"));
+    PYBIND11_TYPE_CASTER(T, const_name("None"));
 };
 
-template <> class type_caster<void_type> : public void_caster<void_type> {};
+template <>
+class type_caster<void_type> : public void_caster<void_type> {};
 
-template <> class type_caster<void> : public type_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()) {
+        }
+        if (h.is_none()) {
             value = nullptr;
             return true;
         }
@@ -1130,7 +277,7 @@ public:
         }
 
         /* Check if this is a C++ type */
-        auto &bases = all_type_info((PyTypeObject *) type::handle_of(h).ptr());
+        const auto &bases = all_type_info((PyTypeObject *) type::handle_of(h).ptr());
         if (bases.size() == 1) { // Only allowing loading from a single-value type
             value = values_and_holders(reinterpret_cast<instance *>(h.ptr())).begin()->value_ptr();
             return true;
@@ -1141,191 +288,247 @@ public:
     }
 
     static handle cast(const void *ptr, return_value_policy /* policy */, handle /* parent */) {
-        if (ptr)
+        if (ptr) {
             return capsule(ptr).release();
-        else
-            return none().inc_ref();
+        }
+        return none().inc_ref();
     }
 
-    template <typename T> using cast_op_type = void*&;
-    operator void *&() { return value; }
-    static constexpr auto name = _("capsule");
+    template <typename T>
+    using cast_op_type = void *&;
+    explicit operator void *&() { return value; }
+    static constexpr auto name = const_name("capsule");
+
 private:
     void *value = nullptr;
 };
 
-template <> class type_caster<std::nullptr_t> : public void_caster<std::nullptr_t> { };
+template <>
+class type_caster<std::nullptr_t> : public void_caster<std::nullptr_t> {};
 
-template <> class type_caster<bool> {
+template <>
+class type_caster<bool> {
 public:
     bool load(handle src, bool convert) {
-        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 if (convert || !strcmp("numpy.bool_", Py_TYPE(src.ptr())->tp_name)) {
+        if (!src) {
+            return false;
+        }
+        if (src.ptr() == Py_True) {
+            value = true;
+            return true;
+        }
+        if (src.ptr() == Py_False) {
+            value = false;
+            return true;
+        }
+        if (convert || (std::strcmp("numpy.bool_", Py_TYPE(src.ptr())->tp_name) == 0)) {
             // (allow non-implicit conversion for numpy booleans)
 
             Py_ssize_t res = -1;
             if (src.is_none()) {
-                res = 0;  // None is implicitly converted to False
+                res = 0; // None is implicitly converted to False
             }
-            #if defined(PYPY_VERSION)
-            // On PyPy, check that "__bool__" (or "__nonzero__" on Python 2.7) attr exists
+#if defined(PYPY_VERSION)
+            // On PyPy, check that "__bool__" attr exists
             else if (hasattr(src, PYBIND11_BOOL_ATTR)) {
                 res = PyObject_IsTrue(src.ptr());
             }
-            #else
+#else
             // Alternate approach for CPython: this does the same as the above, but optimized
             // using the CPython API so as to avoid an unneeded attribute lookup.
-            else if (auto tp_as_number = src.ptr()->ob_type->tp_as_number) {
+            else if (auto *tp_as_number = src.ptr()->ob_type->tp_as_number) {
                 if (PYBIND11_NB_BOOL(tp_as_number)) {
                     res = (*PYBIND11_NB_BOOL(tp_as_number))(src.ptr());
                 }
             }
-            #endif
+#endif
             if (res == 0 || res == 1) {
-                value = (bool) res;
+                value = (res != 0);
                 return true;
-            } else {
-                PyErr_Clear();
             }
+            PyErr_Clear();
         }
         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"));
+    PYBIND11_TYPE_CASTER(bool, const_name("bool"));
 };
 
 // Helper class for UTF-{8,16,32} C++ stl strings:
-template <typename StringType, bool IsView = false> struct string_caster {
+template <typename StringType, bool IsView = false>
+struct string_caster {
     using CharT = typename StringType::value_type;
 
     // Simplify life by being able to assume standard char sizes (the standard only guarantees
     // minimums, but Python requires exact sizes)
-    static_assert(!std::is_same<CharT, char>::value || sizeof(CharT) == 1, "Unsupported char size != 1");
+    static_assert(!std::is_same<CharT, char>::value || sizeof(CharT) == 1,
+                  "Unsupported char size != 1");
 #if defined(PYBIND11_HAS_U8STRING)
-    static_assert(!std::is_same<CharT, char8_t>::value || sizeof(CharT) == 1, "Unsupported char8_t size != 1");
+    static_assert(!std::is_same<CharT, char8_t>::value || sizeof(CharT) == 1,
+                  "Unsupported char8_t size != 1");
 #endif
-    static_assert(!std::is_same<CharT, char16_t>::value || sizeof(CharT) == 2, "Unsupported char16_t size != 2");
-    static_assert(!std::is_same<CharT, char32_t>::value || sizeof(CharT) == 4, "Unsupported char32_t size != 4");
+    static_assert(!std::is_same<CharT, char16_t>::value || sizeof(CharT) == 2,
+                  "Unsupported char16_t size != 2");
+    static_assert(!std::is_same<CharT, char32_t>::value || sizeof(CharT) == 4,
+                  "Unsupported char32_t size != 4");
     // wchar_t can be either 16 bits (Windows) or 32 (everywhere else)
     static_assert(!std::is_same<CharT, wchar_t>::value || sizeof(CharT) == 2 || sizeof(CharT) == 4,
-            "Unsupported wchar_t size != 2/4");
+                  "Unsupported wchar_t size != 2/4");
     static constexpr size_t UTF_N = 8 * sizeof(CharT);
 
     bool load(handle src, bool) {
-#if PY_MAJOR_VERSION < 3
-        object temp;
-#endif
         handle load_src = src;
         if (!src) {
             return false;
-        } else if (!PyUnicode_Check(load_src.ptr())) {
-#if PY_MAJOR_VERSION >= 3
-            return load_bytes(load_src);
-#else
-            if (std::is_same<CharT, char>::value) {
-                return load_bytes(load_src);
-            }
+        }
+        if (!PyUnicode_Check(load_src.ptr())) {
+            return load_raw(load_src);
+        }
 
-            // The below is a guaranteed failure in Python 3 when PyUnicode_Check returns false
-            if (!PYBIND11_BYTES_CHECK(load_src.ptr()))
+        // For UTF-8 we avoid the need for a temporary `bytes` object by using
+        // `PyUnicode_AsUTF8AndSize`.
+        if (PYBIND11_SILENCE_MSVC_C4127(UTF_N == 8)) {
+            Py_ssize_t size = -1;
+            const auto *buffer
+                = reinterpret_cast<const CharT *>(PyUnicode_AsUTF8AndSize(load_src.ptr(), &size));
+            if (!buffer) {
+                PyErr_Clear();
                 return false;
-
-            temp = reinterpret_steal<object>(PyUnicode_FromObject(load_src.ptr()));
-            if (!temp) { PyErr_Clear(); return false; }
-            load_src = temp;
-#endif
+            }
+            value = StringType(buffer, static_cast<size_t>(size));
+            return true;
         }
 
-        auto utfNbytes = reinterpret_steal<object>(PyUnicode_AsEncodedString(
-            load_src.ptr(), UTF_N == 8 ? "utf-8" : UTF_N == 16 ? "utf-16" : "utf-32", nullptr));
-        if (!utfNbytes) { PyErr_Clear(); return false; }
+        auto utfNbytes
+            = reinterpret_steal<object>(PyUnicode_AsEncodedString(load_src.ptr(),
+                                                                  UTF_N == 8    ? "utf-8"
+                                                                  : UTF_N == 16 ? "utf-16"
+                                                                                : "utf-32",
+                                                                  nullptr));
+        if (!utfNbytes) {
+            PyErr_Clear();
+            return false;
+        }
 
-        const auto *buffer = reinterpret_cast<const CharT *>(PYBIND11_BYTES_AS_STRING(utfNbytes.ptr()));
+        const auto *buffer
+            = reinterpret_cast<const CharT *>(PYBIND11_BYTES_AS_STRING(utfNbytes.ptr()));
         size_t length = (size_t) PYBIND11_BYTES_SIZE(utfNbytes.ptr()) / sizeof(CharT);
-        if (UTF_N > 8) { buffer++; length--; } // Skip BOM for UTF-16/32
+        // Skip BOM for UTF-16/32
+        if (PYBIND11_SILENCE_MSVC_C4127(UTF_N > 8)) {
+            buffer++;
+            length--;
+        }
         value = StringType(buffer, length);
 
         // If we're loading a string_view we need to keep the encoded Python object alive:
-        if (IsView)
+        if (IsView) {
             loader_life_support::add_patient(utfNbytes);
+        }
 
         return true;
     }
 
-    static handle cast(const StringType &src, return_value_policy /* policy */, handle /* parent */) {
+    static handle
+    cast(const StringType &src, return_value_policy /* policy */, handle /* parent */) {
         const char *buffer = reinterpret_cast<const char *>(src.data());
         auto nbytes = ssize_t(src.size() * sizeof(CharT));
         handle s = decode_utfN(buffer, nbytes);
-        if (!s) throw error_already_set();
+        if (!s) {
+            throw error_already_set();
+        }
         return s;
     }
 
-    PYBIND11_TYPE_CASTER(StringType, _(PYBIND11_STRING_NAME));
+    PYBIND11_TYPE_CASTER(StringType, const_name(PYBIND11_STRING_NAME));
 
 private:
     static handle decode_utfN(const char *buffer, ssize_t nbytes) {
 #if !defined(PYPY_VERSION)
-        return
-            UTF_N == 8  ? PyUnicode_DecodeUTF8(buffer, nbytes, nullptr) :
-            UTF_N == 16 ? PyUnicode_DecodeUTF16(buffer, nbytes, nullptr, nullptr) :
-                          PyUnicode_DecodeUTF32(buffer, nbytes, nullptr, nullptr);
+        return UTF_N == 8    ? PyUnicode_DecodeUTF8(buffer, nbytes, nullptr)
+               : UTF_N == 16 ? PyUnicode_DecodeUTF16(buffer, nbytes, nullptr, nullptr)
+                             : PyUnicode_DecodeUTF32(buffer, nbytes, nullptr, nullptr);
 #else
-        // PyPy segfaults when on PyUnicode_DecodeUTF16 (and possibly on PyUnicode_DecodeUTF32 as well),
-        // so bypass the whole thing by just passing the encoding as a string value, which works properly:
-        return PyUnicode_Decode(buffer, nbytes, UTF_N == 8 ? "utf-8" : UTF_N == 16 ? "utf-16" : "utf-32", nullptr);
+        // PyPy segfaults when on PyUnicode_DecodeUTF16 (and possibly on PyUnicode_DecodeUTF32 as
+        // well), so bypass the whole thing by just passing the encoding as a string value, which
+        // works properly:
+        return PyUnicode_Decode(buffer,
+                                nbytes,
+                                UTF_N == 8    ? "utf-8"
+                                : UTF_N == 16 ? "utf-16"
+                                              : "utf-32",
+                                nullptr);
 #endif
     }
 
-    // When loading into a std::string or char*, accept a bytes object as-is (i.e.
+    // When loading into a std::string or char*, accept a bytes/bytearray object as-is (i.e.
     // without any encoding/decoding attempt).  For other C++ char sizes this is a no-op.
     // which supports loading a unicode from a str, doesn't take this path.
     template <typename C = CharT>
-    bool load_bytes(enable_if_t<std::is_same<C, char>::value, handle> src) {
+    bool load_raw(enable_if_t<std::is_same<C, char>::value, handle> src) {
         if (PYBIND11_BYTES_CHECK(src.ptr())) {
-            // We were passed a Python 3 raw bytes; accept it into a std::string or char*
+            // We were passed raw bytes; accept it into a std::string or char*
             // without any encoding attempt.
             const char *bytes = PYBIND11_BYTES_AS_STRING(src.ptr());
-            if (bytes) {
-                value = StringType(bytes, (size_t) PYBIND11_BYTES_SIZE(src.ptr()));
-                return true;
+            if (!bytes) {
+                pybind11_fail("Unexpected PYBIND11_BYTES_AS_STRING() failure.");
             }
+            value = StringType(bytes, (size_t) PYBIND11_BYTES_SIZE(src.ptr()));
+            return true;
+        }
+        if (PyByteArray_Check(src.ptr())) {
+            // We were passed a bytearray; accept it into a std::string or char*
+            // without any encoding attempt.
+            const char *bytearray = PyByteArray_AsString(src.ptr());
+            if (!bytearray) {
+                pybind11_fail("Unexpected PyByteArray_AsString() failure.");
+            }
+            value = StringType(bytearray, (size_t) PyByteArray_Size(src.ptr()));
+            return true;
         }
 
         return false;
     }
 
     template <typename C = CharT>
-    bool load_bytes(enable_if_t<!std::is_same<C, char>::value, handle>) { return false; }
+    bool load_raw(enable_if_t<!std::is_same<C, char>::value, handle>) {
+        return false;
+    }
 };
 
 template <typename CharT, class Traits, class Allocator>
-struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_std_char_type<CharT>::value>>
+struct type_caster<std::basic_string<CharT, Traits, Allocator>,
+                   enable_if_t<is_std_char_type<CharT>::value>>
     : string_caster<std::basic_string<CharT, Traits, Allocator>> {};
 
 #ifdef PYBIND11_HAS_STRING_VIEW
 template <typename CharT, class Traits>
-struct type_caster<std::basic_string_view<CharT, Traits>, enable_if_t<is_std_char_type<CharT>::value>>
+struct type_caster<std::basic_string_view<CharT, Traits>,
+                   enable_if_t<is_std_char_type<CharT>::value>>
     : string_caster<std::basic_string_view<CharT, Traits>, true> {};
 #endif
 
 // Type caster for C-style strings.  We basically use a std::string type caster, but also add the
 // ability to use None as a nullptr char* (which the string caster doesn't allow).
-template <typename CharT> struct type_caster<CharT, enable_if_t<is_std_char_type<CharT>::value>> {
+template <typename CharT>
+struct type_caster<CharT, enable_if_t<is_std_char_type<CharT>::value>> {
     using StringType = std::basic_string<CharT>;
-    using StringCaster = type_caster<StringType>;
+    using StringCaster = make_caster<StringType>;
     StringCaster str_caster;
     bool none = false;
     CharT one_char = 0;
+
 public:
     bool load(handle src, bool convert) {
-        if (!src) return false;
+        if (!src) {
+            return false;
+        }
         if (src.is_none()) {
             // Defer accepting None to other overloads (if we aren't in convert mode):
-            if (!convert) return false;
+            if (!convert) {
+                return false;
+            }
             none = true;
             return true;
         }
@@ -1333,45 +536,58 @@ public:
     }
 
     static handle cast(const CharT *src, return_value_policy policy, handle parent) {
-        if (src == nullptr) return pybind11::none().inc_ref();
+        if (src == nullptr) {
+            return pybind11::none().inc_ref();
+        }
         return StringCaster::cast(StringType(src), policy, parent);
     }
 
     static handle cast(CharT src, return_value_policy policy, handle parent) {
         if (std::is_same<char, CharT>::value) {
             handle s = PyUnicode_DecodeLatin1((const char *) &src, 1, nullptr);
-            if (!s) throw error_already_set();
+            if (!s) {
+                throw error_already_set();
+            }
             return s;
         }
         return StringCaster::cast(StringType(1, src), policy, parent);
     }
 
-    operator CharT*() { return none ? nullptr : const_cast<CharT *>(static_cast<StringType &>(str_caster).c_str()); }
-    operator CharT&() {
-        if (none)
+    explicit operator CharT *() {
+        return none ? nullptr : const_cast<CharT *>(static_cast<StringType &>(str_caster).c_str());
+    }
+    explicit operator CharT &() {
+        if (none) {
             throw value_error("Cannot convert None to a character");
+        }
 
         auto &value = static_cast<StringType &>(str_caster);
         size_t str_len = value.size();
-        if (str_len == 0)
+        if (str_len == 0) {
             throw value_error("Cannot convert empty string to a character");
+        }
 
         // If we're in UTF-8 mode, we have two possible failures: one for a unicode character that
-        // is too high, and one for multiple unicode characters (caught later), so we need to figure
-        // out how long the first encoded character is in bytes to distinguish between these two
-        // errors.  We also allow want to allow unicode characters U+0080 through U+00FF, as those
-        // can fit into a single char value.
-        if (StringCaster::UTF_N == 8 && str_len > 1 && str_len <= 4) {
+        // is too high, and one for multiple unicode characters (caught later), so we need to
+        // figure out how long the first encoded character is in bytes to distinguish between these
+        // two errors.  We also allow want to allow unicode characters U+0080 through U+00FF, as
+        // those can fit into a single char value.
+        if (PYBIND11_SILENCE_MSVC_C4127(StringCaster::UTF_N == 8) && str_len > 1 && str_len <= 4) {
             auto v0 = static_cast<unsigned char>(value[0]);
-            size_t char0_bytes = !(v0 & 0x80) ? 1 : // low bits only: 0-127
-                (v0 & 0xE0) == 0xC0 ? 2 : // 0b110xxxxx - start of 2-byte sequence
-                (v0 & 0xF0) == 0xE0 ? 3 : // 0b1110xxxx - start of 3-byte sequence
-                4; // 0b11110xxx - start of 4-byte sequence
+            // low bits only: 0-127
+            // 0b110xxxxx - start of 2-byte sequence
+            // 0b1110xxxx - start of 3-byte sequence
+            // 0b11110xxx - start of 4-byte sequence
+            size_t char0_bytes = (v0 & 0x80) == 0      ? 1
+                                 : (v0 & 0xE0) == 0xC0 ? 2
+                                 : (v0 & 0xF0) == 0xE0 ? 3
+                                                       : 4;
 
             if (char0_bytes == str_len) {
                 // If we have a 128-255 value, we can decode it into a single char:
                 if (char0_bytes == 2 && (v0 & 0xFC) == 0xC0) { // 0x110000xx 0x10xxxxxx
-                    one_char = static_cast<CharT>(((v0 & 3) << 6) + (static_cast<unsigned char>(value[1]) & 0x3F));
+                    one_char = static_cast<CharT>(((v0 & 3) << 6)
+                                                  + (static_cast<unsigned char>(value[1]) & 0x3F));
                     return one_char;
                 }
                 // Otherwise we have a single character, but it's > U+00FF
@@ -1382,36 +598,42 @@ public:
         // UTF-16 is much easier: we can only have a surrogate pair for values above U+FFFF, thus a
         // surrogate pair with total length 2 instantly indicates a range error (but not a "your
         // string was too long" error).
-        else if (StringCaster::UTF_N == 16 && str_len == 2) {
+        else if (PYBIND11_SILENCE_MSVC_C4127(StringCaster::UTF_N == 16) && str_len == 2) {
             one_char = static_cast<CharT>(value[0]);
-            if (one_char >= 0xD800 && one_char < 0xE000)
+            if (one_char >= 0xD800 && one_char < 0xE000) {
                 throw value_error("Character code point not in range(0x10000)");
+            }
         }
 
-        if (str_len != 1)
+        if (str_len != 1) {
             throw value_error("Expected a character, but multi-character string found");
+        }
 
         one_char = value[0];
         return one_char;
     }
 
-    static constexpr auto name = _(PYBIND11_STRING_NAME);
-    template <typename _T> using cast_op_type = pybind11::detail::cast_op_type<_T>;
+    static constexpr auto name = const_name(PYBIND11_STRING_NAME);
+    template <typename _T>
+    using cast_op_type = pybind11::detail::cast_op_type<_T>;
 };
 
 // Base implementation for std::tuple and std::pair
-template <template<typename...> class Tuple, typename... Ts> class tuple_caster {
+template <template <typename...> class Tuple, typename... Ts>
+class tuple_caster {
     using type = Tuple<Ts...>;
     static constexpr auto size = sizeof...(Ts);
     using indices = make_index_sequence<size>;
-public:
 
+public:
     bool load(handle src, bool convert) {
-        if (!isinstance<sequence>(src))
+        if (!isinstance<sequence>(src)) {
             return false;
+        }
         const auto seq = reinterpret_borrow<sequence>(src);
-        if (seq.size() != size)
+        if (seq.size() != size) {
             return false;
+        }
         return load_impl(seq, convert, indices{});
     }
 
@@ -1423,66 +645,83 @@ public:
     // copied from the PYBIND11_TYPE_CASTER macro
     template <typename T>
     static handle cast(T *src, return_value_policy policy, handle parent) {
-        if (!src) return none().release();
+        if (!src) {
+            return none().release();
+        }
         if (policy == return_value_policy::take_ownership) {
-            auto h = cast(std::move(*src), policy, parent); delete src; return h;
-        } else {
-            return cast(*src, policy, parent);
+            auto h = cast(std::move(*src), policy, parent);
+            delete src;
+            return h;
         }
+        return cast(*src, policy, parent);
     }
 
-    static constexpr auto name = _("Tuple[") + concat(make_caster<Ts>::name...) + _("]");
+    static constexpr auto name
+        = const_name("Tuple[") + concat(make_caster<Ts>::name...) + const_name("]");
 
-    template <typename T> using cast_op_type = type;
+    template <typename T>
+    using cast_op_type = type;
 
-    operator type() & { return implicit_cast(indices{}); }
-    operator type() && { return std::move(*this).implicit_cast(indices{}); }
+    explicit operator type() & { return implicit_cast(indices{}); }
+    explicit operator type() && { return std::move(*this).implicit_cast(indices{}); }
 
 protected:
     template <size_t... Is>
-    type implicit_cast(index_sequence<Is...>) & { return type(cast_op<Ts>(std::get<Is>(subcasters))...); }
+    type implicit_cast(index_sequence<Is...>) & {
+        return type(cast_op<Ts>(std::get<Is>(subcasters))...);
+    }
     template <size_t... Is>
-    type implicit_cast(index_sequence<Is...>) && { return type(cast_op<Ts>(std::move(std::get<Is>(subcasters)))...); }
+    type implicit_cast(index_sequence<Is...>) && {
+        return type(cast_op<Ts>(std::move(std::get<Is>(subcasters)))...);
+    }
 
     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...>) {
 #ifdef __cpp_fold_expressions
-        if ((... || !std::get<Is>(subcasters).load(seq[Is], convert)))
+        if ((... || !std::get<Is>(subcasters).load(seq[Is], convert))) {
             return false;
+        }
 #else
-        for (bool r : {std::get<Is>(subcasters).load(seq[Is], convert)...})
-            if (!r)
+        for (bool r : {std::get<Is>(subcasters).load(seq[Is], convert)...}) {
+            if (!r) {
                 return false;
+            }
+        }
 #endif
         return true;
     }
 
     /* Implementation: Convert a C++ tuple into a Python tuple */
     template <typename T, size_t... Is>
-    static handle cast_impl(T &&src, return_value_policy policy, handle parent, index_sequence<Is...>) {
-        std::array<object, size> entries{{
-            reinterpret_steal<object>(make_caster<Ts>::cast(std::get<Is>(std::forward<T>(src)), policy, parent))...
-        }};
-        for (const auto &entry: entries)
-            if (!entry)
+    static handle
+    cast_impl(T &&src, return_value_policy policy, handle parent, index_sequence<Is...>) {
+        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(src, policy, parent);
+        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(policy, parent);
+        std::array<object, size> entries{{reinterpret_steal<object>(
+            make_caster<Ts>::cast(std::get<Is>(std::forward<T>(src)), policy, parent))...}};
+        for (const auto &entry : entries) {
+            if (!entry) {
                 return handle();
+            }
+        }
         tuple result(size);
         int counter = 0;
-        for (auto & entry: entries)
+        for (auto &entry : entries) {
             PyTuple_SET_ITEM(result.ptr(), counter++, entry.release().ptr());
+        }
         return result.release();
     }
 
     Tuple<make_caster<Ts>...> subcasters;
 };
 
-template <typename T1, typename T2> class type_caster<std::pair<T1, T2>>
-    : public tuple_caster<std::pair, T1, T2> {};
+template <typename T1, typename T2>
+class type_caster<std::pair<T1, T2>> : public tuple_caster<std::pair, T1, T2> {};
 
-template <typename... Ts> class type_caster<std::tuple<Ts...>>
-    : public tuple_caster<std::tuple, Ts...> {};
+template <typename... Ts>
+class type_caster<std::tuple<Ts...>> : public tuple_caster<std::tuple, Ts...> {};
 
 /// Helper class which abstracts away certain actions. Users can provide specializations for
 /// custom holders, but it's only necessary if the type has a non-standard interface.
@@ -1492,12 +731,16 @@ struct holder_helper {
 };
 
 /// Type caster for holder types like std::shared_ptr, etc.
-template <typename type, typename holder_type>
+/// The SFINAE hook is provided to help work around the current lack of support
+/// for smart-pointer interoperability. Please consider it an implementation
+/// detail that may change in the future, as formal support for smart-pointer
+/// interoperability is added into pybind11.
+template <typename type, typename holder_type, typename SFINAE = void>
 struct copyable_holder_caster : public type_caster_base<type> {
 public:
     using base = type_caster_base<type>;
     static_assert(std::is_base_of<base, type_caster<type>>::value,
-            "Holder classes are only supported for custom types");
+                  "Holder classes are only supported for custom types");
     using base::base;
     using base::cast;
     using base::typeinfo;
@@ -1507,12 +750,12 @@ public:
         return base::template load_impl<copyable_holder_caster<type, holder_type>>(src, convert);
     }
 
-    explicit operator type*() { return this->value; }
+    explicit operator type *() { return this->value; }
     // static_cast works around compiler error with MSVC 17 and CUDA 10.2
     // see issue #2180
-    explicit operator type&() { return *(static_cast<type *>(this->value)); }
-    explicit operator holder_type*() { return std::addressof(holder); }
-    explicit operator holder_type&() { return holder; }
+    explicit operator type &() { return *(static_cast<type *>(this->value)); }
+    explicit operator holder_type *() { return std::addressof(holder); }
+    explicit operator holder_type &() { return holder; }
 
     static handle cast(const holder_type &src, return_value_policy, handle) {
         const auto *ptr = holder_helper<holder_type>::get(src);
@@ -1522,8 +765,9 @@ public:
 protected:
     friend class type_caster_generic;
     void check_holder_compat() {
-        if (typeinfo->default_holder)
+        if (typeinfo->default_holder) {
             throw cast_error("Unable to load a custom holder type from a default-holder instance");
+        }
     }
 
     bool load_value(value_and_holder &&v_h) {
@@ -1531,20 +775,25 @@ protected:
             value = v_h.value_ptr();
             holder = v_h.template holder<holder_type>();
             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)");
+        }
+        throw cast_error("Unable to cast from non-held to held instance (T& to Holder<T>) "
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+                         "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for "
+                         "type information)");
 #else
-                             "of type '" + type_id<holder_type>() + "''");
+                         "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, bool) {
+        return false;
+    }
 
-    template <typename T = holder_type, detail::enable_if_t<std::is_constructible<T, const T &, type*>::value, int> = 0>
+    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) {
             copyable_holder_caster sub_caster(*cast.first);
@@ -1559,18 +808,20 @@ protected:
 
     static bool try_direct_conversions(handle) { return false; }
 
-
     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 copyable_holder_caster<T, std::shared_ptr<T>> { };
+class type_caster<std::shared_ptr<T>> : public copyable_holder_caster<T, std::shared_ptr<T>> {};
 
-template <typename type, typename holder_type>
+/// Type caster for holder types like std::unique_ptr.
+/// Please consider the SFINAE hook an implementation detail, as explained
+/// in the comment for the copyable_holder_caster.
+template <typename type, typename holder_type, typename SFINAE = void>
 struct move_only_holder_caster {
     static_assert(std::is_base_of<type_caster_base<type>, type_caster<type>>::value,
-            "Holder classes are only supported for custom types");
+                  "Holder classes are only supported for custom types");
 
     static handle cast(holder_type &&src, return_value_policy, handle) {
         auto *ptr = holder_helper<holder_type>::get(src);
@@ -1581,50 +832,101 @@ struct move_only_holder_caster {
 
 template <typename type, typename deleter>
 class type_caster<std::unique_ptr<type, deleter>>
-    : public move_only_holder_caster<type, std::unique_ptr<type, deleter>> { };
+    : public move_only_holder_caster<type, std::unique_ptr<type, deleter>> {};
 
 template <typename type, typename holder_type>
 using type_caster_holder = conditional_t<is_copy_constructible<holder_type>::value,
                                          copyable_holder_caster<type, holder_type>,
                                          move_only_holder_caster<type, holder_type>>;
 
-template <typename T, bool Value = false> struct always_construct_holder { static constexpr bool value = Value; };
+template <typename T, bool Value = false>
+struct always_construct_holder {
+    static constexpr bool value = Value;
+};
 
 /// Create a specialization for custom holder types (silently ignores std::shared_ptr)
-#define PYBIND11_DECLARE_HOLDER_TYPE(type, holder_type, ...) \
-    namespace pybind11 { namespace detail { \
-    template <typename type> \
-    struct always_construct_holder<holder_type> : always_construct_holder<void, ##__VA_ARGS__>  { }; \
-    template <typename type> \
-    class type_caster<holder_type, enable_if_t<!is_shared_ptr<holder_type>::value>> \
-        : public type_caster_holder<type, holder_type> { }; \
-    }}
+#define PYBIND11_DECLARE_HOLDER_TYPE(type, holder_type, ...)                                      \
+    PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)                                                  \
+    namespace detail {                                                                            \
+    template <typename type>                                                                      \
+    struct always_construct_holder<holder_type> : always_construct_holder<void, ##__VA_ARGS__> {  \
+    };                                                                                            \
+    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_NAMESPACE_END(PYBIND11_NAMESPACE)
 
 // 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>> {};
+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 constexpr auto name = _<T>(); };
-template <> struct handle_type_name<bytes> { static constexpr auto name = _(PYBIND11_BYTES_NAME); };
-template <> struct handle_type_name<int_> { static constexpr auto name = _("int"); };
-template <> struct handle_type_name<iterable> { static constexpr auto name = _("Iterable"); };
-template <> struct handle_type_name<iterator> { static constexpr auto name = _("Iterator"); };
-template <> struct handle_type_name<none> { static constexpr auto name = _("None"); };
-template <> struct handle_type_name<args> { static constexpr auto name = _("*args"); };
-template <> struct handle_type_name<kwargs> { static constexpr auto name = _("**kwargs"); };
+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 constexpr auto name = const_name<T>();
+};
+template <>
+struct handle_type_name<bool_> {
+    static constexpr auto name = const_name("bool");
+};
+template <>
+struct handle_type_name<bytes> {
+    static constexpr auto name = const_name(PYBIND11_BYTES_NAME);
+};
+template <>
+struct handle_type_name<int_> {
+    static constexpr auto name = const_name("int");
+};
+template <>
+struct handle_type_name<iterable> {
+    static constexpr auto name = const_name("Iterable");
+};
+template <>
+struct handle_type_name<iterator> {
+    static constexpr auto name = const_name("Iterator");
+};
+template <>
+struct handle_type_name<float_> {
+    static constexpr auto name = const_name("float");
+};
+template <>
+struct handle_type_name<none> {
+    static constexpr auto name = const_name("None");
+};
+template <>
+struct handle_type_name<args> {
+    static constexpr auto name = const_name("*args");
+};
+template <>
+struct handle_type_name<kwargs> {
+    static constexpr auto name = const_name("**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); }
+    pyobject_caster() : value() {}
 
+    // `type` may not be default constructible (e.g. frozenset, anyset).  Initializing `value`
+    // to a nil handle is safe since it will only be accessed if `load` succeeds.
     template <typename T = type, enable_if_t<std::is_base_of<object, T>::value, int> = 0>
+    pyobject_caster() : value(reinterpret_steal<type>(handle())) {}
+
+    template <typename T = type, enable_if_t<std::is_same<T, handle>::value, int> = 0>
     bool load(handle src, bool /* convert */) {
-        if (!isinstance<type>(src))
+        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;
     }
@@ -1636,7 +938,7 @@ struct pyobject_caster {
 };
 
 template <typename T>
-class type_caster<T, enable_if_t<is_pyobject<T>::value>> : public pyobject_caster<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:
@@ -1647,65 +949,83 @@ class type_caster<T, enable_if_t<is_pyobject<T>::value>> : public pyobject_caste
 // - 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> using move_is_plain_type = satisfies_none_of<T,
-    std::is_void, std::is_pointer, std::is_reference, std::is_const
->;
-template <typename T, typename SFINAE = void> struct move_always : std::false_type {};
-template <typename T> struct move_always<T, enable_if_t<all_of<
-    move_is_plain_type<T>,
-    negation<is_copy_constructible<T>>,
-    std::is_move_constructible<T>,
-    std::is_same<decltype(std::declval<make_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<all_of<
-    move_is_plain_type<T>,
-    negation<move_always<T>>,
-    std::is_move_constructible<T>,
-    std::is_same<decltype(std::declval<make_caster<T>>().operator T&()), T&>
->::value>> : std::true_type {};
-template <typename T> using move_never = none_of<move_always<T>, move_if_unreferenced<T>>;
+template <typename T>
+using move_is_plain_type
+    = satisfies_none_of<T, std::is_void, std::is_pointer, std::is_reference, std::is_const>;
+template <typename T, typename SFINAE = void>
+struct move_always : std::false_type {};
+template <typename T>
+struct move_always<
+    T,
+    enable_if_t<
+        all_of<move_is_plain_type<T>,
+               negation<is_copy_constructible<T>>,
+               std::is_move_constructible<T>,
+               std::is_same<decltype(std::declval<make_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<
+        all_of<move_is_plain_type<T>,
+               negation<move_always<T>>,
+               std::is_move_constructible<T>,
+               std::is_same<decltype(std::declval<make_caster<T>>().operator T &()), T &>>::value>>
+    : std::true_type {};
+template <typename T>
+using move_never = none_of<move_always<T>, move_if_unreferenced<T>>;
 
 // 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 &&
-    !std::is_same<intrinsic_t<type>, void>::value
->;
+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
+                    && !std::is_same<intrinsic_t<type>, void>::value>;
 
 // When a value returned from a C++ function is being cast back to Python, we almost always want to
 // force `policy = move`, regardless of the return value policy the function/method was declared
 // with.
-template <typename Return, typename SFINAE = void> struct return_value_policy_override {
+template <typename Return, typename SFINAE = void>
+struct return_value_policy_override {
     static return_value_policy policy(return_value_policy p) { return p; }
 };
 
-template <typename Return> struct return_value_policy_override<Return,
-        detail::enable_if_t<std::is_base_of<type_caster_generic, make_caster<Return>>::value, void>> {
+template <typename Return>
+struct return_value_policy_override<
+    Return,
+    detail::enable_if_t<std::is_base_of<type_caster_generic, make_caster<Return>>::value, void>> {
     static return_value_policy policy(return_value_policy p) {
-        return !std::is_lvalue_reference<Return>::value &&
-               !std::is_pointer<Return>::value
-                   ? return_value_policy::move : p;
+        return !std::is_lvalue_reference<Return>::value && !std::is_pointer<Return>::value
+                   ? return_value_policy::move
+                   : p;
     }
 };
 
 // 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) {
+template <typename T, typename SFINAE>
+type_caster<T, SFINAE> &load_type(type_caster<T, SFINAE> &conv, const handle &handle) {
+    static_assert(!detail::is_pyobject<T>::value,
+                  "Internal error: type_caster should only be used for C++ types");
     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)");
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+        throw cast_error("Unable to cast Python instance to C++ type (#define "
+                         "PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
 #else
-        throw cast_error("Unable to cast Python instance of type " +
-            (std::string) str(type::handle_of(handle)) + " to C++ type '" + type_id<T>() + "'");
+        throw cast_error("Unable to cast Python instance of type "
+                         + (std::string) str(type::handle_of(handle)) + " 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) {
+template <typename T>
+make_caster<T> load_type(const handle &handle) {
     make_caster<T> conv;
     load_type(conv, handle);
     return conv;
@@ -1718,44 +1038,60 @@ template <typename T, detail::enable_if_t<!detail::is_pyobject<T>::value, int> =
 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");
+                  "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)); }
+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(T &&value, return_value_policy policy = return_value_policy::automatic_reference,
+object cast(T &&value,
+            return_value_policy policy = return_value_policy::automatic_reference,
             handle parent = handle()) {
     using no_ref_T = typename std::remove_reference<T>::type;
-    if (policy == return_value_policy::automatic)
-        policy = std::is_pointer<no_ref_T>::value ? return_value_policy::take_ownership :
-                 std::is_lvalue_reference<T>::value ? return_value_policy::copy : return_value_policy::move;
-    else if (policy == return_value_policy::automatic_reference)
-        policy = std::is_pointer<no_ref_T>::value ? return_value_policy::reference :
-                 std::is_lvalue_reference<T>::value ? return_value_policy::copy : return_value_policy::move;
-    return reinterpret_steal<object>(detail::make_caster<T>::cast(std::forward<T>(value), policy, parent));
+    if (policy == return_value_policy::automatic) {
+        policy = std::is_pointer<no_ref_T>::value     ? return_value_policy::take_ownership
+                 : std::is_lvalue_reference<T>::value ? return_value_policy::copy
+                                                      : return_value_policy::move;
+    } else if (policy == return_value_policy::automatic_reference) {
+        policy = std::is_pointer<no_ref_T>::value     ? return_value_policy::reference
+                 : std::is_lvalue_reference<T>::value ? return_value_policy::copy
+                                                      : return_value_policy::move;
+    }
+    return reinterpret_steal<object>(
+        detail::make_caster<T>::cast(std::forward<T>(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>
+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_never<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)");
+    if (obj.ref_count() > 1) {
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+        throw cast_error(
+            "Unable to cast Python instance to C++ rvalue: instance has multiple references"
+            " (#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
 #else
-        throw cast_error("Unable to move from Python " + (std::string) str(type::handle_of(obj)) +
-                " instance to C++ " + type_id<T>() + " instance: instance has multiple references");
+        throw cast_error("Unable to move from Python " + (std::string) str(type::handle_of(obj))
+                         + " 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&());
+    T ret = std::move(detail::load_type<T>(obj).operator T &());
     return ret;
 }
 
@@ -1764,96 +1100,164 @@ detail::enable_if_t<!detail::move_never<T>::value, T> move(object &&obj) {
 //   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) {
+template <typename T>
+detail::enable_if_t<!detail::is_pyobject<T>::value && 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)
+template <typename T>
+detail::enable_if_t<!detail::is_pyobject<T>::value && 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));
+    }
+    return move<T>(std::move(object));
 }
-template <typename T> detail::enable_if_t<detail::move_never<T>::value, T> cast(object &&object) {
+template <typename T>
+detail::enable_if_t<!detail::is_pyobject<T>::value && 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; }
+// pytype rvalue -> pytype (calls converting constructor)
+template <typename T>
+detail::enable_if_t<detail::is_pyobject<T>::value, T> cast(object &&object) {
+    return T(std::move(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;
+}
 
 PYBIND11_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)); }
+object object_or_cast(T &&o) {
+    return pybind11::cast(std::forward<T>(o));
+}
 
-struct override_unused {}; // Placeholder type for the unneeded (and dead code) static variable in the PYBIND11_OVERRIDE_OVERRIDE macro
-template <typename ret_type> using override_caster_t = conditional_t<
-    cast_is_temporary_value_reference<ret_type>::value, make_caster<ret_type>, override_unused>;
+// Placeholder type for the unneeded (and dead code) static variable in the
+// PYBIND11_OVERRIDE_OVERRIDE macro
+struct override_unused {};
+template <typename ret_type>
+using override_caster_t = conditional_t<cast_is_temporary_value_reference<ret_type>::value,
+                                        make_caster<ret_type>,
+                                        override_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) {
+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 &&, override_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 &&) {}
+template <typename T>
+enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&,
+                                                                      override_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 &&) {
+    pybind11_fail("Internal error: cast_safe fallback invoked");
+}
+template <typename T>
+enable_if_t<std::is_same<void, intrinsic_t<T>>::value, void> cast_safe(object &&) {}
+template <typename T>
+enable_if_t<detail::none_of<cast_is_temporary_value_reference<T>,
+                            std::is_same<void, intrinsic_t<T>>>::value,
+            T>
+cast_safe(object &&o) {
+    return pybind11::cast<T>(std::move(o));
+}
 
 PYBIND11_NAMESPACE_END(detail)
 
+// The overloads could coexist, i.e. the #if is not strictly speaking needed,
+// but it is an easy minor optimization.
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+inline cast_error cast_error_unable_to_convert_call_arg() {
+    return cast_error("Unable to convert call argument to Python object (#define "
+                      "PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
+}
+#else
+inline cast_error cast_error_unable_to_convert_call_arg(const std::string &name,
+                                                        const std::string &type) {
+    return cast_error("Unable to convert call argument '" + name + "' of type '" + type
+                      + "' to Python object");
+}
+#endif
+
 template <return_value_policy policy = return_value_policy::automatic_reference>
-tuple make_tuple() { return tuple(0); }
+tuple make_tuple() {
+    return tuple(0);
+}
 
-template <return_value_policy policy = return_value_policy::automatic_reference,
-          typename... Args> tuple make_tuple(Args&&... args_) {
+template <return_value_policy policy = return_value_policy::automatic_reference, typename... Args>
+tuple make_tuple(Args &&...args_) {
     constexpr size_t size = sizeof...(Args);
-    std::array<object, size> args {
-        { reinterpret_steal<object>(detail::make_caster<Args>::cast(
-            std::forward<Args>(args_), policy, nullptr))... }
-    };
+    std::array<object, size> args{{reinterpret_steal<object>(
+        detail::make_caster<Args>::cast(std::forward<Args>(args_), policy, nullptr))...}};
     for (size_t i = 0; i < args.size(); i++) {
         if (!args[i]) {
-#if defined(NDEBUG)
-            throw cast_error("make_tuple(): unable to convert arguments to Python object (compile in debug mode for details)");
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+            throw cast_error_unable_to_convert_call_arg();
 #else
-            std::array<std::string, size> argtypes { {type_id<Args>()...} };
-            throw cast_error("make_tuple(): unable to convert argument of type '" +
-                argtypes[i] + "' to Python object");
+            std::array<std::string, size> argtypes{{type_id<Args>()...}};
+            throw cast_error_unable_to_convert_call_arg(std::to_string(i), argtypes[i]);
 #endif
         }
     }
     tuple result(size);
     int counter = 0;
-    for (auto &arg_value : args)
+    for (auto &arg_value : args) {
         PyTuple_SET_ITEM(result.ptr(), counter++, arg_value.release().ptr());
+    }
     return result;
 }
 
 /// \ingroup annotations
 /// Annotation for arguments
 struct arg {
-    /// Constructs an argument with the name of the argument; if null or omitted, this is a positional argument.
-    constexpr explicit arg(const char *name = nullptr) : name(name), flag_noconvert(false), flag_none(true) { }
+    /// Constructs an argument with the name of the argument; if null or omitted, this is a
+    /// positional argument.
+    constexpr explicit arg(const char *name = nullptr)
+        : name(name), flag_noconvert(false), flag_none(true) {}
     /// Assign a value to this argument
-    template <typename T> arg_v operator=(T &&value) const;
+    template <typename T>
+    arg_v operator=(T &&value) const;
     /// Indicate that the type should not be converted in the type caster
-    arg &noconvert(bool flag = true) { flag_noconvert = flag; return *this; }
+    arg &noconvert(bool flag = true) {
+        flag_noconvert = flag;
+        return *this;
+    }
     /// Indicates that the argument should/shouldn't allow None (e.g. for nullable pointer args)
-    arg &none(bool flag = true) { flag_none = flag; return *this; }
+    arg &none(bool flag = true) {
+        flag_none = flag;
+        return *this;
+    }
 
-    const char *name; ///< If non-null, this is a named kwargs argument
-    bool flag_noconvert : 1; ///< If set, do not allow conversion (requires a supporting type caster!)
-    bool flag_none : 1; ///< If set (the default), allow None to be passed to this argument
+    const char *name;        ///< If non-null, this is a named kwargs argument
+    bool flag_noconvert : 1; ///< If set, do not allow conversion (requires a supporting type
+                             ///< caster!)
+    bool flag_none : 1;      ///< If set (the default), allow None to be passed to this argument
 };
 
 /// \ingroup annotations
@@ -1862,58 +1266,73 @@ struct arg_v : arg {
 private:
     template <typename T>
     arg_v(arg &&base, T &&x, const char *descr = nullptr)
-        : arg(base),
-          value(reinterpret_steal<object>(
-              detail::make_caster<T>::cast(x, return_value_policy::automatic, {})
-          )),
+        : arg(base), value(reinterpret_steal<object>(detail::make_caster<T>::cast(
+                         std::forward<T>(x), return_value_policy::automatic, {}))),
           descr(descr)
-#if !defined(NDEBUG)
-        , type(type_id<T>())
+#if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+          ,
+          type(type_id<T>())
 #endif
-    { }
+    {
+        // Workaround! See:
+        // https://github.com/pybind/pybind11/issues/2336
+        // https://github.com/pybind/pybind11/pull/2685#issuecomment-731286700
+        if (PyErr_Occurred()) {
+            PyErr_Clear();
+        }
+    }
 
 public:
     /// Direct construction with name, default, and description
     template <typename T>
     arg_v(const char *name, T &&x, const char *descr = nullptr)
-        : arg_v(arg(name), std::forward<T>(x), descr) { }
+        : arg_v(arg(name), std::forward<T>(x), descr) {}
 
     /// Called internally when invoking `py::arg("a") = value`
     template <typename T>
     arg_v(const arg &base, T &&x, const char *descr = nullptr)
-        : arg_v(arg(base), std::forward<T>(x), descr) { }
+        : arg_v(arg(base), std::forward<T>(x), descr) {}
 
     /// Same as `arg::noconvert()`, but returns *this as arg_v&, not arg&
-    arg_v &noconvert(bool flag = true) { arg::noconvert(flag); return *this; }
+    arg_v &noconvert(bool flag = true) {
+        arg::noconvert(flag);
+        return *this;
+    }
 
     /// Same as `arg::nonone()`, but returns *this as arg_v&, not arg&
-    arg_v &none(bool flag = true) { arg::none(flag); return *this; }
+    arg_v &none(bool flag = true) {
+        arg::none(flag);
+        return *this;
+    }
 
     /// The default value
     object value;
     /// The (optional) description of the default value
     const char *descr;
-#if !defined(NDEBUG)
+#if defined(PYBIND11_DETAILED_ERROR_MESSAGES)
     /// The C++ type name of the default value (only available when compiled in debug mode)
     std::string type;
 #endif
 };
 
 /// \ingroup annotations
-/// Annotation indicating that all following arguments are keyword-only; the is the equivalent of an
-/// unnamed '*' argument (in Python 3)
+/// Annotation indicating that all following arguments are keyword-only; the is the equivalent of
+/// an unnamed '*' argument
 struct kw_only {};
 
 /// \ingroup annotations
-/// Annotation indicating that all previous arguments are positional-only; the is the equivalent of an
-/// unnamed '/' argument (in Python 3.8)
+/// Annotation indicating that all previous arguments are positional-only; the is the equivalent of
+/// an unnamed '/' argument (in Python 3.8)
 struct pos_only {};
 
 template <typename T>
-arg_v arg::operator=(T &&value) const { return {std::move(*this), std::forward<T>(value)}; }
+arg_v arg::operator=(T &&value) const {
+    return {*this, std::forward<T>(value)};
+}
 
 /// Alias for backward compatibility -- to be removed in version 2.0
-template <typename /*unused*/> using arg_t = arg_v;
+template <typename /*unused*/>
+using arg_t = arg_v;
 
 inline namespace literals {
 /** \rst
@@ -1924,6 +1343,11 @@ constexpr arg operator"" _a(const char *name, size_t) { return arg(name); }
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
+template <typename T>
+using is_kw_only = std::is_same<intrinsic_t<T>, kw_only>;
+template <typename T>
+using is_pos_only = std::is_same<intrinsic_t<T>, pos_only>;
+
 // forward declaration (definition in attr.h)
 struct function_record;
 
@@ -1951,56 +1375,63 @@ struct function_call {
     handle init_self;
 };
 
-
 /// Helper class which loads arguments for C++ functions called from Python
 template <typename... Args>
 class argument_loader {
     using indices = make_index_sequence<sizeof...(Args)>;
 
-    template <typename Arg> using argument_is_args   = std::is_same<intrinsic_t<Arg>, args>;
-    template <typename Arg> using argument_is_kwargs = std::is_same<intrinsic_t<Arg>, kwargs>;
-    // Get args/kwargs argument positions relative to the end of the argument list:
-    static constexpr auto args_pos = constexpr_first<argument_is_args, Args...>() - (int) sizeof...(Args),
-                        kwargs_pos = constexpr_first<argument_is_kwargs, Args...>() - (int) sizeof...(Args);
-
-    static constexpr bool args_kwargs_are_last = kwargs_pos >= - 1 && args_pos >= kwargs_pos - 1;
+    template <typename Arg>
+    using argument_is_args = std::is_same<intrinsic_t<Arg>, args>;
+    template <typename Arg>
+    using argument_is_kwargs = std::is_same<intrinsic_t<Arg>, kwargs>;
+    // Get kwargs argument position, or -1 if not present:
+    static constexpr auto kwargs_pos = constexpr_last<argument_is_kwargs, Args...>();
 
-    static_assert(args_kwargs_are_last, "py::args/py::kwargs are only permitted as the last argument(s) of a function");
+    static_assert(kwargs_pos == -1 || kwargs_pos == (int) sizeof...(Args) - 1,
+                  "py::kwargs is only permitted as the last argument of a function");
 
 public:
-    static constexpr bool has_kwargs = kwargs_pos < 0;
-    static constexpr bool has_args = args_pos < 0;
+    static constexpr bool has_kwargs = kwargs_pos != -1;
+
+    // py::args argument position; -1 if not present.
+    static constexpr int args_pos = constexpr_last<argument_is_args, Args...>();
+
+    static_assert(args_pos == -1 || args_pos == constexpr_first<argument_is_args, Args...>(),
+                  "py::args cannot be specified more than once");
 
     static constexpr auto arg_names = concat(type_descr(make_caster<Args>::name)...);
 
-    bool load_args(function_call &call) {
-        return load_impl_sequence(call, indices{});
-    }
+    bool load_args(function_call &call) { return load_impl_sequence(call, indices{}); }
 
     template <typename Return, typename Guard, typename Func>
+    // NOLINTNEXTLINE(readability-const-return-type)
     enable_if_t<!std::is_void<Return>::value, Return> call(Func &&f) && {
-        return std::move(*this).template call_impl<Return>(std::forward<Func>(f), indices{}, Guard{});
+        return std::move(*this).template call_impl<remove_cv_t<Return>>(
+            std::forward<Func>(f), indices{}, Guard{});
     }
 
     template <typename Return, typename Guard, typename Func>
     enable_if_t<std::is_void<Return>::value, void_type> call(Func &&f) && {
-        std::move(*this).template call_impl<Return>(std::forward<Func>(f), indices{}, Guard{});
+        std::move(*this).template call_impl<remove_cv_t<Return>>(
+            std::forward<Func>(f), indices{}, Guard{});
         return void_type();
     }
 
 private:
-
     static bool load_impl_sequence(function_call &, index_sequence<>) { return true; }
 
     template <size_t... Is>
     bool load_impl_sequence(function_call &call, index_sequence<Is...>) {
 #ifdef __cpp_fold_expressions
-        if ((... || !std::get<Is>(argcasters).load(call.args[Is], call.args_convert[Is])))
+        if ((... || !std::get<Is>(argcasters).load(call.args[Is], call.args_convert[Is]))) {
             return false;
+        }
 #else
-        for (bool r : {std::get<Is>(argcasters).load(call.args[Is], call.args_convert[Is])...})
-            if (!r)
+        for (bool r : {std::get<Is>(argcasters).load(call.args[Is], call.args_convert[Is])...}) {
+            if (!r) {
                 return false;
+            }
+        }
 #endif
         return true;
     }
@@ -2020,7 +1451,7 @@ class simple_collector {
 public:
     template <typename... Ts>
     explicit simple_collector(Ts &&...values)
-        : m_args(pybind11::make_tuple<policy>(std::forward<Ts>(values)...)) { }
+        : m_args(pybind11::make_tuple<policy>(std::forward<Ts>(values)...)) {}
 
     const tuple &args() const & { return m_args; }
     dict kwargs() const { return {}; }
@@ -2030,8 +1461,9 @@ public:
     /// Call a Python function and pass the collected arguments
     object call(PyObject *ptr) const {
         PyObject *result = PyObject_CallObject(ptr, m_args.ptr());
-        if (!result)
+        if (!result) {
             throw error_already_set();
+        }
         return reinterpret_steal<object>(result);
     }
 
@@ -2048,8 +1480,8 @@ public:
         // 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(_);
+        using expander = int[];
+        (void) expander{0, (process(args_list, std::forward<Ts>(values)), 0)...};
 
         m_args = std::move(args_list);
     }
@@ -2063,61 +1495,66 @@ public:
     /// 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)
+        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, {}));
+        auto o = reinterpret_steal<object>(
+            detail::make_caster<T>::cast(std::forward<T>(x), policy, {}));
         if (!o) {
-#if defined(NDEBUG)
-            argument_cast_error();
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+            throw cast_error_unable_to_convert_call_arg();
 #else
-            argument_cast_error(std::to_string(args_list.size()), type_id<T>());
+            throw cast_error_unable_to_convert_call_arg(std::to_string(args_list.size()),
+                                                        type_id<T>());
 #endif
         }
-        args_list.append(o);
+        args_list.append(std::move(o));
     }
 
     void process(list &args_list, detail::args_proxy ap) {
-        for (auto a : ap)
+        for (auto a : ap) {
             args_list.append(a);
+        }
     }
 
-    void process(list &/*args_list*/, arg_v a) {
-        if (!a.name)
-#if defined(NDEBUG)
+    void process(list & /*args_list*/, arg_v a) {
+        if (!a.name) {
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
             nameless_argument_error();
 #else
             nameless_argument_error(a.type);
 #endif
-
+        }
         if (m_kwargs.contains(a.name)) {
-#if defined(NDEBUG)
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
             multiple_values_error();
 #else
             multiple_values_error(a.name);
 #endif
         }
         if (!a.value) {
-#if defined(NDEBUG)
-            argument_cast_error();
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
+            throw cast_error_unable_to_convert_call_arg();
 #else
-            argument_cast_error(a.name, a.type);
+            throw cast_error_unable_to_convert_call_arg(a.name, a.type);
 #endif
         }
         m_kwargs[a.name] = a.value;
     }
 
-    void process(list &/*args_list*/, detail::kwargs_proxy kp) {
-        if (!kp)
+    void process(list & /*args_list*/, detail::kwargs_proxy kp) {
+        if (!kp) {
             return;
+        }
         for (auto k : reinterpret_borrow<dict>(kp)) {
             if (m_kwargs.contains(k.first)) {
-#if defined(NDEBUG)
+#if !defined(PYBIND11_DETAILED_ERROR_MESSAGES)
                 multiple_values_error();
 #else
                 multiple_values_error(str(k.first));
@@ -2128,62 +1565,71 @@ private:
     }
 
     [[noreturn]] static void nameless_argument_error() {
-        throw type_error("Got kwargs without a name; only named arguments "
-                         "may be passed via py::arg() to a python function call. "
-                         "(compile in debug mode for details)");
+        throw type_error(
+            "Got kwargs without a name; only named arguments "
+            "may be passed via py::arg() to a python function call. "
+            "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
     }
-    [[noreturn]] static void nameless_argument_error(std::string type) {
-        throw type_error("Got kwargs without a name of type '" + type + "'; only named "
-                         "arguments may be passed via py::arg() to a python function call. ");
+    [[noreturn]] static void nameless_argument_error(const std::string &type) {
+        throw type_error("Got kwargs without a name of type '" + type
+                         + "'; only named "
+                           "arguments may be passed via py::arg() to a python function call. ");
     }
     [[noreturn]] static void multiple_values_error() {
-        throw type_error("Got multiple values for keyword argument "
-                         "(compile in debug mode for details)");
+        throw type_error(
+            "Got multiple values for keyword argument "
+            "(#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)");
     }
 
-    [[noreturn]] static void multiple_values_error(std::string name) {
+    [[noreturn]] static void multiple_values_error(const 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;
 };
 
+// [workaround(intel)] Separate function required here
+// We need to put this into a separate function because the Intel compiler
+// fails to compile enable_if_t<!all_of<is_positional<Args>...>::value>
+// (tested with ICC 2021.1 Beta 20200827).
+template <typename... Args>
+constexpr bool args_are_all_positional() {
+    return all_of<is_positional<Args>...>::value;
+}
+
 /// Collect only positional arguments for a Python function call
-template <return_value_policy policy, typename... Args,
-          typename = enable_if_t<all_of<is_positional<Args>...>::value>>
+template <return_value_policy policy,
+          typename... Args,
+          typename = enable_if_t<args_are_all_positional<Args...>()>>
 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<is_positional<Args>...>::value>>
+template <return_value_policy policy,
+          typename... Args,
+          typename = enable_if_t<!args_are_all_positional<Args...>()>>
 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"
-    );
+    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 {
+#ifndef NDEBUG
+    if (!PyGILState_Check()) {
+        pybind11_fail("pybind11::object_api<>::operator() PyGILState_Check() failure.");
+    }
+#endif
     return detail::collect_arguments<policy>(std::forward<Args>(args)...).call(derived().ptr());
 }
 
@@ -2195,25 +1641,25 @@ object object_api<Derived>::call(Args &&...args) const {
 
 PYBIND11_NAMESPACE_END(detail)
 
-
-template<typename T>
+template <typename T>
 handle type::handle_of() {
-   static_assert(
-      std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>::value,
-      "py::type::of<T> only supports the case where T is a registered C++ types."
-    );
+    static_assert(std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>::value,
+                  "py::type::of<T> only supports the case where T is a registered C++ types.");
 
     return detail::get_type_handle(typeid(T), true);
 }
 
-
-#define PYBIND11_MAKE_OPAQUE(...) \
-    namespace pybind11 { namespace detail { \
-        template<> class type_caster<__VA_ARGS__> : public type_caster_base<__VA_ARGS__> { }; \
-    }}
+#define PYBIND11_MAKE_OPAQUE(...)                                                                 \
+    PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)                                                  \
+    namespace detail {                                                                            \
+    template <>                                                                                   \
+    class type_caster<__VA_ARGS__> : public type_caster_base<__VA_ARGS__> {};                     \
+    }                                                                                             \
+    PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 
 /// Lets you pass a type containing a `,` through a macro parameter without needing a separate
-/// typedef, e.g.: `PYBIND11_OVERRIDE(PYBIND11_TYPE(ReturnType<A, B>), PYBIND11_TYPE(Parent<C, D>), f, arg)`
+/// typedef, e.g.:
+/// `PYBIND11_OVERRIDE(PYBIND11_TYPE(ReturnType<A, B>), PYBIND11_TYPE(Parent<C, D>), f, arg)`
 #define PYBIND11_TYPE(...) __VA_ARGS__
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)