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Since the argument loader split off from the tuple converter, it is
never called with a `convert` argument set to anything but true.  This
removes the argument entirely, passing a literal `true` from within
`argument_loader` to the individual value casters.

Current g++ 7 snapshot fails to compile pybind under -std=c++17 with:

```
$ make
[  3%] Building CXX object tests/CMakeFiles/pybind11_tests.dir/pybind11_tests.cpp.o
In file included from /home/jagerman/src/pybind11/tests/pybind11_tests.h:2:0,
                 from /home/jagerman/src/pybind11/tests/pybind11_tests.cpp:10:
/home/jagerman/src/pybind11/include/pybind11/pybind11.h: In instantiation of 'pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_record*, pybind11::handle, pybind11::handle, pybind11::handle)> [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]':
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:120:22:   required from 'struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(struct pybind11::detail::function_record*, class pybind11::handle, class pybind11::handle, class pybind11::handle)>'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:120:19:   required from 'void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:62:9:   required from 'pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:984:22:   required from 'pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const char*, Func&&, const Extra& ...) [with Func = int (ConstructorStats::*)(); Extra = {}; type_ = ConstructorStats; options = {}]'
/home/jagerman/src/pybind11/tests/pybind11_tests.cpp:24:47:   required from here
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:147:9: sorry, unimplemented: unexpected AST of kind cleanup_stmt
         };
         ^
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:147:9: internal compiler error: in potential_constant_expression_1, at cp/constexpr.c:5593
0x84c52a potential_constant_expression_1
	../../src/gcc/cp/constexpr.c:5593
0x84c3c0 potential_constant_expression_1
	../../src/gcc/cp/constexpr.c:5154
0x645511 finish_function(int)
	../../src/gcc/cp/decl.c:15527
0x66e80b instantiate_decl(tree_node*, int, bool)
	../../src/gcc/cp/pt.c:22558
0x6b61e2 instantiate_class_template_1
	../../src/gcc/cp/pt.c:10444
0x6b61e2 instantiate_class_template(tree_node*)
	../../src/gcc/cp/pt.c:10514
0x75a676 complete_type(tree_node*)
	../../src/gcc/cp/typeck.c:133
0x67d5a4 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool, bool)
	../../src/gcc/cp/pt.c:17516
0x67ca19 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool, bool)
	../../src/gcc/cp/pt.c:16655
0x672cce tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:16140
0x6713dc tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15408
0x671915 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15394
0x671fc0 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15618
0x66e97f tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15379
0x66e97f instantiate_decl(tree_node*, int, bool)
	../../src/gcc/cp/pt.c:22536
0x6ba0cb instantiate_pending_templates(int)
	../../src/gcc/cp/pt.c:22653
0x6fd7f8 c_parse_final_cleanups()
	../../src/gcc/cp/decl2.c:4512
```

which looks a lot like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77545.

The error seems to be that it gets confused about the `std::tuple<...>
value` in argument_loader: it is apparently not being initialized
properly.  Adding a default constructor with an explicit
default-initialization of `value` works around the problem.

This adds automatic casting when assigning to python types like dict,
list, and attributes.  Instead of:

    dict["key"] = py::cast(val);
    m.attr("foo") = py::cast(true);
    list.append(py::cast(42));

you can now simply write:

    dict["key"] = val;
    m.attr("foo") = true;
    list.append(42);

Casts needing extra parameters (e.g. for a non-default rvp) still
require the py::cast() call. set::add() is also supported.

All usage is channeled through a SFINAE implementation which either just returns or casts. 

Combined non-converting handle and autocasting template methods via a
helper method that either just returns (handle) or casts (C++ type).

Newer standard libraries use compiler intrinsics for std::index_sequence
which makes it ‘free’. This prevents hitting instantiation limits for
recursive templates (-ftemplate-depth).

This is more Pythonic and compliments the std::vector and std::list
casters which also accept sequences.

This is needed in order to allow the tuple caster to accept any sequence
while keeping the argument loader fast. There is also very little overlap
between the two classes which makes the separation clean. It’s also good
practice not to have completely new functionality in a specialization.

Using a complicated declval here was pointlessly complicated: we
already know the type, because that's what cast_op_type<T> is in the
first place.  (The declval also broke MSVC).

This adds a `detail::cast_op<T>(caster)` function which handles the
rather verbose:

    caster.operator typename CasterType::template cast_op_type<T>()

which allows various places to use the shorter and clearer:

    cast_op<T>(caster)

instead of the full verbose cast operator invocation.

Fixes #509.

The move policy was already set for rvalues in PR #473, but this only
applied to directly cast user-defined types. The problem is that STL
containers cast values indirectly and the rvalue information is lost.
Therefore the move policy was not set correctly. This commit fixes it.

This also makes an additional adjustment to remove the `copy` policy
exception: rvalues now always use the `move` policy. This is also safe
for copy-only rvalues because the `move` policy has an internal fallback
to copying.

The pytype converting constructors are convenient and safe for user
code, but for library internals the additional type checks and possible
conversions are sometimes not desired. `reinterpret_borrow<T>()` and
`reinterpret_steal<T>()` serve as the low-level unsafe counterparts
of `cast<T>()`.

This deprecates the `object(handle, bool)` constructor.

Renamed `borrowed` parameter to `is_borrowed` to avoid shadowing
warnings on MSVC.

* Deprecate the `py::object::str()` member function since `py::str(obj)`
  is now equivalent and preferred

* Make `py::repr()` a free function

* Make sure obj.cast<T>() works as expected when T is a Python type

`obj.cast<T>()` should be the same as `T(obj)`, i.e. it should convert
the given object to a different Python type. However, `obj.cast<T>()`
usually calls `type_caster::load()` which only checks the type without
doing any actual conversion. That causes a very unexpected `cast_error`.
This commit makes it so that `obj.cast<T>()` and `T(obj)` are the same
when T is a Python type.

* Simplify pytypes converting constructor implementation

It's not necessary to maintain a full set of converting constructors
and assignment operators + const& and &&. A single converting const&
constructor will work and there is no impact on binary size. On the
other hand, the conversion functions can be significantly simplified.

Allows checking the Python types before creating an object instead of
after. For example:
```c++
auto l = list(ptr, true);
if (l.check())
   // ...
```
The above is replaced with:
```c++
if (isinstance<list>(ptr)) {
    auto l = reinterpret_borrow(ptr);
    // ...
}
```

This deprecates `py::object::check()`. `py::isinstance()` covers the
same use case, but it can also check for user-defined types:
```c++
class Pet { ... };
py::class_<Pet>(...);

m.def("is_pet", [](py::object obj) {
    return py::isinstance<Pet>(obj); // works as expected
});
```

* Incref returned None in std::optional type caster

* Add type casters for nullopt_t

* Add a test for nullopt_t

This commit includes the following changes:

* Don't provide make_copy_constructor for non-copyable container

make_copy_constructor currently fails for various stl containers (e.g.
std::vector, std::unordered_map, std::deque, etc.) when the container's
value type (e.g. the "T" or the std::pair<K,T> for a map) is
non-copyable.  This adds an override that, for types that look like
containers, also requires that the value_type be copyable.

* stl_bind.h: make bind_{vector,map} work for non-copy-constructible types

Most stl_bind modifiers require copying, so if the type isn't copy
constructible, we provide a read-only interface instead.

In practice, this means that if the type is non-copyable, it will be,
for all intents and purposes, read-only from the Python side (but
currently it simply fails to compile with such a container).

It is still possible for the caller to provide an interface manually
(by defining methods on ...

* Make reference(_internal) the default return value policy for properties

Before this, all `def_property*` functions used `automatic` as their
default return value policy. This commit makes it so that:

 * Non-static properties use `reference_interal` by default, thus
   matching `def_readonly` and `def_readwrite`.

 * Static properties use `reference` by default, thus matching
   `def_readonly_static` and `def_readwrite_static`.

In case `cpp_function` is passed to any `def_property*`, its policy will
be used instead of any defaults. User-defined arguments in `extras`
still have top priority and will override both the default policies and
the ones from `cpp_function`.

Resolves #436.

* Almost always use return_value_policy::move for rvalues

For functions which return rvalues or rvalue references, the only viable
return value policies are `copy` and `move`. `reference(_internal)` and
`take_ownership` would take the address of a temporary which is always
an error.

This commit prevents possible user errors by overriding the bad rvalue
policies with `move`. Besides `move`, only `copy` is allowed, and only
if it's explicitly selected by the user.

This is also a necessary safety feature to support the new default
return value policies for properties: `reference(_internal)`.

The current integer caster was unnecessarily strict and rejected
various kinds of NumPy integer types when calling C++ functions
expecting normal integers. This relaxes the current behavior.

This reduces direct access to internals.registered_types_cpp to
just a few places.

This avoid a hashmap lookup since the pointer to the list of
direct converters is now cached in the typeinfo.

type_caster_generic::cast(): The values of

    wrapper->value
    wrapper->owned

are incorrect in the case that a return value policy of 'copy' is
requested but there is no copy-constructor.  (Similarly 'move'.)  In
particular, if the source object is a static instance, the destructor of
the 'object' 'inst' leads to class_::dealloc() which incorrectly
attempts to 'delete' the static instance.

This commit re-arranges the code to be clearer as to what the values of
'value' and 'owned' should be in the various cases.  Behaviour is
different to previous code only in two situations:

policy = copy but no copy-ctor: Old code leaves 'value = src, owned =
true', which leads to trouble.  New code leaves 'value = nullptr, owned
= false', which is correct.

policy = move but no move- or copy-ctor: old code leaves 'value = src,
owned = true', which leads to trouble.  New code leaves 'value =
nullptr, owned = false', which is correct.

With this there is no more need for manual user declarations like
`PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)`. Existing ones
will still compile without error -- they will just be ignored silently.

Resolves #446.

We have various classes that have non-explicit constructors that accept
a single argument, which is implicitly making them implicitly
convertible from the argument.  In a few cases, this is desirable (e.g.
implicit conversion of std::string to py::str, or conversion of double
to py::float_); in many others, however, it is unintended (e.g. implicit
conversion of size_t to some pre-declared py::array_t<T> type).

This disables most of the unwanted implicit conversions by marking them
`explicit`, and comments the ones that are deliberately left implicit.

This fixes an issue that can arise when forwarding (*args, **kwargs)
captured from a pybind11-bound function call to another Python function.
When the initial function call includes no keyword arguments, the
py::kwargs field is set to nullptr and causes a crash later on.

This also adds the `hasattr` and `getattr` functions which are needed
with the new attribute behavior. The new functions behave exactly like
their Python counterparts.

Similarly `object` gets a `contains` method which calls `__contains__`,
i.e. it's the same as the `in` keyword in Python.

Add unit tests and documentation for the chrono cast.