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This fixes #856. Instead of the weakref trick, the internals structure
holds an unordered_map from PyObject* to a vector of references. To
avoid the cost of the unordered_map lookup for objects that don't have
any keep_alive patients, a flag is added to each instance to indicate
whether there is anything to do.

Using `std::type_info::operator==` fails under libc++ because the .so
is loaded with RTLD_LOCAL.  libc++ considers types under such .sos
distinct, and so comparing typeid() values directly isn't going to work.

This adds a custom hasher and equality class for the type lookup maps
when not under stdlibc++, and adds a `detail::same_type` function to
perform the equality test.  It also converts a few pointer arguments to
const lvalue references, particularly since doing the pointer
comparison wasn't technically valid to being with (though in practice,
appeared to work everywhere).

This fixes #912.

Fixes a race condition when multiple threads try to acquire the GIL
before `detail::internals` have been initialized. `gil_scoped_release`
is now tasked with initializing `internals` (guaranteed single-threaded)
to ensure the safety of subsequent `acquire` calls from multiple threads.

This commit allows multiple inheritance of pybind11 classes from
Python, e.g.

    class MyType(Base1, Base2):
        def __init__(self):
            Base1.__init__(self)
            Base2.__init__(self)

where Base1 and Base2 are pybind11-exported classes.

This requires collapsing the various builtin base objects
(pybind11_object_56, ...) introduced in 2.1 into a single
pybind11_object of a fixed size; this fixed size object allocates enough
space to contain either a simple object (one base class & small* holder
instance), or a pointer to a new allocation that can contain an
arbitrary number of base classes and holders, with holder size
unrestricted.

* "small" here means having a sizeof() of at most 2 pointers, which is
enough to fit unique_ptr (sizeof is 1 ptr) and shared_ptr (sizeof is 2
ptrs).

To minimize the performance impact, this repurposes
`internals::registered_types_py` to store a vector of pybind-registered
base types.  For direct-use pybind types (e.g. the `PyA` for a C++ `A`)
this is simply storing the same thing as before, but now in a vector;
for Python-side inherited types, the map lets us avoid having to do a
base class traversal as long as we've seen the class before.  The
change to vector is needed for multiple inheritance: Python types
inheriting from multiple registered bases have one entry per base.

Fixes #896.

From Python docs: "Once an iterator’s `__next__()` method raises
`StopIteration`, it must continue to do so on subsequent calls.
Implementations that do not obey this property are deemed broken."

Fixes #887.

Adds `remove_reference_t` and converts various `typename
std::remove_reference<...>::type` to using it.

This allows calling of functions (typically void) over a parameter
pack, replacing usage such as:

    bool unused[] = { (voidfunc(param_pack_arg), false)..., false };
    (void) unused;

with a much cleaner:

    PYBIND11_EXPAND_SIDE_EFFECTS(voidfunc(param_pack_arg));

This attribute lets you disable (or explicitly enable) passing None to
an argument that otherwise would allow it by accepting
a value by raw pointer or shared_ptr.

This commit allows type_casters to allow their local values to be moved
away, rather than copied, when the type caster instance itself is an rvalue.

This only applies (automatically) to type casters using
PYBIND11_TYPE_CASTER; the generic type type casters don't own their own
pointer, and various value casters (e.g. std::string, std::pair,
arithmetic types) already cast to an rvalue (i.e. they return by value).

This updates various calling code to attempt to get a movable value
whenever the value is itself coming from a type caster about to be
destroyed: for example, when constructing an std::pair or various stl.h
containers.  For types that don't support value moving, the cast_op
falls back to an lvalue cast.

There wasn't an obvious place to add the tests, so I added them to
test_copy_move_policies, but also renamed it to drop the _policies as it
now tests more than just policies.

Closes #857, by adding overloads to def_buffer that match pointers to
member functions and wrap them in lambdas.

This changes javadoc-style documenting comments from:

    /** Text starts here
     * and continues here
     */

to:

    /**
     * Test starts here
     * and continues here
     */

which looks a little better, and also matches the javadoc-recommended
way of writing documenting comments.

Using a dynamic_cast instead of a static_cast is needed to safely cast
from a base to a derived type.  The previous static_pointer_cast isn't
safe, however, when downcasting (and fails to compile when downcasting
with virtual inheritance).

Switching this to always use a dynamic_pointer_cast shouldn't incur any
additional overhead when a static_pointer_cast is safe (i.e. when
upcasting, or self-casting): compilers don't need RTTI checks in those
cases.

The PYBIND11_CPP14 macro started out as a guard for the compile-time
path code in `descr.h`, but has since come to mean other things.  This
means that while the `descr.h` check has just checked the
`PYBIND11_CPP14` macro, various other places now check `PYBIND11_CPP14
|| _MSC_VER`.  This reverses that by now setting the CPP14 macro when
MSVC is trying to support C++14, but disabling the `descr.h` C++14 code
(which still fails under MSVC 2017).

The CPP17 macro also gets enabled when MSVC 2017 is compiling with
/std:c++latest (the default is /std:c++14), which enables
`std::optional` and `std::variant` support under MSVC.

GCC 7 generates (when compiling in C++11/14 mode) warnings such as:

    mangled name for ‘pybind11::class_<type_, options>&
    pybind11::class_<type_, options>::def(const char*, Func&&, const Extra&
    ...) [with Func = int (test_exc_sp::C::*)(int) noexcept; Extra = {};
    type_ = test_exc_sp::C; options = {}]’ will change in C++17 because the
    exception specification is part of a function type [-Wnoexcept-type]

There's nothing we can actually do in the code to avoid this, so just
disable the warning.

Enumerations on Python 2.7 were not always implicitly converted to
integers (depending on the target size). This patch adds a __long__
conversion function (only enabled on 2.7) which fixes this issue.

The attached test case fails without this patch.

Python 3's `PyInstanceMethod_Type` hides itself via its `tp_descr_get`,
which prevents aliasing methods via `cls.attr("m2") = cls.attr("m1")`:
instead the `tp_descr_get` returns a plain function, when called on a
class, or a `PyMethod`, when called on an instance.  Override that
behaviour for pybind11 types with a special bypass for
`PyInstanceMethod_Types`.

This commits adds base class pointers of offset base classes (i.e. due
to multiple inheritance) to `registered_instances` so that if such a
pointer is returned we properly recognize it as an existing instance.

Without this, returning a base class pointer will cast to the existing
instance if the pointer happens to coincide with the instance pointer,
but constructs a new instance (quite possibly with a segfault, if
ownership is applied) for unequal base class pointers due to multiple
inheritance.

We currently fail at runtime when trying to call a method that is
overloaded with both static and non-static methods.  This is something
python won't allow: the object is either a function or an instance, and
can't be both.

```c++
m.def("foo", foo, py::call_guard<T>());
```

is equivalent to:

```c++
m.def("foo", [](args...) {
    T scope_guard;
    return foo(args...); // forwarded arguments
});
```

* nicer py::capsule destructor mechanism
* added destructor-only version of capsule & tests
* added documentation for module destructors (fixes #733)

Fixes #754.

py::arg() doesn't only specify named arguments anymore, so the error
message was misleading (e.g. when using `py::arg().noconvert()` and
forgetting `py::arg()` for a second positional argument).

This adds brief API documentation for make_iterator/make_key_iterator,
specifically mentioning that it requires InputIterators.

Closes #734.

[skip ci] (no code change here)

We can't support this for classes from imported modules (which is the
primary purpose of a ctor argument base class) because we *have* to
have both parent and derived to properly extract a multiple-inheritance
base class pointer from a derived class pointer.

We could support this for actual `class_<Base, ...> instances, but since
in that case the `Base` is already present in the code, it seems more
consistent to simply always require MI to go via template options.

Fixes #697

The `decltype(...)` in the template parameter that gives us SFINAE
matching for a lambda makes MSVC 2017 ICE; this works around if by
changing the test to an explicit not-a-function-or-pointer test, which
seems to work everywhere.

When using pybind::options to disable function signatures, user-defined
docstrings only get appended if they exist, but newlines were getting
appended unconditionally, so the docstring could end up with blank lines
(depending on which overloads, in particular, provided docstrings).

This commit suppresses the empty lines by only adding newlines for
overloads when needed.

Added in 6fb48490

The second constructor can't be doing anything--the signatures are
exactly the same, and so the first is always going to be the one
invoked by the dispatcher.

* The definition of `PySequence_Fast` is more restrictive on PyPy, so
  use the slow path instead.
* `PyDict_Next` has been fixed in PyPy -> remove workaround.

Eigen::Ref objects, when returned, are almost always returned as
rvalues; what's important is the data they reference, not the outer
shell, and so we want to be able to use `::copy`,
`::reference_internal`, etc. to refer to the data the Eigen::Ref
references (in the following commits), rather than the Eigen::Ref
instance itself.

This moves the policy override into a struct so that code that wants to
avoid it (or wants to provide some other Return-type-conditional
override) can create a specialization of
return_value_policy_override<Return> in order to override the override.

This lets an Eigen::Ref-returning function be bound with `rvp::copy`,
for example, to specify that the data should be copied into a new numpy
array rather than referenced, or `rvp::reference_internal` to indicate
that it should be referenced, but a keep-alive used (actually, we used
the array's `base` rather than a py::keep_alive in such a case, but it
accomplishes the same thing).

With the previous commit, output can be very confusing because you only
see positional arguments in the "invoked with" line, but you can have a
failure from kwargs as well (in particular, when a value is invalidly
specified via both via positional and kwargs).  This commits adds
kwargs to the output, and updates the associated tests to match.

Fixes #688.

My (commented) assumption that such an error is "highly likely to be a
caller mistake" was proven false by #688.

Now that only one shared metaclass is ever allocated, it's extremely
cheap to enable it for all pybind11 types.

* Deprecate the default py::metaclass() since it's not needed anymore.
* Allow users to specify a custom metaclass via py::metaclass(handle).