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The previous commit to address #392 triggers a compiler warning about
returning a reference to a local variable, which is *not* a false alarm:
the following:

    py::cast<int &>(o)

(which happens internally in an overload declaration) really is
returning a reference to a local, because the cast operators for the
type_caster for numeric types returns a reference to its own member.

This commit adds a static_assert to make that a compilation failure
rather than returning a reference into about-to-be-freed memory.

Incidentally, this is also a fix for #219, which is exactly the same
issue: we can't reference numeric primitives that are cast from
wrappers around python numeric types.

Need to use the intrinsic type, not the raw type.

Fixes #392.

Allow arbitrary class_ template option ordering

Replace std::cout with py::print in tests

This allows a slightly cleaner base type specification of:

    py::class_<Type, Base>("Type")

as an alternative to

    py::class_<Type>("Type", py::base<Base>())

As with the other template parameters, the order relative to the holder
or trampoline types doesn't matter.

This also includes a compile-time assertion failure if attempting to
specify more than one base class (but is easily extendible to support
multiple inheritance, someday, by updating the class_selector::set_bases
function to set multiple bases).

With this change both C++ and Python write to sys.stdout which resolves
the capture issues noted in #351. Therefore, the related workarounds are
removed.

The current pybind11::class_<Type, Holder, Trampoline> fixed template
ordering results in a requirement to repeat the Holder with its default
value (std::unique_ptr<Type>) argument, which is a little bit annoying:
it needs to be specified not because we want to override the default,
but rather because we need to specify the third argument.

This commit removes this limitation by making the class_ template take
the type name plus a parameter pack of options.  It then extracts the
first valid holder type and the first subclass type for holder_type and
trampoline type_alias, respectively.  (If unfound, both fall back to
their current defaults, `std::unique_ptr<type>` and `type`,
respectively).  If any unmatched template arguments are provided, a
static assertion fails.

What this means is that you can specify or omit the arguments in any
order:

    py::class_<A, PyA> c1(m, "A");
    py::class_<B, PyB, std::shared_ptr<B>> c2(m, "B");
    py::class_<C, std::shared_ptr<C>, PyB> c3(m, "C");

It also allows future class attributes (such as base types in the next
commit) to be passed as class template types rather than needing to use
a py::base<> wrapper.

Keyword arguments and generalized unpacking for C++ API

With this change arg_t is no longer a template, but it must remain so
for backward compatibility. Thus, a non-template arg_v is introduced,
while a dummy template alias arg_t is there to keep old code from
breaking. This can be remove in the next major release.

The implementation of arg_v also needed to be placed a little earlier in
the headers because it's not a template any more and unpacking_collector
needs more than a forward declaration.

MSVC fails to compile if the constructor is defined out-of-line.
The error states that it cannot deduce the type of the default template
parameter which is used for SFINAE.

The variadic handle::operator() offers the same functionality as well
as mixed positional, keyword, * and ** arguments. The tests are also
superseded by the ones in `test_callbacks`.

Replicates Python API including keyword arguments.

A Python function can be called with the syntax:
```python
foo(a1, a2, *args, ka=1, kb=2, **kwargs)
```
This commit adds support for the equivalent syntax in C++:
```c++
foo(a1, a2, *args, "ka"_a=1, "kb"_a=2, **kwargs)
```

In addition, generalized unpacking is implemented, as per PEP 448,
which allows calls with multiple * and ** unpacking:
```python
bar(*args1, 99, *args2, 101, **kwargs1, kz=200, **kwargs2)
```
and
```c++
bar(*args1, 99, *args2, 101, **kwargs1, "kz"_a=200, **kwargs2)
```

Adding bind_map. Adding key_error exception.

enum serialization support (fixes #380)

Make unique_ptr's with non-default deleters work

Currently pybind11 only supports std::unique_ptr<T> holders by default
(other holders can, of course, be declared using the macro).  PR #368
added a `py::nodelete` that is intended to be used as:

    py::class_<Type, std::unique_ptr<Type, py::nodelete>> c("Type");

but this doesn't work out of the box.  (You could add an explicit
holder type declaration, but this doesn't appear to have been the
intention of the commit).

This commit fixes it by generalizing the unique_ptr type_caster to take
both the type and deleter as template arguments, so that *any*
unique_ptr instances are now automatically handled by pybind.  It also
adds a test to test_smart_ptr, testing both that py::nodelete (now)
works, and that the object is indeed not deleted as intended.

Buffer info improvements

Make test initialization self-registering

Adding or removing tests is a little bit cumbersome currently: the test
needs to be added to CMakeLists.txt, the init function needs to be
predeclared in pybind11_tests.cpp, then called in the plugin
initialization.  While this isn't a big deal for tests that are being
committed, it's more of a hassle when working on some new feature or
test code for which I temporarily only care about building and linking
the test being worked on rather than the entire test suite.

This commit changes tests to self-register their initialization by
having each test initialize a local object (which stores the
initialization function in a static variable).  This makes changing the
set of tests being build easy: one only needs to add or comment out
test names in tests/CMakeLists.txt.

A couple other minor changes that go along with this:

- test_eigen.cpp is now included in the test list, then removed if eigen
  isn't available.  This lets you disable the eigen tests by commenting
  it out, just like all the other tests, but keeps the build working
  without eigen eigen isn't available.  (Also, if it's commented out, we
  don't even bother looking for and reporting the building with/without
  eigen status message).

- pytest is now invoked with all the built test names (with .cpp changed
  to .py) so that it doesn't try to run tests that weren't built.