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PR #880 changed the implementation of keep_alive to avoid weak
references when the nurse is pybind11-registered, but the documentation
didn't get updated to match.

[skip ci]

:exc: isn't valid.

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.

```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
});
```

This commit largely rewrites the Eigen dense matrix support to avoid
copying in many cases: Eigen arguments can now reference numpy data, and
numpy objects can now reference Eigen data (given compatible types).

Eigen::Ref<...> arguments now also make use of the new `convert`
argument use (added in PR #634) to avoid conversion, allowing
`py::arg().noconvert()` to be used when binding a function to prohibit
copying when invoking the function.  Respecting `convert` also means
Eigen overloads that avoid copying will be preferred during overload
resolution to ones that require copying.

This commit also rewrites the Eigen documentation and test suite to
explain and test the new capabilities.

* changed return_value:: to return_value_policy::

* Update functions.rst

This changes the function dispatching code for overloaded functions into
a two-pass procedure where we first try all overloads with
`convert=false` for all arguments.  If no function calls succeeds in the
first pass, we then try a second pass where we allow arguments to have
`convert=true` (unless, of course, the argument was explicitly specified
with `py::arg().noconvert()`).

For non-overloaded methods, the two-pass procedure is skipped (we just
make the overload-allowed call).  The second pass is also skipped if it
would result in the same thing (i.e. where all arguments are
`.noconvert()` arguments).

This adds support for controlling the `convert` flag of arguments
through the py::arg annotation.  This then allows arguments to be
flagged as non-converting, which the type_caster is able to use to
request different behaviour.

Currently, AFAICS `convert` is only used for type converters of regular
pybind11-registered types; all of the other core type_casters ignore it.
We can, however, repurpose it to control internal conversion of
converters like Eigen and `array`: most usefully to give callers a way
to disable the conversion that would otherwise occur when a
`Eigen::Ref<const Eigen::Matrix>` argument is passed a numpy array that
requires conversion (either because it has an incompatible stride or the
wrong dtype).

Specifying a noconvert looks like one of these:

    m.def("f1", &f, "a"_a.noconvert() = "default"); // Named, default, noconvert
    m.def("f2", &f, "a"_a.noconvert()); // Named, no default, no converting
    m.def("f3", &f, py::arg().noconvert()); // Unnamed, no default, no converting

(The last part--being able to declare a py::arg without a name--is new:
previous py::arg() only accepted named keyword arguments).

Such an non-convert argument is then passed `convert = false` by the
type caster when loading the argument.  Whether this has an effect is up
to the type caster itself, but as mentioned above, this would be
extremely helpful for the Eigen support to give a nicer way to specify
a "no-copy" mode than the custom wrapper in the current PR, and
moreover isn't an Eigen-specific hack.

This commit rewrites the function dispatcher code to support mixing
regular arguments with py::args/py::kwargs arguments.  It also
simplifies the argument loader noticeably as it no longer has to worry
about args/kwargs: all of that is now sorted out in the dispatcher,
which now simply appends a tuple/dict if the function takes
py::args/py::kwargs, then passes all the arguments in a vector.

When the argument loader hit a py::args or py::kwargs, it doesn't do
anything special: it just calls the appropriate type_caster just like it
does for any other argument (thus removing the previous special cases
for args/kwargs).

Switching to passing arguments in a single std::vector instead of a pair
of tuples also makes things simpler, both in the dispatch and the
argument_loader: since this argument list is strictly pybind-internal
(i.e. it never goes to Python) we have no particular reason to use a
Python tuple here.

Some (intentional) restrictions:
- you may not bind a function that has args/kwargs somewhere other than
  the end (this somewhat matches Python, and keeps the dispatch code a
  little cleaner by being able to not worry about where to inject the
  args/kwargs in the argument list).
- If you specify an argument both positionally and via a keyword
  argument, you get a TypeError alerting you to this (as you do in
  Python).

* Make 'any' the default markup role for Sphinx docs

* Automate generation of reference docs with doxygen and breathe

* Improve reference docs coverage

* Some clarifications to section on virtual fns

Primarily, I made it clear that PYBIND11_OVERLOAD_PURE_NAME is not "useful" but required in renaming situations. Also clarified that one should not bind to the trampoline helper class which I found tempting since it seems more explicit.

* Remove :emphasize-lines: from cpp block, seems to suppress formatting

* docs: emphasize default policy, clarify keep_alive

Emphasize the default return value policy since this statement is hidden in a wall of text. 

Add a hint that call policies are probably required for container objects.

* [Doc] Fix several errors of examples from the doc

* Add missing operator def.

* Added missing `()`

* Add missing `namespace`.

* 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)`.