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When compiling in C++17 mode the noexcept specifier is part of the
function type.  This causes a failure in pybind11 because, by omitting
a noexcept specifier when deducing function return and argument types,
we are implicitly making `noexcept(false)` part of the type.

This means that functions with `noexcept` fail to match the function
templates in cpp_function (and other places), and we get compilation
failure (we end up trying to fit it into the lambda function version,
which fails since a function pointer has no `operator()`).

We can, however, deduce the true/false `B` in noexcept(B), so we don't
need to add a whole other set of overloads, but need to deduce the extra
argument when under C++17.  That will *not* work under pre-C++17,
however.

This commit adds two macros to fix the problem: under C++17 (with the
appropriate feature macro set) they provide an extra `bool NoExceptions`
template argument and provide the `noexcept(NoExceptions)` deduced
specifier.  Under pre-C++17 they expand to nothing.

This is needed to compile pybind11 with gcc7 under -std=c++17.

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.

-Wint-in-bool-context triggers many warnings when compiling eigen code,
so disable it locally in eigen.h.

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).

* Added ternary support with descr args

Current the `_<bool>(a, b)` ternary support only works for `char[]` `a`
and `b`; this commit allows it to work for `descr` `a` and `b` arguments
as well.

* Add support for std::valarray to stl.h

This abstracts the std::array into a `array_caster` which can then be
used with either std::array or std::valarray, the main difference being
that std::valarray is resizable.  (It also lets the array_caster be
potentially used for other std::array-like interfaces, much as the
list_caster and map_caster currently provide).

* Small stl.h cleanups

- Remove redundant `type` typedefs
- make internal list_caster methods private

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.

stl casters were using a value cast to (Value) or (Key), but that isn't
always appropriate.  This changes it to use the appropriate value
converter's cast_op_type.

C++ exceptions are destructed in the context of the code that catches
them. At this point, the Python GIL may not be held, which could lead
to crashes with the previous implementation.

PyErr_Fetch and PyErr_Restore should always occur in pairs, which was
not the case for the previous implementation. To clear the exception,
the new approach uses PyErr_Restore && PyErr_Clear instead of simply
decreasing the reference counts of the exception objects.

Previously all types are marked unaligned in buffer format strings,
now we test for alignment before adding the '=' marker.

This gives more informative output, often including the type (or at
least some hint about the type).

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.

Following commit 90d278, the object code generated by the python
bindings of nanogui (github.com/wjakob/nanogui) went up by a whopping
12%. It turns out that that project has quite a few enums where we don't
really care about arithmetic operators.

This commit thus partially reverts the effects of #503 by introducing
an additional attribute py::arithmetic() that must be specified if the
arithmetic operators are desired.

* `array_t(const object &)` now throws on error
* `array_t::ensure()` is intended for casters —- old constructor is
  deprecated
* `array` and `array_t` get default constructors (empty array)
* `array` gets a converting constructor
* `py::isinstance<array_T<T>>()` checks the type (but not flags)

There is only one special thing which must remain: `array_t` gets
its own `type_caster` specialization which uses `ensure` instead
of a simple check.

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
});
```

* Also added unsafe version without checks

* Allow enums to be ordered
* Support binary operators

* Incref returned None in std::optional type caster

* Add type casters for nullopt_t

* Add a test for nullopt_t

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.

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 the returned class_ object), but this isn't
something stl_bind can handle because the C++ code to construct values
is going to be highly dependent on the container value_type.

* stl_bind: copy only for arithmetic value types

For non-primitive types, we may well be copying some complex type, when
returning by reference is more appropriate.  This commit returns by
internal reference for all but basic arithmetic types.

* Return by reference whenever possible

Only if we definitely can't--i.e. std::vector<bool>--because v[i]
returns something that isn't a T& do we copy; for everything else, we
return by reference.

For the map case, we can always return by reference (at least for the
default stl map/unordered_map).