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* fix: the types for return_value_policy_override in optional_caster

`return_value_policy_override` was not being applied correctly in
`optional_caster` in two ways:
- The `is_lvalue_reference` condition referenced `T`, which was the
`optional<T>` type parameter from the class, when it should have used `T_`,
which was the parameter to the `cast` function. `T_` can potentially be a
reference type, but `T` will never be.
- The type parameter passed to `return_value_policy_override` should be
`T::value_type`, not `T`. This matches the way that the other STL container
type casters work.

The result of these issues was that a method/property definition which used a
`reference` or `reference_internal` return value policy would create a Python
value that's bound by reference to a temporary C++ object, resulting in
undefined behavior. For reasons that I was not able to figure out fully, it
seems like this causes problems when using old versions of `boost::optional`,
but not with recent versions of `boost::optional` or the `libstdc++`
implementation of `std::optional`. The issue (that the override to
`return_value_policy::move` is never being applied) is present for all
implementations, it just seems like that somehow doesn't result in problems for
the some implementation of `optional`. This change includes a regression type
with a custom optional-like type which was able to reproduce the issue.

Part of the issue with using the wrong types may have stemmed from the type
variables `T` and `T_` having very similar names. This also changes the type
variables in `optional_caster` to use slightly more descriptive names, which
also more closely follow the naming convention used by the other STL casters.

Fixes #3330

* Fix clang-tidy complaints

* Add missing NOLINT

* Apply a couple more fixes

* fix: support GCC 4.8

* tests: avoid warning about unknown compiler for compilers missing C++17

* Remove unneeded test module attribute

* Change test enum to have more unique int values
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

* Adding google-explicit-constructor to .clang-tidy

* clang-tidy explicit attr.h (all automatic)

* clang-tidy explicit cast.h (all automatic)

* clang-tidy detail/init.h (1 NOLINT)

* clang-tidy detail/type_caster_base.h (2 NOLINT)

* clang-tidy pybind11.h (7 NOLINT)

* clang-tidy detail/common.h (3 NOLINT)

* clang-tidy detail/descr.h (2 NOLINT)

* clang-tidy pytypes.h (23 NOLINT, only 1 explicit)

* clang-tidy eigen.h (7 NOLINT, 0 explicit)

* Adding 2 explicit in functional.h

* Adding 4 explicit in iostream.h

* clang-tidy numpy.h (1 NOLINT, 1 explicit)

* clang-tidy embed.h (0 NOLINT, 1 explicit)

* clang-tidy tests/local_bindings.h (0 NOLINT, 4 explicit)

* clang-tidy tests/pybind11_cross_module_tests.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/pybind11_tests.h (0 NOLINT, 2 explicit)

* clang-tidy tests/test_buffers.cpp (0 NOLINT, 2 explicit)

* clang-tidy tests/test_builtin_casters.cpp (0 NOLINT, 4 explicit)

* clang-tidy tests/test_class.cpp (0 NOLINT, 6 explicit)

* clang-tidy tests/test_copy_move.cpp (0 NOLINT, 7 explicit)

* clang-tidy tests/test_embed/external_module.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/test_embed/test_interpreter.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/object.h (0 NOLINT, 2 explicit)

* clang-tidy batch of fully automatic fixes.

* Workaround for MSVC 19.16.27045.0 C++17 Python 2 C++ syntax error.

* maint(clang-tidy) Improve code readability

* Fix minor typos

* Revert optimization that removed test case

* Fix comment formatting

* Revert another optimization to repro an issue

* Remove make_unique since it C++14 and newer only

* eformat comments

* Fix unsignedness of comparison

* Update comment

* Adding test_return_vector_bool_raw_ptr to test_stl.py.

* First attempt to make the documentation more accurate, but not trying to be comprehensive, to not bloat the reference table with too many details.

* Fixing minor oversights.

* Applying reviewer suggestion.

* Copying from prework_no_rst branch (PR #3087): test_numpy_array.cpp, test_stl.cpp

* Manual changes reducing NOLINTs.

* clang-format-diff.py

* Minor adjustment to avoid MSVC warning C4702: unreachable code

* Splitting out pybind11/stl/filesystem.h.

To solve breakages like: https://github.com/deepmind/open_spiel/runs/2999582108

Mostly following the suggestion here: https://github.com/pybind/pybind11/pull/2730#issuecomment-750507575

Except using pybind11/stl/filesystem.h instead of pybind11/stlfs.h, as decided via chat.

stl.h restored to the exact state before merging PR #2730 via:
```
git checkout 733f8de2 stl.h
```

* Properly including new stl subdirectory in pip wheel config.

This now passes interactively:
```
pytest tests/extra_python_package/
```

* iwyu cleanup.

iwyuh.py -c -std=c++17 -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/include/python3.9 -I/usr/include/eigen3 include/pybind11/stl/filesystem.h

* Adding PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL.

* Eliminating else after return.

* Initial fixes

* Whoops

* Finish clang-tidy manual fixes

* Add two missing fixes

* Revert

* Update clang-tidy

* Try to fix unreachable code error

* Move nolint comment

* Apply missing fix

* Don't override clang-tidy config

* Does this fix clang-tidy?

* Make all clang-tidy errors visible

* Add comments about NOLINTs and remove a few

* Fix typo

The variables PYBIND11_HAS_OPTIONAL, PYBIND11_HAS_EXP_OPTIONAL, PYBIND11_HAS_VARIANT,
__clang__, __APPLE__ were not checked for defined in a minortity of instances.

If the project using pybind11 sets -Wundef, the warnings will show.

The test build is also modified to catch the problem.

* Adds std::deque to the types supported by list_caster in stl.h.
* Adds a new test_deque test in test_stl.{py,cpp}.
* Updates the documentation to include std::deque as a default
  supported type.

This PR brings the std::array<> caster in sync with the other STL type
casters: to accept an arbitrary sequence as input (rather than a list,
which is too restrictive).

* Fix for Issue #1258

list_caster::load method will now check for a Python string and prevent its automatic conversion to a list.
This should fix the issue "pybind11/stl.h converts string to vector<string> #1258" (https://github.com/pybind/pybind11/issues/1258)

* Added tests for fix of issue #1258

* Changelog: stl string auto-conversion

* stl.h: propagate return value policies to type-specific casters

Return value policies for containers like those handled in in 'stl.h'
are currently broken.

The problem is that detail::return_value_policy_override<C>::policy()
always returns 'move' when given a non-pointer/reference type, e.g.
'std::vector<...>'.

This is sensible behavior for custom types that are exposed via
'py::class_<>', but it does not make sense for types that are handled by
other type casters (STL containers, Eigen matrices, etc.).

This commit changes the behavior so that
detail::return_value_policy_override only becomes active when the type
caster derives from type_caster_generic.

Furthermore, the override logic is called recursively in STL type
casters to enable key/value-specific behavior.

To avoid an ODR violation in the test suite while testing
both `stl.h` and `std_bind.h` with `std::vector<bool>`,
the `py::bind_vector<std::vector<bool>>` test is moved to
the secondary module (which does not include `stl.h`).

In C++11 mode, `boost::apply_visitor` requires an explicit `result_type`.
This also adds optional tests for `boost::variant` in C++11/14, if boost
is available. In C++17 mode, `std::variant` is tested instead.

This udpates all the remaining tests to the new test suite code and
comment styles started in #898.  For the most part, the test coverage
here is unchanged, with a few minor exceptions as noted below.

- test_constants_and_functions: this adds more overload tests with
  overloads with different number of arguments for more comprehensive
  overload_cast testing.  The test style conversion broke the overload
  tests under MSVC 2015, prompting the additional tests while looking
  for a workaround.

- test_eigen: this dropped the unused functions `get_cm_corners` and
  `get_cm_corners_const`--these same tests were duplicates of the same
  things provided (and used) via ReturnTester methods.

- test_opaque_types: this test had a hidden dependence on ExampleMandA
  which is now fixed by using the global UserType which suffices for the
  relevant test.

- test_methods_and_attributes: this required some additions to UserType
  to make it usable as a replacement for the test's previous SimpleType:
  UserType gained a value mutator, and the `value` property is not
  mutable (it was previously readonly).  Some overload tests were also
  added to better test overload_cast (as described above).

- test_numpy_array: removed the untemplated mutate_data/mutate_data_t:
  the templated versions with an empty parameter pack expand to the same
  thing.

- test_stl: this was already mostly in the new style; this just tweaks
  things a bit, localizing a class, and adding some missing
  `// test_whatever` comments.

- test_virtual_functions: like `test_stl`, this was mostly in the new
  test style already, but needed some `// test_whatever` comments.
  This commit also moves the inherited virtual example code to the end
  of the file, after the main set of tests (since it is less important
  than the other tests, and rather length); it also got renamed to
  `test_inherited_virtuals` (from `test_inheriting_repeat`) because it
  tests both inherited virtual approaches, not just the repeat approach.

PR #936 broke the ability to return a pointer to a stl container (and,
likewise, to a tuple) because the added deduced type matched a
non-const pointer argument: the pointer-accepting `cast` in
PYBIND11_TYPE_CASTER had a `const type *`, which is a worse match for a
non-const pointer than the universal reference template #936 added.

This changes the provided TYPE_CASTER cast(ptr) to take the pointer by
template arg (so that it will accept either const or non-const pointer).
It has two other effects: it slightly reduces .so size (because many
type casters never actually need the pointer cast at all), and it allows
type casters to provide their untemplated pointer `cast()` that will
take precedence over the templated version provided in the macro.

This updates the std::tuple, std::pair and `stl.h` type casters to
forward their contained value according to whether the container being
cast is an lvalue or rvalue reference.  This fixes an issue where
subcaster casts were always called with a const lvalue which meant
nested type casters didn't have the desired `cast()` overload invoked.
For example, this caused Eigen values in a tuple to end up with a
readonly flag (issue #935) and made it impossible to return a container
of move-only types (issue #853).

This fixes both issues by adding templated universal reference `cast()`
methods to the various container types that forward container elements
according to the container reference type.

`nullptr` is not expected to work in this case.