This variable is a CMake community standard to set the C++
standard of a build. Document it in favor of the previous variable,
which stays as a legacy flag for existing projects.

https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html

With this change, and cast.h as-is in master, test_tagbased_polymorphic.cpp fails to compile with the error message below.
With the cast.h change in pull/2016, building and testing succeeds.

cd pybind11/build/tests && /usr/bin/c++  -DPYBIND11_TEST_BOOST -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -Ipybind11/include -I/usr/include/python3.7m -isystem /usr/include/eigen3  -Os -DNDEBUG -fPIC -fvisibility=hidden   -std=c++2a -flto -fno-fat-lto-objects -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -o CMakeFiles/pybind11_tests.dir/test_tagbased_polymorphic.cpp.o -c pybind11/tests/test_tagbased_polymorphic.cpp
In file included from pybind11/include/pybind11/attr.h:13,
                 from pybind11/include/pybind11/pybind11.h:44,
                 from pybind11/tests/pybind11_tests.h:2,
                 from pybind11/tests/test_tagbased_polymorphic.cpp:10:
pybind11/include/pybind11/cast.h: In instantiation of ‘static std::pair<const void*, const pybind11::detail::type_info*> pybind11::detail::type_caster_base<type>::src_and_type(const itype*) [with type = Animal; pybind11::detail::type_caster_base<type>::itype = Animal]’:
pybind11/include/pybind11/cast.h:906:31:   required from ‘static pybind11::handle pybind11::detail::type_caster_base<type>::cast_holder(const itype*, const void*) [with type = Animal; pybind11::detail::type_caster_base<type>::itype = Animal]’
pybind11/include/pybind11/cast.h:1566:51:   required from ‘static pybind11::handle pybind11::detail::move_only_holder_caster<type, holder_type>::cast(holder_type&&, pybind11::return_value_policy, pybind11::handle) [with type = Animal; holder_type = std::unique_ptr<Animal>]’
pybind11/include/pybind11/stl.h:175:69:   required from ‘static pybind11::handle pybind11::detail::list_caster<Type, Value>::cast(T&&, pybind11::return_value_policy, pybind11::handle) [with T = std::vector<std::unique_ptr<Animal> >; Type = std::vector<std::unique_ptr<Animal> >; Value = std::unique_ptr<Animal>]’
pybind11/include/pybind11/pybind11.h:159:43:   required from ‘void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = std::vector<std::unique_ptr<Animal> > (*&)(); Return = std::vector<std::unique_ptr<Animal> >; Args = {}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}]’
pybind11/include/pybind11/pybind11.h:64:9:   required from ‘pybind11::cpp_function::cpp_function(Return (*)(Args ...), const Extra& ...) [with Return = std::vector<std::unique_ptr<Animal> >; Args = {}; Extra = {pybind11::name, pybind11::scope, pybind11::sibling}]’
pybind11/include/pybind11/pybind11.h:819:22:   required from ‘pybind11::module& pybind11::module::def(const char*, Func&&, const Extra& ...) [with Func = std::vector<std::unique_ptr<Animal> > (*)(); Extra = {}]’
pybind11/tests/test_tagbased_polymorphic.cpp:141:36:   required from here
pybind11/include/pybind11/cast.h:880:61: error: ambiguous template instantiation for ‘struct pybind11::polymorphic_type_hook<Animal, void>’
         const void *vsrc = polymorphic_type_hook<itype>::get(src, instance_type);
                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
pybind11/include/pybind11/cast.h:844:8: note: candidates are: ‘template<class itype> struct pybind11::polymorphic_type_hook<itype, typename std::enable_if<std::is_polymorphic<_Tp>::value, void>::type> [with itype = Animal]’
 struct polymorphic_type_hook<itype, detail::enable_if_t<std::is_polymorphic<itype>::value>>
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pybind11/tests/test_tagbased_polymorphic.cpp:115:12: note:                 ‘template<class itype> struct pybind11::polymorphic_type_hook<itype, typename std::enable_if<std::is_base_of<Animal, itype>::value, void>::type> [with itype = Animal]’
     struct polymorphic_type_hook<itype, detail::enable_if_t<std::is_base_of<Animal, itype>::value>>
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from pybind11/include/pybind11/attr.h:13,
                 from pybind11/include/pybind11/pybind11.h:44,
                 from pybind11/tests/pybind11_tests.h:2,
                 from pybind11/tests/test_tagbased_polymorphic.cpp:10:
pybind11/include/pybind11/cast.h:880:61: error: incomplete type ‘pybind11::polymorphic_type_hook<Animal, void>’ used in nested name specifier
         const void *vsrc = polymorphic_type_hook<itype>::get(src, instance_type);
                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~

Currently user specializations of the form

template <typename itype> struct polymorphic_type_hook<itype, std::enable_if_t<...>> { ... };

will fail if itype is also polymorphic, because the existing specialization will also
be enabled, which leads to 2 equally viable candidates. With this change, user provided
specializations have higher priority than the built in specialization for polymorphic types.

When binding code immediately throws an exception of type
py::error_already_set (e.g. via py::module::import that fails), the
catch block sets an import error as expected. Unfortunately, following
this, the deconstructor of py::error_already_set decides to call
py::detail::get_internals() and set up various internal data structures
of pybind11, which fails given that the error flag is active. The call
stack of this looks as follows:

Py_init_mymodule() -> __cxa_decrement_exception_refcount ->
error_already_set::~error_already_set() ->
gil_scoped_acquire::gil_scoped_acquire() -> detail::get_internals() ->
... -> pybind11::detail::simple_collector() -> uh oh..

The solution is simple: we call detail::get_internals() once before
running any binding code to make sure that the internal data structures
are ready.

This commit introduces the use of C++17-style fold expressions when
casting tuples & the argument lists of functions.

This change can improve performance of the resulting bindings: because
fold expressions have short-circuiting semantics, pybind11 e.g. won't
try to cast the second argument of a function if the first one failed.
This is particularly effective when working with functions that have
many overloads with long argument lists.

* Fix test build in C++20

* Add C++20 char8_t/u8string support

If a debug interpreter is detected, we allow linking with
pythonXX_d.lib under windows.

This reverts commit 819802da.

This reverts commit 61e4f118.

* Add FAQ entry, with code example, for dealing with long functions interruption

Fixing SystemError when nb_bool/nb_nonzero sets a Python exception in type_caster<bool>::load (#1976)

Add Python 3.8 to considered versions in CMake for additional
hints.
https://cmake.org/cmake/help/v3.2/module/FindPythonLibs.html

* test pair-copyability on C++17 upwards

The stdlib falsely detects containers like M=std::map<T, U>
as copyable, even when one of T and U is not copyable.
Therefore we cannot rely on the stdlib dismissing std::pair<T, M>
by itself, even on C++17.

* fix is_copy_assignable

bind_map used std::is_copy_assignable which suffers from the same problems
as std::is_copy_constructible, therefore the same fix has been applied.

* created tests for copyability

Don't assume that just because the language version is C++17 that the
standard library offers all C++17 features, too.  When using clang-6.0
and --std=c++17 on Ubuntu 18.04 with libstdc++, __cpp_sized_deallocation
is false.