Uncommenting `#define PYBIND11_USE_SMART_HOLDER_AS_DEFAULT` to run GitHub CI with smart_holder as default. NOT meant for merging.

Systematically replacing `detail::enable_if_t<...smart_holder...>` with `typename std::enable_if<...smart_holder...>::type`. Attempt to work around MSVC 2015 issues, to be tested via GitHub CI. The idea for this change originates from this comment: https://github.com/pybind/pybind11/issues/1616#issuecomment-444536813

Also removing `operator T()` from smart_holder_type_caster, to fix gcc compilation errors. The only loss is pass_rref in test_class_sh_basic.

Using @pytest.mark.parametrize to run each assert separately (to see all errors, not just the first).

Fixing unfortunate editing mishap. This reverts the last remaining test manipulation in commit 249df7cb and makes all existing unit tests pass with smart_holder as default holder.

Removing `operator T&&() &&` from smart_holder_type_caster, for compatibility with the behavior of type_caster_base. Enables reverting 2 of 3 test manipulations applied under commit 249df7cb. The manipulation in test_factory_constructors.py is NOT reverted in this commit.

[skip ci]

Adding copy constructor and move constructor tracking to atyp. Preparation for a follow-up change in smart_holder_type_caster, to make this test sensitive to the changing behavior.

[skip ci]

This version of the code is ASAN clean with unique_ptr or smart_holder as the default.

This change needs to be reverted after adopting the existing move-only-if-refcount-is-1
logic used by type_caster_base.

Adding return_value_policy::move to permissible policies for unique_ptr returns. New stats for all tests combined: 3 failed, 467 passed.

Changing std::shared_ptr pointer/reference to const pointer/reference. New stats for all tests combined: 4 failed, 466 passed.

Adding "Lazy allocation for unallocated values" (for old-style __init__) into load_value_and_holder. Deferring destruction of disowned holder until clear_instance, to remain inspectable for "uninitialized" or "disowned" detection. New stats for all tests combined: 5 failed, 465 passed.

Adding unowned_void_ptr_from_direct_conversion to modified_type_caster_generic_load_impl. This fixes the last remaining segfault (test_numpy_dtypes). New stats for all tests combined: 12 failed, 458 passed.

Bug fix: Adding have_value() to smart_holder_type_caster_load. With this test_builtin_casters succeeds. (All 42 tests build, 36 tests succeed, 5 run but have some failures, 1 segfault.)

Introducing check_is_smart_holder_type_caster() function for runtime check, and reinterpreting record.default_holder as "uses_unique_ptr_holder". With this test_smart_ptr succeeds. (All 42 tests build, 35 tests succeed, 5 run but have some failures, 2 segfault.)

Adding remaining PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS. static_assert for "necessary conditions" for both types of default holder, static_assert for "strict conditions" guarded by new PYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX. All tests build & run as before with unique_ptr as the default holder, all tests build for smart_holder as the default holder, even with the strict static_assert.

Adding PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS in test_class.cpp (with this all but one test succeed with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT).

Softening static_assert, to only check specifically that smart_holder is not mixed with type_caster_base, and unique_ptr/shared_ptr holders are not mixed with smart_holder_type_casters.

Introducing 1. type_caster_for_class_, used in PYBIND11_MAKE_OPAQUE, and 2. default_holder_type, used in stl_bind.h.

Introducing PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS. Using it in test_smart_ptr.cpp. With this test_smart_ptr builds with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT and all but one test run successfully.

Simplifying from_unique_ptr(): typename D = std::default_delete<T> is not needed. Factoring out is_std_default_delete<T>() for consistentcy between ensure_compatible_rtti_uqp_del() and from_unique_ptr().

Factoring out struct and class definitions into anonymous namespace. Preparation for building with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT.

Disabling shared_ptr&, shared_ptr* tests when building with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT for now, pending work on smart_holder_type_caster<shared_ptr>.

Adding enable_if !is_smart_holder_type_caster to existing initimpl::construct(). With this test_factory_constructors.cpp builds with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT.

Adding initimpl::construct() overloads, resulting in test_class_sh_factory_constructors feature parity for py::class_ and py::classh.

Removing include/pybind11/detail/smart_holder_type_casters.h from CMakeLists.txt, test_files.py (since it does not exist in this branch).

Adding test_class_sh_factory_constructors, reproducing test_factory_constructors failure. Using py::class_ in this commit, to be changed to py::classh for debugging.

Commenting out const in def_buffer(... const). With this, test_buffers builds and runs with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT. Explanation why the const needs to be removed, or fix elsewhere, is still needed, but left for later.

With this test_class.cpp builds and even mostly runs, except
`test_multiple_instances_with_same_pointer`, which segfaults because it is
using a `unique_ptr` holder but `smart_holder` `type_caster`.

Also adding `static_assert`s to generate build errors for such situations,
but guarding with `#if 0` to first pivot to test_factory_constructors.cpp.