Adding py::smart_holder (for smart-pointer interoperability). (#2672)

* Adding test_unique_ptr_member (for desired PyCLIF behavior).

See also: #2583

Does not build with upstream master or
#2047, but builds with
https://github.com/RobotLocomotion/pybind11 and almost runs:

```
Running tests in directory "/usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests":
================================================================================= test session starts =================================================================================
platform linux -- Python 3.8.5, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests, inifile: pytest.ini
collected 2 items

test_unique_ptr_member.py .F                                                                                                                                                    [100%]

====================================================================================== FAILURES =======================================================================================
_____________________________________________________________________________ test_pointee_and_ptr_owner ______________________________________________________________________________

    def test_pointee_and_ptr_owner():
        obj = m.pointee()
        assert obj.get_int() == 213
        m.ptr_owner(obj)
        with pytest.raises(ValueError) as exc_info:
>           obj.get_int()
E           Failed: DID NOT RAISE <class 'ValueError'>

test_unique_ptr_member.py:17: Failed
============================================================================= 1 failed, 1 passed in 0.06s =============================================================================
```

* unique_ptr or shared_ptr return

* new test_variant_unique_shared with vptr_holder prototype

* moving prototype code to pybind11/vptr_holder.h, adding type_caster specialization to make the bindings involving unique_ptr passing compile, but load and cast implementations are missing

* disabling GitHub Actions on pull_request (for this PR)

* disabling AppVeyor (for this PR)

* TRIGGER_SEGSEV macro, annotations for GET_STACK (vptr::get), GET_INT_STACK (pointee)

* adding test_promotion_of_disowned_to_shared

* Copying tests as-is from xxx_value_ptr_xxx_holder branch.

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising returning and passing unique_ptr<T>, shared_ptr<T>
with unique_ptr, shared_ptr holder.

Observations:

test_holder_unique_ptr:
  make_unique_pointee  OK
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  Abort free(): double free detected
  pass_shared_pointee  RuntimeError: Unable to load a custom holder type from a default-holder instance

test_holder_shared_ptr:
  make_unique_pointee  Segmentation fault (#1138)
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  OK
  pass_shared_pointee  OK

* Copying tests as-is from xxx_value_ptr_xxx_holder branch.

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising casting between shared_ptr<base>, shared_ptr<derived>.

* Demonstration of Undefined Behavior in handling of shared_ptr holder.

Based on https://godbolt.org/z/4fdjaW by jorgbrown@ (thanks Jorg!).

* Additional demonstration of Undefined Behavior in handling of shared_ptr holder.

* fixing up-down mixup in comment

* Demonstration of Undefined Behavior in handling of polymorphic pointers.

(This demo does NOT involve smart pointers at all, unlike the otherwise similar test_smart_ptr_private_first_base.)

* minor test_private_first_base.cpp simplification (after discovering that this can be wrapped with Boost.Python, using boost::noncopyable)

* pybind11 equivalent of Boost.Python test similar to reproducer under #1333

* Snapshot of WIP, TODO: shared_ptr deleter with on/off switch

* Adding vptr_deleter.

* Adding from/as unique_ptr<T> and unique_ptr<T, D>.

* Adding from_shared_ptr. Some polishing.

* New tests/core/smart_holder_poc_test.cpp, using Catch2.

* Adding in vptr_deleter_guard_flag.

* Improved labeling of TEST_CASEs.

* Shuffling existing TEST_CASEs into systematic matrix.

* Implementing all [S]uccess tests.

* Implementing all [E]xception tests.

* Testing of exceptions not covered by the from-as matrix.

* Adding top-level comment.

* Converting from methods to factory functions (no functional change).

* Removing obsolete and very incomplete test (replaced by Catch2-based test).

* Removing stray file.

* Adding type_caster_bare_interface_demo.

* Adding shared_ptr<mpty>, shared_ptr<mpty const> casters.

* Adding unique_ptr<mpty>, unique_ptr<mpty const> casters.

* Pure copy of `class class_` implementation in pybind11.h (master commit 98f1bbb).

* classh.h: renaming of class_ to classh + namespace; forking test_classh_wip from test_type_caster_bare_interface_demo.

* Hard-coding smart_holder into classh.

* Adding mpty::mtxt string member.

* Adding isinstance<mpty> in type_caster::load functions.

* Adding rvalue_ref, renaming const_value_ref to lvalue_ref & removing const.

* Retrieving smart_holder pointer in type_caster<mpty>::load, and using it cast_op operators.

* Factoring out smart_holder_type_caster_load.

* Retrieving smart_holder pointer in type_caster<std::shared_ptr<mpty[ const]>>::load, and using it cast_op operators.

* Improved error messaging: Cannot disown nullptr (as_unique_ptr).

* Retrieving smart_holder pointer in type_caster<std::unique_ptr<mpty[ const]>>::load, and using it cast_op operators.

* Pure `clang-format --style=file -i` change.

* Pure `clang-format --style=file -i` change, with two `clang-format off` directives.

* Fixing oversight (discovered by flake8).

* flake8 cleanup

* Systematically setting mtxt for all rtrn_mpty_* functions (preparation, the values are not actually used yet).

* static cast handle for rtrn_cptr works by simply dropping in code from type_caster_base (marked with comments).

* static cast handle for rtrn_cref works by simply dropping in code from type_caster_base (marked with comments). rtrn_mref and rtrn_mptr work via const_cast (to add const).

* static cast handle for rtrn_valu works by simply dropping in code from type_caster_base (marked with comments). rtrn_rref raises a RuntimeError, to be investigated.

* Copying type_caster_generic::cast into type_caster<mpty> as-is (preparation for handling smart pointers).

* Pure clang-format change (applied to original type_caster_generic::cast).

* Adding comment re potential use_count data race.

* static handle cast implementations for rtrn_shmp, rtrn_shcp.

* Adding MISSING comments in operator std::unique_ptr<mpty[ const]>.

* static handle cast implementations for rtrn_uqmp, rtrn_uqcp.

* Bug fix: vptr_deleter_armed_flag_ptr has to live on the heap.

See new bullet point in comment section near the top.

The variable was also renamed to reflect its function more accurately.

* Fixing bugs discovered by ASAN. The code is now ASAN, MSAN, UBSAN clean.

* Making test_type_caster_bare_interface_demo.cpp slightly more realistic, ASAN, MSAN, UBSAN clean.

* Calling deregister_instance after disowning via unique_ptr.

* Removing enable_shared_from_this stub, simplifying existing code, clang-format.

Open question, with respect to the original code:
https://github.com/pybind/pybind11/blob/76a160070b369f8d82b945c97924227e8b835c94/include/pybind11/pybind11.h#L1510
To me it looks like the exact situation marked as `std::shared_ptr<Good> gp1 = not_so_good.getptr();` here: https://en.cppreference.com/w/cpp/memory/enable_shared_from_this
The comment there is: `// undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)`
Does the existing code have UB pre C++17?

I'll leave handling of enable_shared_from_this for later, as the need arises.

* Cosmetical change around helper functions.

* Using type_caster_base<mpty>::src_and_type directly, removing copy. Also renaming one cast to cast_const_raw_ptr, for clarity.

* Fixing clang-format oversight.

* Using factored-out make_constructor (PR #2798), removing duplicate code.

* Inserting additional assert to ensure a returned unique_ptr is always a new Python instance.

* Adding minor comment (change to internals needed to distinguish uninitialized/disowned in error message).

* Factoring out find_existing_python_instance().

* Moving factored-out make_constructor to test_classh_wip.cpp, restoring previous version of cast.h. This is currently the most practical approach. See PR #2798 for background.

* Copying classh type_casters from test_classh_wip.cpp UNMODIFIED, as a baseline for generalizing the code.

* Using pybind11/detail/classh_type_casters.h from test_classh_wip.cpp.

* Adding & using PYBIND11_CLASSH_TYPE_CASTERS define.

* Adding test_classh_inheritance, currently failing (passes with class_).

* Removing .clang-format before git rebase master (where the file was added).

* Bringing back .clang-format, the previous rm was a bad idea.

* Folding in modified_type_caster_generic_load_impl, just enough to pass test_class_wip. test_classh_inheritance is still failing, but with a different error: [RuntimeError: Incompatible type (as_raw_ptr_unowned).]

* Minimal changes needed to pass test_classh_inheritance.

* First pass adjusting try_implicit_casts and try_load_foreign_module_local to capture loaded_v_h, but untested and guarded with pybind11_failure("Untested"). This was done mainly to determine general feasibility. Note the TODO in pybind11.h, where type_caster_generic::local_load is currently hard-coded. test_classh_wip and test_classh_inheritance still pass, as before.

* Decoupling generic_type from type_caster_generic.

* Changes and tests covering classh_type_casters try_implicit_casts.

* Minimal test covering classh_type_casters load_impl Case 2b.

* Removing stray isinstance<T>(src): it interferes with the py::module_local feature. Adding missing #includes.

* Tests for classh py::module_local() feature.

* Pure renaming of function names in test_classh_inheritance, similar to the systematic approach used in test_class_wip. NO functional changes.

* Pure renaming of function and variable names, for better generalization when convoluting with inheritance. NO functional changes.

* Adopting systematic naming scheme from test_classh_wip. NO functional changes.

* Moving const after type name, for functions that cover a systematic scheme. NO functional changes.

* Adding smart_holder_type_caster_load::loaded_as_shared_ptr, currently bypassing smart_holder shared_ptr tracking completely, but the tests pass and are sanitizer clean.

* Removing rtti_held from smart_holder. See updated comment.

* Cleaning up loaded_as_raw_ptr_unowned, loaded_as_shared_ptr.

* Factoring out convert_type and folding into loaded_as_unique_ptr.

* Folding convert_type into lvalue_ref and rvalue_ref paths. Some smart_holder_type_caster_load cleanup.

* Using unique_ptr in local_load to replace static variable. Also adding local_load_safety_guard.

* Converting test_unique_ptr_member to using classh: fully working, ASAN, MSAN, UBSAN clean.

* Removing debugging comments (GET_STACK, GET_INT_STACK). cast.h is identical to current master again, pybind11.h only has the generic_type::initialize(..., &type_caster_generic::local_load) change.

* Purging obsolete pybind11/vptr_holder.h and associated test.

* Moving several tests to github.com/rwgk/rwgk_tbx/tree/main/pybind11_tests

rwgk/rwgk_tbx@a2c2f88

These tests are from experimenting, and for demonstrating UB in pybind11 multiple inheritance handling ("first_base"), to be fixed later.

* Adding py::smart_holder support to py::class_, purging py::classh completely.

* Renaming files in include directory, creating pybind11/smart_holder.h.

* Renaming all "classh" to "smart_holder" in pybind11/detail/smart_holder_type_casters.h.

The user-facing macro is now PYBIND11_SMART_HOLDER_TYPE_CASTERS.

* Systematically renaming tests to use "class_sh" in the name.

* Renaming test_type_caster_bare_interface_demo to test_type_caster_bare_interface.

* Renaming new tests/core subdirectory to tests/pure_cpp.

* Adding new tests to CMake config, resetting CI config.

* Changing CMake file so that test_class_sh_module_local.py actually runs.

* clang-tidy fixes.

* 32-bit compatibility.

* Reusing type_caster_base make_copy_constructor, make_move_constructor with a trick.

* CMake COMPARE NATURAL is not available with older versions.

* Adding copyright notices to new header files.

* Explicitly define copy/move constructors/assignments.

* Adding new header files to tests/extra_python_package/test_files.py.

* Adding tests/pure_cpp/CMakeLists.txt.

* Making use of the new find_existing_python_instance() function factored out with PR #2822.

* Moving define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T) down in the file. NO functional changes. Preparation for follow-up work (to keep that diff smaller).

* Reintroducing py::classh, this time as a simple alias for py::class_<U, py::smart_holder>.

* Replacing detail::is_smart_holder<H> in cast.h with detail::is_smart_holder_type_caster<T>.
Moving get_local_load_function_ptr, init_instance_for_type to smart_holder_type_caster_class_hooks.
Expanding static_assert in py::type::handle_of<> to accommodate smart_holder_type_casters.

* Fixing oversight.

* Adding classu alias for class_<U, std::unique_ptr<U>>.

* Giving up on idea to use legacy init_instance only if is_base_of<type_caster_generic, type_caster<T>. There are use cases in the wild that define both a custom type_caster and class_.

* Removing test_type_caster_bare_interface, which was moved to the separate PR #2834.

* Moving up is_smart_holder_type_caster, to also use in cast_is_temporary_value_reference.

* Adding smart_holder_type_casters for unique_ptr with custom deleter. SEVERE CODE DUPLICATION. This commit is to establish a baseline for consolidating the unique_ptr code.

* Unification of unique_ptr, unique_ptr_with_deleter code in smart_holder_poc.h. Leads to more fitting error messages. Enables use of unique_ptr<T, D> smart_holder_type_casters also for unique_ptr<T>.

* Copying files as-is from branch test_unique_ptr_member (PR #2672).

* Adding comment, simplifying naming, cmake addition.

* Introducing PYBIND11_USE_SMART_HOLDER_AS_DEFAULT macro (tested only undefined; there are many errors with the macro defined).

* Removing test_type_caster_bare_interface, which was moved to the separate PR #2834.

* Fixing oversight introduced with commit 95425f1.

* Setting record.default_holder correctly for PYBIND11_USE_SMART_HOLDER_AS_DEFAULT.

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.

* Fixing up cast.h and smart_holder.h after rebase.

* Removing detail/smart_holder_type_casters.h in separate commit.

* 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.

* 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.

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

* Adding // DANGER ZONE reminders.

* Converting as many py::class_ to py::classh as possible, not breaking tests.

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

* 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.

* 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>.

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

* 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().

* 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.

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

* Using __VA_ARGS__ in PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS.

* Replacing condense_for_macro with much simpler approach.

* 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.

* 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).

* 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.

* 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.)

* 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.)

* 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.

* 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.

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

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

* Overlooked flake8 fixes.

* Manipulating failing ConstructorStats test to pass, to be able to run all tests with ASAN.

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 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]

* 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 249df7c. The manipulation in test_factory_constructors.py is NOT reverted in this commit.

[skip ci]

* Fixing unfortunate editing mishap. This reverts the last remaining test manipulation in commit 249df7c and makes all existing unit tests pass with smart_holder as default holder.

* GitHub CI clang-tidy fixes.

* Adding messages to terse `static_assert`s, for pre-C++17 compatibility.

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

* Systematically removing _atyp from function names, to make the test code simpler.

* Using re.match to accommodate variable number of intermediate MvCtor.

* 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.

* 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: #1616 (comment)

* Importing re before pytest after observing a PyPy CI flake when importing pytest first.

* Copying MSVC 2015 compatibility change from branch pr2672_use_smart_holder_as_default.

* Introducing is_smart_holder_type_caster_base_tag, to keep smart_holder code more disconnected.

* Working around MSVC 2015 bug.

* Expanding comment for MSVC 2015 workaround.

* Systematically changing std::enable_if back to detail::enable_if_t, effectively reverting commit 5d4b689.

* Removing unused smart_holder_type_caster_load::loaded_as_rvalue_ref (it was an oversight that it was not removed with commit 23036a4).

* Removing py::classu, because it does not seem useful enough.

* Reverting commit 6349531 by un-commenting `const` in `def_buffer(...)`. To make this possible, `operator T const&` and `operator T const*` in `smart_holder_type_caster` need to be marked as `const` member functions.

* Adding construct() overloads for constructing smart_holder from alias unique_ptr, shared_ptr returns.

* Adding test_class_sh_factory_constructors.cpp to tests/CMakeLists.txt (fixes oversight, this should have been added long before).

* Compatibility with old clang versions (clang 3.6, 3.7 C++11).

* Cleaning up changes to existing unit tests.

* Systematically adding SMART_HOLDER_WIP tag. Removing minor UNTESTED tags (only the throw are not actually exercised, investing time there has a high cost but very little benefit).

* Splitting out smart_holder_type_casters again, into new detail/smart_holder_type_casters_inline_include.h.

* Splitting out smart_holder_init_inline_include.h.

* Adding additional new include files to CMakeLists.txt, tests/extra_python_package/test_files.py.

* clang-format cleanup of most smart_holder code.

* Adding source code comments in response to review.

* Simple micro-benchmark ("ubench") comparing runtime performance for several holders.

Tested using github.com/rwgk/pybind11_scons and Google-internal build system.
Sorry, no cmake support at the moment.

First results: https://docs.google.com/spreadsheets/d/1InapCYws2Gt-stmFf_Bwl33eOMo3aLE_gc9adveY7RU/edit#gid=0

* Breaking out number_bucket.h, adding hook for also collecting performance data for PyCLIF.

* Accounting for ubench in MANIFEST.in (simply prune, for now).

* Smarter determination of call_repetitions.

[skip ci]

* Also scaling performance data to PyCLIF.

[skip ci]

* Adding ubench/python/number_bucket.clif here for general visibility.

* Fix after rebase

* Merging detail/smart_holder_init_inline_include.h into detail/init.h.

* Renaming detail/is_smart_holder_type_caster.h -> detail/smart_holder_sfinae_hooks_only.h.

* Renaming is_smart_holder_type_caster -> type_uses_smart_holder_type_caster for clarity.

* Renaming type_caster_type_is_smart_holder_type_caster -> wrapped_type_uses_smart_holder_type_caster for clarity.

* Renaming is_smart_holder_type_caster_base_tag -> smart_holder_type_caster_base_tag for simplicity.

* Adding copyright notices and minor colateral cleanup.

* iwyu cleanup (comprehensive only for cast.h and smart_holder*.h files).

* Fixing `git rebase master` accident.

* Moving large `pragma warning` block from pybind11.h to detail/common.h.

* Fixing another `git rebase master` accident.

Author: rwgk

CMakeLists.txt

@@ -105,6 +105,9 @@ set(PYBIND11_HEADERS
     include/pybind11/detail/descr.h
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h
+    include/pybind11/detail/smart_holder_poc.h
+    include/pybind11/detail/smart_holder_sfinae_hooks_only.h
+    include/pybind11/detail/smart_holder_type_casters.h
     include/pybind11/detail/type_caster_base.h
     include/pybind11/detail/typeid.h
     include/pybind11/attr.h
@@ -124,6 +127,7 @@ set(PYBIND11_HEADERS
     include/pybind11/operators.h
     include/pybind11/pybind11.h
     include/pybind11/pytypes.h
+    include/pybind11/smart_holder.h
     include/pybind11/stl.h
     include/pybind11/stl_bind.h)
 

MANIFEST.in

@@ -4,3 +4,4 @@ recursive-include pybind11 py.typed
 recursive-include pybind11 *.pyi
 include pybind11/share/cmake/pybind11/*.cmake
 include LICENSE README.rst pyproject.toml setup.py setup.cfg
+prune ubench

include/pybind11/cast.h

@@ -1,3 +1,4 @@
+// clang-format off
 /*
     pybind11/cast.h: Partial template specializations to cast between
     C++ and Python types
@@ -13,6 +14,7 @@
 #include "pytypes.h"
 #include "detail/common.h"
 #include "detail/descr.h"
+#include "detail/smart_holder_sfinae_hooks_only.h"
 #include "detail/type_caster_base.h"
 #include "detail/typeid.h"
 #include <array>
@@ -27,6 +29,10 @@
 #include <utility>
 #include <vector>
 
+#ifdef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+#include "detail/smart_holder_type_casters.h"
+#endif
+
 #if defined(PYBIND11_CPP17)
 #  if defined(__has_include)
 #    if __has_include(<string_view>)
@@ -47,8 +53,24 @@
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
-template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
-template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
+// clang-format on
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+template <typename T>
+class type_caster_for_class_ : public type_caster_base<T> {};
+#endif
+
+template <typename type, typename SFINAE = void>
+class type_caster : public type_caster_for_class_<type> {};
+
+template <typename type>
+using make_caster = type_caster<intrinsic_t<type>>;
+
+template <typename T>
+struct type_uses_smart_holder_type_caster {
+    static constexpr bool value
+        = std::is_base_of<smart_holder_type_caster_base_tag, make_caster<T>>::value;
+};
+// clang-format off
 
 // Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
 template <typename T> typename make_caster<T>::template cast_op_type<T> cast_op(make_caster<T> &caster) {
@@ -696,9 +718,11 @@ struct copyable_holder_caster : public type_caster_base<type> {
     holder_type holder;
 };
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
 /// Specialize for the common std::shared_ptr, so users don't need to
 template <typename T>
 class type_caster<std::shared_ptr<T>> : public copyable_holder_caster<T, std::shared_ptr<T>> { };
+#endif
 
 /// Type caster for holder types like std::unique_ptr.
 /// Please consider the SFINAE hook an implementation detail, as explained
@@ -715,9 +739,11 @@ struct move_only_holder_caster {
     static constexpr auto name = type_caster_base<type>::name;
 };
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
 template <typename type, typename deleter>
 class type_caster<std::unique_ptr<type, deleter>>
     : public move_only_holder_caster<type, std::unique_ptr<type, deleter>> { };
+#endif
 
 template <typename type, typename holder_type>
 using type_caster_holder = conditional_t<is_copy_constructible<holder_type>::value,
@@ -820,6 +846,7 @@ template <typename T> using move_never = none_of<move_always<T>, move_if_unrefer
 template <typename type> using cast_is_temporary_value_reference = bool_constant<
     (std::is_reference<type>::value || std::is_pointer<type>::value) &&
     !std::is_base_of<type_caster_generic, make_caster<type>>::value &&
+    !type_uses_smart_holder_type_caster<intrinsic_t<type>>::value &&
     !std::is_same<intrinsic_t<type>, void>::value
 >;
 
@@ -1362,18 +1389,18 @@ PYBIND11_NAMESPACE_END(detail)
 
 template<typename T>
 handle type::handle_of() {
-   static_assert(
-      std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>::value,
-      "py::type::of<T> only supports the case where T is a registered C++ types."
-    );
+    static_assert(
+        detail::any_of<std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>,
+                       detail::type_uses_smart_holder_type_caster<T>>::value,
+        "py::type::of<T> only supports the case where T is a registered C++ types.");
 
     return detail::get_type_handle(typeid(T), true);
 }
 
 
 #define PYBIND11_MAKE_OPAQUE(...) \
     namespace pybind11 { namespace detail { \
-        template<> class type_caster<__VA_ARGS__> : public type_caster_base<__VA_ARGS__> { }; \
+        template<> class type_caster<__VA_ARGS__> : public type_caster_for_class_<__VA_ARGS__> { }; \
     }}
 
 /// Lets you pass a type containing a `,` through a macro parameter without needing a separate

include/pybind11/detail/class.h

@@ -398,6 +398,8 @@ inline void clear_instance(PyObject *self) {
 
             if (instance->owned || v_h.holder_constructed())
                 v_h.type->dealloc(v_h);
+        } else if (v_h.holder_constructed()) {
+            v_h.type->dealloc(v_h); // Disowned instance.
         }
     }
     // Deallocate the value/holder layout internals:

include/pybind11/detail/common.h

@@ -13,6 +13,38 @@
 #define PYBIND11_VERSION_MINOR 6
 #define PYBIND11_VERSION_PATCH 3.dev1
 
+#if defined(__INTEL_COMPILER)
+#  pragma warning push
+#  pragma warning disable 68    // integer conversion resulted in a change of sign
+#  pragma warning disable 186   // pointless comparison of unsigned integer with zero
+#  pragma warning disable 878   // incompatible exception specifications
+#  pragma warning disable 1334  // the "template" keyword used for syntactic disambiguation may only be used within a template
+#  pragma warning disable 1682  // implicit conversion of a 64-bit integral type to a smaller integral type (potential portability problem)
+#  pragma warning disable 1786  // function "strdup" was declared deprecated
+#  pragma warning disable 1875  // offsetof applied to non-POD (Plain Old Data) types is nonstandard
+#  pragma warning disable 2196  // warning #2196: routine is both "inline" and "noinline"
+#elif defined(_MSC_VER)
+#  pragma warning(push)
+#  pragma warning(disable: 4100) // warning C4100: Unreferenced formal parameter
+#  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
+#  pragma warning(disable: 4512) // warning C4512: Assignment operator was implicitly defined as deleted
+#  pragma warning(disable: 4800) // warning C4800: 'int': forcing value to bool 'true' or 'false' (performance warning)
+#  pragma warning(disable: 4996) // warning C4996: The POSIX name for this item is deprecated. Instead, use the ISO C and C++ conformant name
+#  pragma warning(disable: 4702) // warning C4702: unreachable code
+#  pragma warning(disable: 4522) // warning C4522: multiple assignment operators specified
+#  pragma warning(disable: 4505) // warning C4505: 'PySlice_GetIndicesEx': unreferenced local function has been removed (PyPy only)
+#elif defined(__GNUG__) && !defined(__clang__)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wunused-but-set-parameter"
+#  pragma GCC diagnostic ignored "-Wunused-but-set-variable"
+#  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
+#  pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#  pragma GCC diagnostic ignored "-Wattributes"
+#  if __GNUC__ >= 7
+#    pragma GCC diagnostic ignored "-Wnoexcept-type"
+#  endif
+#endif
+
 #define PYBIND11_NAMESPACE_BEGIN(name) namespace name {
 #define PYBIND11_NAMESPACE_END(name) }
 

include/pybind11/detail/init.h

@@ -1,3 +1,4 @@
+// clang-format off
 /*
     pybind11/detail/init.h: init factory function implementation and support code.
 
@@ -10,6 +11,7 @@
 #pragma once
 
 #include "class.h"
+#include "smart_holder_sfinae_hooks_only.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
@@ -105,11 +107,13 @@ void construct(value_and_holder &v_h, Cpp<Class> *ptr, bool need_alias) {
         // the holder and destruction happens when we leave the C++ scope, and the holder
         // class gets to handle the destruction however it likes.
         v_h.value_ptr() = ptr;
-        v_h.set_instance_registered(true); // To prevent init_instance from registering it
+        v_h.set_instance_registered(true); // SHORTCUT To prevent init_instance from registering it
+        // DANGER ZONE BEGIN: exceptions will leave v_h in an invalid state.
         v_h.type->init_instance(v_h.inst, nullptr); // Set up the holder
         Holder<Class> temp_holder(std::move(v_h.holder<Holder<Class>>())); // Steal the holder
         v_h.type->dealloc(v_h); // Destroys the moved-out holder remains, resets value ptr to null
         v_h.set_instance_registered(false);
+        // DANGER ZONE END.
 
         construct_alias_from_cpp<Class>(is_alias_constructible<Class>{}, v_h, std::move(*ptr));
     } else {
@@ -129,7 +133,8 @@ void construct(value_and_holder &v_h, Alias<Class> *alias_ptr, bool) {
 // Holder return: copy its pointer, and move or copy the returned holder into the new instance's
 // holder.  This also handles types like std::shared_ptr<T> and std::unique_ptr<T> where T is a
 // derived type (through those holder's implicit conversion from derived class holder constructors).
-template <typename Class>
+template <typename Class,
+          detail::enable_if_t<!detail::type_uses_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
 void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
     no_nullptr(ptr);
@@ -166,6 +171,66 @@ void construct(value_and_holder &v_h, Alias<Class> &&result, bool) {
     v_h.value_ptr() = new Alias<Class>(std::move(result));
 }
 
+// clang-format on
+template <
+    typename Class,
+    typename D = std::default_delete<Cpp<Class>>,
+    detail::enable_if_t<detail::type_uses_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_ptr, bool need_alias) {
+    auto *ptr = unq_ptr.get();
+    no_nullptr(ptr);
+    if (Class::has_alias && need_alias)
+        throw type_error("pybind11::init(): construction failed: returned std::unique_ptr pointee "
+                         "is not an alias instance");
+    auto smhldr
+        = type_caster<Cpp<Class>>::template smart_holder_from_unique_ptr(std::move(unq_ptr));
+    v_h.value_ptr() = ptr;
+    v_h.type->init_instance(v_h.inst, &smhldr);
+}
+
+template <
+    typename Class,
+    typename D = std::default_delete<Alias<Class>>,
+    detail::enable_if_t<detail::type_uses_smart_holder_type_caster<Alias<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h,
+               std::unique_ptr<Alias<Class>, D> &&unq_ptr,
+               bool /*need_alias*/) {
+    auto *ptr = unq_ptr.get();
+    no_nullptr(ptr);
+    auto smhldr
+        = type_caster<Alias<Class>>::template smart_holder_from_unique_ptr(std::move(unq_ptr));
+    v_h.value_ptr() = ptr;
+    v_h.type->init_instance(v_h.inst, &smhldr);
+}
+
+template <
+    typename Class,
+    detail::enable_if_t<detail::type_uses_smart_holder_type_caster<Cpp<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h, std::shared_ptr<Cpp<Class>> &&shd_ptr, bool need_alias) {
+    auto *ptr = shd_ptr.get();
+    no_nullptr(ptr);
+    if (Class::has_alias && need_alias)
+        throw type_error("pybind11::init(): construction failed: returned std::shared_ptr pointee "
+                         "is not an alias instance");
+    auto smhldr     = type_caster<Cpp<Class>>::template smart_holder_from_shared_ptr(shd_ptr);
+    v_h.value_ptr() = ptr;
+    v_h.type->init_instance(v_h.inst, &smhldr);
+}
+
+template <
+    typename Class,
+    detail::enable_if_t<detail::type_uses_smart_holder_type_caster<Alias<Class>>::value, int> = 0>
+void construct(value_and_holder &v_h,
+               std::shared_ptr<Alias<Class>> &&shd_ptr,
+               bool /*need_alias*/) {
+    auto *ptr = shd_ptr.get();
+    no_nullptr(ptr);
+    auto smhldr     = type_caster<Alias<Class>>::template smart_holder_from_shared_ptr(shd_ptr);
+    v_h.value_ptr() = ptr;
+    v_h.type->init_instance(v_h.inst, &smhldr);
+}
+// clang-format off
+
 // Implementing class for py::init<...>()
 template <typename... Args>
 struct constructor {