diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a7ff6b3be..4060ed713 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,8 @@ jobs:
         - 3.5
         - 3.6
         - 3.9
-        - 3.10-dev
+        # Broken b/o https://github.com/pytest-dev/pytest/issues/8539
+        # - 3.10-dev
         - pypy2
         - pypy3
 
diff --git a/README_smart_holder.rst b/README_smart_holder.rst
index 4dc769e76..ba2c97748 100644
--- a/README_smart_holder.rst
+++ b/README_smart_holder.rst
@@ -101,28 +101,48 @@ holder:
        py::classh<Foo>(m, "Foo");
    }
 
-There are three small differences compared to classic pybind11:
+There are three small differences compared to Classic pybind11:
 
 - ``#include <pybind11/smart_holder.h>`` is used instead of
   ``#include <pybind11/pybind11.h>``.
 
-- ``py::classh`` is used instead of ``py::class_``.
-
 - The ``PYBIND11_SMART_HOLDER_TYPE_CASTERS(Foo)`` macro is needed.
 
-To the 2nd bullet point, ``py::classh<Foo>`` is simply a shortcut for
-``py::class_<Foo, py::smart_holder>``. The shortcut makes it possible to
-switch to using ``py::smart_holder`` without messing up the indentation of
-existing code. However, when migrating code that uses ``py::class_<Foo,
-std::shared_ptr<Foo>>``, currently ``std::shared_ptr<Foo>`` needs to be
-removed manually when switching to ``py::classh`` (#HelpAppreciated this
-could probably be avoided with a little bit of template metaprogramming).
+- ``py::classh`` is used instead of ``py::class_``.
 
-To the 3rd bullet point, the macro also needs to appear in other translation
-units with pybind11 bindings that involve Python⇄C++ conversions for
-`Foo`. This is the biggest inconvenience of the Conservative mode. Practically,
-at a larger scale it is best to work with a pair of `.h` and `.cpp` files
-for the bindings code, with the macros in the `.h` files.
+To the 2nd bullet point, the ``PYBIND11_SMART_HOLDER_TYPE_CASTERS`` macro
+needs to appear in all translation units with pybind11 bindings that involve
+Python⇄C++ conversions for `Foo`. This is the biggest inconvenience of the
+Conservative mode. Practically, at a larger scale it is best to work with a
+pair of `.h` and `.cpp` files for the bindings code, with the macros in the
+`.h` files.
+
+To the 3rd bullet point, ``py::classh<Foo>`` is simply a shortcut for
+``py::class_<Foo, py::smart_holder>``. The shortcut makes it possible to
+switch to using ``py::smart_holder`` without disturbing the indentation of
+existing code.
+
+When migrating code that uses ``py::class_<Bar, std::shared_ptr<Bar>>``
+there are two alternatives. The first one is to use ``py::classh<Bar>``:
+
+.. code-block:: diff
+
+   - py::class_<Bar, std::shared_ptr<Bar>>(m, "Bar");
+   + py::classh<Bar>(m, "Bar");
+
+This is clean and simple, but makes it difficult to fall back to Classic
+mode if needed. The second alternative is to replace ``std::shared_ptr<Bar>``
+with ``PYBIND11_SH_AVL(Bar)``:
+
+.. code-block:: diff
+
+   - py::class_<Bar, std::shared_ptr<Bar>>(m, "Bar");
+   + py::class_<Bar, PYBIND11_SH_AVL(Bar)>(m, "Bar");
+
+The ``PYBIND11_SH_AVL`` macro substitutes ``py::smart_holder``
+in Conservative mode, or ``std::shared_ptr<Bar>`` in Classic mode.
+See tests/test_classh_mock.cpp for an example. Note that the macro is also
+designed to not disturb the indentation of existing code.
 
 
 Progressive mode
@@ -132,47 +152,63 @@ To work in Progressive mode:
 
 - Add ``-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT`` to the compilation commands.
 
-- Remove any ``std::shared_ptr<...>`` holders from existing ``py::class_``
-  instantiations.
+- Remove or replace (see below) ``std::shared_ptr<...>`` holders.
 
 - Only if custom smart-pointers are used: the
-  `PYBIND11_TYPE_CASTER_BASE_HOLDER` macro is needed [`example
-  <https://github.com/pybind/pybind11/blob/2f624af1ac8571d603df2d70cb54fc7e2e3a356a/tests/test_multiple_inheritance.cpp#L72>`_].
+  `PYBIND11_TYPE_CASTER_BASE_HOLDER` macro is needed (see
+  tests/test_smart_ptr.cpp for examples).
 
 Overall this is probably easier to work with than the Conservative mode, but
 
 - the macro inconvenience is shifted from ``py::smart_holder`` to custom
-  smart-pointers (but probably much more rarely needed).
+  smart-pointer holders (which are probably much more rare).
 
 - it will not interoperate with other extensions built against master or
   stable, or extensions built in Conservative mode (see the cross-module
   compatibility section below).
 
+When migrating code that uses ``py::class_<Bar, std::shared_ptr<Bar>>`` there
+are the same alternatives as for the Conservative mode (see previous section).
+An additional alternative is to use the ``PYBIND11_SH_DEF(...)`` macro:
 
-Transition from Conservative to Progressive mode
-------------------------------------------------
+.. code-block:: diff
+
+   - py::class_<Bar, std::shared_ptr<Bar>>(m, "Bar");
+   + py::class_<Bar, PYBIND11_SH_DEF(Bar)>(m, "Bar");
+
+The ``PYBIND11_SH_DEF`` macro substitutes ``py::smart_holder`` only in
+Progressive mode, or ``std::shared_ptr<Bar>`` in Classic or Conservative
+mode. See tests/test_classh_mock.cpp for an example. Note that the
+``PYBIND11_SMART_HOLDER_TYPE_CASTERS`` macro is never needed in combination
+with the ``PYBIND11_SH_DEF`` macro, which is an advantage compared to the
+``PYBIND11_SH_AVL`` macro. Please review tests/test_classh_mock.cpp for a
+concise overview of all available options.
+
+
+Transition from Classic to Progressive mode
+-------------------------------------------
 
 This still has to be tried out more in practice, but in small-scale situations
 it may be feasible to switch directly to Progressive mode in a break-fix
 fashion. In large-scale situations it seems more likely that an incremental
 approach is needed, which could mean incrementally converting ``py::class_``
-to ``py::classh`` including addition of the macros, then flip the switch,
-and convert ``py::classh`` back to ``py:class_`` combined with removal of the
-macros if desired (at that point it will work equivalently either way). It
-may be smart to delay the final cleanup step until all third-party projects
-of interest have made the switch, because then the code will continue to
-work in either mode.
+to ``py::classh`` and using the family of related macros, then flip the switch
+to Progressive mode, and convert ``py::classh`` back to ``py:class_`` combined
+with removal of the macros if desired (at that point it will work equivalently
+either way). It may be smart to delay the final cleanup step until all
+third-party projects of interest have made the switch, because then the code
+will continue to work in all modes.
 
 
-Using py::classh but with fallback to classic pybind11
-------------------------------------------------------
+Using py::smart_holder but with fallback to Classic pybind11
+------------------------------------------------------------
 
-This could be viewed as super-conservative mode, for situations in which
-compatibility with classic pybind11 (without smart_holder) is needed for
-some period of time. The main idea is to enable use of ``py::classh``
-and the associated ``PYBIND11_SMART_HOLDER_TYPE_CASTERS`` macro while
-still being able to build the same code with classic pybind11. Please see
-tests/test_classh_mock.cpp for an example.
+For situations in which compatibility with Classic pybind11
+(without smart_holder) is needed for some period of time, fallback
+to Classic mode can be enabled by copying the ``BOILERPLATE`` code
+block from tests/test_classh_mock.cpp. This code block provides mock
+implementations of ``py::classh`` and the family of related macros
+(e.g. ``PYBIND11_SMART_HOLDER_TYPE_CASTERS``).
 
 
 Classic / Conservative / Progressive cross-module compatibility
@@ -268,7 +304,7 @@ inherit from ``py::trampoline_self_life_support``, for example:
        ...
    };
 
-This is the only difference compared to classic pybind11. A fairly
+This is the only difference compared to Classic pybind11. A fairly
 minimal but complete example is tests/test_class_sh_trampoline_unique_ptr.cpp.
 
 
@@ -277,7 +313,7 @@ Ideas for the long-term
 
 The macros are clearly an inconvenience in many situations. Highly
 speculative: to avoid the need for the macros, a potential approach would
-be to combine the classic implementation (``type_caster_base``) with
+be to combine the Classic implementation (``type_caster_base``) with
 the ``smart_holder_type_caster``, but this will probably be very messy and
 not great as a long-term solution. The ``type_caster_base`` code is very
 complex already. A more maintainable approach long-term could be to work
diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index 2c1c77ae9..0aea1f52c 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -179,7 +179,7 @@ template <
 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)
+    if (Class::has_alias && need_alias && !is_alias<Class>(ptr))
         throw type_error("pybind11::init(): construction failed: returned std::unique_ptr pointee "
                          "is not an alias instance");
     auto smhldr
@@ -209,7 +209,7 @@ template <
 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)
+    if (Class::has_alias && need_alias && !is_alias<Class>(ptr))
         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);
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 12f79d42e..ed21b8fd4 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1217,12 +1217,30 @@ template <typename T>
 
 using default_holder_type = std::unique_ptr<T>;
 
+#    ifndef PYBIND11_SH_AVL
+#        define PYBIND11_SH_AVL(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if AVaiLable"
+// -------- std::shared_ptr(...) -- same length by design, to not disturb the indentation
+// of existing code.
+#    endif
+
+#    define PYBIND11_SH_DEF(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if DEFault"
+// -------- std::shared_ptr(...) -- same length by design, to not disturb the indentation
+// of existing code.
+
 #    define PYBIND11_TYPE_CASTER_BASE_HOLDER(T, ...)
 
 #else
 
 using default_holder_type = smart_holder;
 
+#    ifndef PYBIND11_SH_AVL
+#        define PYBIND11_SH_AVL(...) ::pybind11::smart_holder // "Smart_Holder if AVaiLable"
+// -------- std::shared_ptr(...) -- same length by design, to not disturb the indentation
+// of existing code.
+#    endif
+
+#    define PYBIND11_SH_DEF(...) ::pybind11::smart_holder // "Smart_Holder if DEFault"
+
 // This define could be hidden away inside detail/smart_holder_type_casters.h, but is kept here
 // for clarity.
 #    define PYBIND11_TYPE_CASTER_BASE_HOLDER(T, ...)                                              \
diff --git a/include/pybind11/smart_holder.h b/include/pybind11/smart_holder.h
index bf7cfb9dd..f852f77e2 100644
--- a/include/pybind11/smart_holder.h
+++ b/include/pybind11/smart_holder.h
@@ -8,6 +8,12 @@
 #include "detail/smart_holder_type_casters.h"
 #include "pybind11.h"
 
+#undef PYBIND11_SH_AVL // Undoing #define in pybind11.h
+
+#define PYBIND11_SH_AVL(...) ::pybind11::smart_holder // "Smart_Holder if AVaiLable"
+// ---- std::shared_ptr(...) -- same length by design, to not disturb the indentation
+// of existing code.
+
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 // Supports easier switching between py::class_<T> and py::class_<T, py::smart_holder>:
diff --git a/tests/test_classh_mock.cpp b/tests/test_classh_mock.cpp
index af016c7f8..38e765fb0 100644
--- a/tests/test_classh_mock.cpp
+++ b/tests/test_classh_mock.cpp
@@ -14,6 +14,12 @@ PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 template <typename type_, typename... options>
 using classh = class_<type_, options...>;
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
+#    ifndef PYBIND11_SH_AVL
+#        define PYBIND11_SH_AVL(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if AVaiLable"
+#    endif
+#    ifndef PYBIND11_SH_DEF
+#        define PYBIND11_SH_DEF(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if DEFault"
+#    endif
 #    ifndef PYBIND11_SMART_HOLDER_TYPE_CASTERS
 #        define PYBIND11_SMART_HOLDER_TYPE_CASTERS(...)
 #    endif
@@ -24,23 +30,42 @@ PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 // BOILERPLATE END
 
 namespace {
-struct Foo0 {};
-struct Foo1 {};
-struct Foo2 {};
+struct FooUc {};
+struct FooUp {};
+struct FooSa {};
+struct FooSc {};
+struct FooSp {};
 } // namespace
 
-PYBIND11_TYPE_CASTER_BASE_HOLDER(Foo1, std::shared_ptr<Foo1>)
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(Foo2)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooUp)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooSp)
+
+PYBIND11_TYPE_CASTER_BASE_HOLDER(FooSa, std::shared_ptr<FooSa>)
 
 TEST_SUBMODULE(classh_mock, m) {
-    // Uses std::unique_ptr<Foo0> as holder in conservative mode, py::smart_holder in progressive
-    // mode (if available).
-    py::class_<Foo0>(m, "Foo0").def(py::init<>());
+    // Please see README_smart_holder.rst, in particular section
+    // Classic / Conservative / Progressive cross-module compatibility
 
-    // Always uses std::shared_ptr<Foo1> as holder.
-    py::class_<Foo1, std::shared_ptr<Foo1>>(m, "Foo1").def(py::init<>());
+    // Uses std::unique_ptr<FooUc> as holder in Classic or Conservative mode, py::smart_holder in
+    // Progressive mode.
+    py::class_<FooUc>(m, "FooUc").def(py::init<>());
 
-    // Uses py::smart_holder if available, or std::unique_ptr<Foo2> if only pybind11 classic is
-    // available.
-    py::classh<Foo2>(m, "Foo2").def(py::init<>());
+    // Uses std::unique_ptr<FooUp> as holder in Classic mode, py::smart_holder in Conservative or
+    // Progressive mode.
+    py::classh<FooUp>(m, "FooUp").def(py::init<>());
+
+    // Always uses std::shared_ptr<FooSa> as holder.
+    py::class_<FooSa, std::shared_ptr<FooSa>>(m, "FooSa").def(py::init<>());
+
+    // Uses std::shared_ptr<FooSc> as holder in Classic or Conservative mode, py::smart_holder in
+    // Progressive mode.
+    py::class_<FooSc, PYBIND11_SH_DEF(FooSc)>(m, "FooSc").def(py::init<>());
+    // -------------- std::shared_ptr<FooSc> -- same length by design, to not disturb the
+    // indentation of existing code.
+
+    // Uses std::shared_ptr<FooSp> as holder in Classic mode, py::smart_holder in Conservative or
+    // Progressive mode.
+    py::class_<FooSp, PYBIND11_SH_AVL(FooSp)>(m, "FooSp").def(py::init<>());
+    // -------------- std::shared_ptr<FooSp> -- same length by design, to not disturb the
+    // indentation of existing code.
 }
diff --git a/tests/test_classh_mock.py b/tests/test_classh_mock.py
index 2c1d48754..03d323eae 100644
--- a/tests/test_classh_mock.py
+++ b/tests/test_classh_mock.py
@@ -6,6 +6,8 @@ from pybind11_tests import classh_mock as m
 def test_foobar():
     # Not really testing anything in particular. The main purpose of this test is to ensure the
     # suggested BOILERPLATE code block in test_classh_mock.cpp is correct.
-    assert m.Foo0()
-    assert m.Foo1()
-    assert m.Foo2()
+    assert m.FooUc()
+    assert m.FooUp()
+    assert m.FooSa()
+    assert m.FooSc()
+    assert m.FooSp()
diff --git a/tests/test_factory_constructors.cpp b/tests/test_factory_constructors.cpp
index a4e4829e5..db1a1b18a 100644
--- a/tests/test_factory_constructors.cpp
+++ b/tests/test_factory_constructors.cpp
@@ -139,11 +139,6 @@ public:
     static std::shared_ptr<TestFactory3> construct3(int a) { return std::shared_ptr<TestFactory3>(new TestFactory3(a)); }
 };
 
-PYBIND11_TYPE_CASTER_BASE_HOLDER(TestFactory3, std::shared_ptr<TestFactory3>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(TestFactory4, std::shared_ptr<TestFactory4>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(TestFactory5, std::shared_ptr<TestFactory5>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(TestFactory7, std::shared_ptr<TestFactory7>)
-
 TEST_SUBMODULE(factory_constructors, m) {
 
     // Define various trivial types to allow simpler overload resolution:
@@ -188,7 +183,7 @@ TEST_SUBMODULE(factory_constructors, m) {
     auto c4a = [c](pointer_tag, TF4_tag, int a) { (void) c; return new TestFactory4(a);};
 
     // test_init_factory_basic, test_init_factory_casting
-    py::class_<TestFactory3, std::shared_ptr<TestFactory3>> pyTestFactory3(m, "TestFactory3");
+    py::class_<TestFactory3, PYBIND11_SH_DEF(TestFactory3)> pyTestFactory3(m, "TestFactory3");
     pyTestFactory3
         .def(py::init([](pointer_tag, int v) { return TestFactoryHelper::construct3(v); }))
         .def(py::init([](shared_ptr_tag) { return TestFactoryHelper::construct3(); }));
@@ -212,12 +207,12 @@ TEST_SUBMODULE(factory_constructors, m) {
         ;
 
     // test_init_factory_casting
-    py::class_<TestFactory4, TestFactory3, std::shared_ptr<TestFactory4>>(m, "TestFactory4")
+    py::class_<TestFactory4, TestFactory3, PYBIND11_SH_DEF(TestFactory4)>(m, "TestFactory4")
         .def(py::init(c4a)) // pointer
         ;
 
     // Doesn't need to be registered, but registering makes getting ConstructorStats easier:
-    py::class_<TestFactory5, TestFactory3, std::shared_ptr<TestFactory5>>(m, "TestFactory5");
+    py::class_<TestFactory5, TestFactory3, PYBIND11_SH_DEF(TestFactory5)>(m, "TestFactory5");
 
     // test_init_factory_alias
     // Alias testing
@@ -238,7 +233,7 @@ TEST_SUBMODULE(factory_constructors, m) {
 
     // test_init_factory_dual
     // Separate alias constructor testing
-    py::class_<TestFactory7, PyTF7, std::shared_ptr<TestFactory7>>(m, "TestFactory7")
+    py::class_<TestFactory7, PyTF7, PYBIND11_SH_DEF(TestFactory7)>(m, "TestFactory7")
         .def(py::init(
             [](int i) { return TestFactory7(i); },
             [](int i) { return PyTF7(i); }))
diff --git a/tests/test_factory_constructors.py b/tests/test_factory_constructors.py
index 8c6ca173a..ed8e6d52c 100644
--- a/tests/test_factory_constructors.py
+++ b/tests/test_factory_constructors.py
@@ -280,10 +280,11 @@ def test_init_factory_dual():
     assert not g1.has_alias()
     with pytest.raises(TypeError) as excinfo:
         PythFactory7(tag.shared_ptr, tag.invalid_base, 14)
-    assert (
-        str(excinfo.value)
-        == "pybind11::init(): construction failed: returned holder-wrapped instance is not an "
-        "alias instance"
+    assert str(excinfo.value) in (
+        "pybind11::init(): construction failed: returned holder-wrapped instance is not an "
+        "alias instance",
+        "pybind11::init(): construction failed: returned std::shared_ptr pointee is not an "
+        "alias instance",
     )
 
     assert [i.alive() for i in cstats] == [13, 7]
diff --git a/tests/test_methods_and_attributes.cpp b/tests/test_methods_and_attributes.cpp
index d4c2c1466..ddf3b4526 100644
--- a/tests/test_methods_and_attributes.cpp
+++ b/tests/test_methods_and_attributes.cpp
@@ -113,7 +113,7 @@ class NoneTester { public: int answer = 42; };
 int none1(const NoneTester &obj) { return obj.answer; }
 int none2(NoneTester *obj) { return obj ? obj->answer : -1; }
 int none3(const std::shared_ptr<NoneTester> &obj) { return obj ? obj->answer : -1; }
-int none4(const std::shared_ptr<NoneTester> *obj) { return obj && *obj ? (*obj)->answer : -1; }
+int none4(std::shared_ptr<NoneTester> *obj) { return obj && *obj ? (*obj)->answer : -1; }
 int none5(std::shared_ptr<NoneTester> obj) { return obj ? obj->answer : -1; }
 
 struct StrIssue {
@@ -148,8 +148,6 @@ struct RefQualified {
     int constRefQualified(int other) const & { return value + other; }
 };
 
-PYBIND11_TYPE_CASTER_BASE_HOLDER(NoneTester, std::shared_ptr<NoneTester>)
-
 TEST_SUBMODULE(methods_and_attributes, m) {
     // test_methods_and_attributes
     py::class_<ExampleMandA> emna(m, "ExampleMandA");
@@ -327,19 +325,23 @@ TEST_SUBMODULE(methods_and_attributes, m) {
     // [workaround(intel)] ICC 20/21 breaks with py::arg().stuff, using py::arg{}.stuff works.
 
     // test_accepts_none
-    py::class_<NoneTester, std::shared_ptr<NoneTester>>(m, "NoneTester")
+    py::class_<NoneTester, PYBIND11_SH_DEF(NoneTester)>(m, "NoneTester")
         .def(py::init<>());
     m.def("no_none1", &none1, py::arg{}.none(false));
     m.def("no_none2", &none2, py::arg{}.none(false));
     m.def("no_none3", &none3, py::arg{}.none(false));
-    m.def("no_none4", &none4, py::arg{}.none(false));
     m.def("no_none5", &none5, py::arg{}.none(false));
     m.def("ok_none1", &none1);
     m.def("ok_none2", &none2, py::arg{}.none(true));
     m.def("ok_none3", &none3);
-    m.def("ok_none4", &none4, py::arg{}.none(true));
     m.def("ok_none5", &none5);
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+    // smart_holder_type_caster does not support conversion to `const shared_ptr<T> *`.
+    m.def("no_none4", &none4, py::arg{}.none(false));
+    m.def("ok_none4", &none4, py::arg{}.none(true));
+#endif
+
     m.def("no_none_kwarg", &none2, "a"_a.none(false));
     m.def("no_none_kwarg_kw_only", &none2, py::kw_only(), "a"_a.none(false));
 
diff --git a/tests/test_methods_and_attributes.py b/tests/test_methods_and_attributes.py
index 2aaf9331f..8328dcd4f 100644
--- a/tests/test_methods_and_attributes.py
+++ b/tests/test_methods_and_attributes.py
@@ -374,12 +374,10 @@ def test_accepts_none(msg):
     assert m.no_none1(a) == 42
     assert m.no_none2(a) == 42
     assert m.no_none3(a) == 42
-    assert m.no_none4(a) == 42
     assert m.no_none5(a) == 42
     assert m.ok_none1(a) == 42
     assert m.ok_none2(a) == 42
     assert m.ok_none3(a) == 42
-    assert m.ok_none4(a) == 42
     assert m.ok_none5(a) == 42
 
     with pytest.raises(TypeError) as excinfo:
@@ -391,9 +389,6 @@ def test_accepts_none(msg):
     with pytest.raises(TypeError) as excinfo:
         m.no_none3(None)
     assert "incompatible function arguments" in str(excinfo.value)
-    with pytest.raises(TypeError) as excinfo:
-        m.no_none4(None)
-    assert "incompatible function arguments" in str(excinfo.value)
     with pytest.raises(TypeError) as excinfo:
         m.no_none5(None)
     assert "incompatible function arguments" in str(excinfo.value)
@@ -414,7 +409,6 @@ def test_accepts_none(msg):
     # The rest take the argument as pointer or holder, and accept None:
     assert m.ok_none2(None) == -1
     assert m.ok_none3(None) == -1
-    assert m.ok_none4(None) == -1
     assert m.ok_none5(None) == -1
 
     with pytest.raises(TypeError) as excinfo:
@@ -430,6 +424,14 @@ def test_accepts_none(msg):
         m.no_none_kwarg_kw_only(a=None)
     assert "incompatible function arguments" in str(excinfo.value)
 
+    if hasattr(m, "no_none4"):
+        assert m.no_none4(a) == 42
+        assert m.ok_none4(a) == 42
+        with pytest.raises(TypeError) as excinfo:
+            m.no_none4(None)
+        assert "incompatible function arguments" in str(excinfo.value)
+        assert m.ok_none4(None) == -1
+
 
 def test_str_issue(msg):
     """#283: __str__ called on uninitialized instance when constructor arguments invalid"""
diff --git a/tests/test_multiple_inheritance.cpp b/tests/test_multiple_inheritance.cpp
index 23e6341dd..6243deab5 100644
--- a/tests/test_multiple_inheritance.cpp
+++ b/tests/test_multiple_inheritance.cpp
@@ -69,14 +69,6 @@ struct I801D : I801C {}; // Indirect MI
 
 } // namespace
 
-PYBIND11_TYPE_CASTER_BASE_HOLDER(Base1a, std::shared_ptr<Base1a>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(Base2a, std::shared_ptr<Base2a>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(Base12a, std::shared_ptr<Base12a>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(I801B1, std::shared_ptr<I801B1>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(I801B2, std::shared_ptr<I801B2>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(I801C, std::shared_ptr<I801C>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(I801D, std::shared_ptr<I801D>)
-
 TEST_SUBMODULE(multiple_inheritance, m) {
     // Please do not interleave `struct` and `class` definitions with bindings code,
     // but implement `struct`s and `class`es in the anonymous namespace above.
@@ -136,16 +128,16 @@ TEST_SUBMODULE(multiple_inheritance, m) {
     // test_multiple_inheritance_virtbase
     // Test the case where not all base classes are specified, and where pybind11 requires the
     // py::multiple_inheritance flag to perform proper casting between types.
-    py::class_<Base1a, std::shared_ptr<Base1a>>(m, "Base1a")
+    py::class_<Base1a, PYBIND11_SH_DEF(Base1a)>(m, "Base1a")
         .def(py::init<int>())
         .def("foo", &Base1a::foo);
 
-    py::class_<Base2a, std::shared_ptr<Base2a>>(m, "Base2a")
+    py::class_<Base2a, PYBIND11_SH_DEF(Base2a)>(m, "Base2a")
         .def(py::init<int>())
         .def("bar", &Base2a::bar);
 
     py::class_<Base12a, /* Base1 missing */ Base2a,
-               std::shared_ptr<Base12a>>(m, "Base12a", py::multiple_inheritance())
+               PYBIND11_SH_DEF(Base12a)>(m, "Base12a", py::multiple_inheritance())
         .def(py::init<int, int>());
 
     m.def("bar_base2a", [](Base2a *b) { return b->bar(); });
@@ -158,10 +150,10 @@ TEST_SUBMODULE(multiple_inheritance, m) {
     struct I801B3 { int c = 3; virtual ~I801B3() = default; };
     struct I801E : I801B3, I801D {};
 
-    py::class_<I801B1, std::shared_ptr<I801B1>>(m, "I801B1").def(py::init<>()).def_readonly("a", &I801B1::a);
-    py::class_<I801B2, std::shared_ptr<I801B2>>(m, "I801B2").def(py::init<>()).def_readonly("b", &I801B2::b);
-    py::class_<I801C, I801B1, I801B2, std::shared_ptr<I801C>>(m, "I801C").def(py::init<>());
-    py::class_<I801D, I801C, std::shared_ptr<I801D>>(m, "I801D").def(py::init<>());
+    py::class_<I801B1, PYBIND11_SH_DEF(I801B1)>(m, "I801B1").def(py::init<>()).def_readonly("a", &I801B1::a);
+    py::class_<I801B2, PYBIND11_SH_DEF(I801B2)>(m, "I801B2").def(py::init<>()).def_readonly("b", &I801B2::b);
+    py::class_<I801C, I801B1, I801B2, PYBIND11_SH_DEF(I801C)>(m, "I801C").def(py::init<>());
+    py::class_<I801D, I801C, PYBIND11_SH_DEF(I801D)>(m, "I801D").def(py::init<>());
 
     // Two separate issues here: first, we want to recognize a pointer to a base type as being a
     // known instance even when the pointer value is unequal (i.e. due to a non-first