diff --git a/include/pybind11/detail/class.h b/include/pybind11/detail/class.h
index d30621c88..33a1e8a12 100644
--- a/include/pybind11/detail/class.h
+++ b/include/pybind11/detail/class.h
@@ -431,6 +431,8 @@ 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:
diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index 4509bd131..9c09cfbcb 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -10,6 +10,7 @@
 #pragma once
 
 #include "class.h"
+#include "smart_holder_sfinae_hooks_only.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
@@ -128,11 +129,14 @@ 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.type->init_instance(v_h.inst, nullptr); // Set up the holder
+        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 {
@@ -153,7 +157,9 @@ void construct(value_and_holder &v_h, Alias<Class> *alias_ptr, bool) {
 // 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) {
     PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
@@ -195,6 +201,73 @@ void construct(value_and_holder &v_h, Alias<Class> &&result, bool) {
     v_h.value_ptr() = new Alias<Class>(std::move(result));
 }
 
+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) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
+    auto *ptr = unq_ptr.get();
+    no_nullptr(ptr);
+    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");
+    }
+    // Here and below: if the new object is a trampoline, the shared_from_this mechanism needs
+    // to be prevented from accessing the smart_holder vptr, because it does not keep the
+    // trampoline Python object alive. For types that don't inherit from enable_shared_from_this
+    // it does not matter if void_cast_raw_ptr is true or false, therefore it's not necessary
+    // to also inspect the type.
+    auto smhldr = type_caster<Cpp<Class>>::smart_holder_from_unique_ptr(
+        std::move(unq_ptr), /*void_cast_raw_ptr*/ Class::has_alias && is_alias<Class>(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>>::smart_holder_from_unique_ptr(
+        std::move(unq_ptr), /*void_cast_raw_ptr*/ true);
+    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) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
+    auto *ptr = shd_ptr.get();
+    no_nullptr(ptr);
+    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>>::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>>::smart_holder_from_shared_ptr(shd_ptr);
+    v_h.value_ptr() = ptr;
+    v_h.type->init_instance(v_h.inst, &smhldr);
+}
+
 // Implementing class for py::init<...>()
 template <typename... Args>
 struct constructor {