diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index e7b3a5f49..062036da5 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -10,7 +10,7 @@
 #pragma once
 
 #include "class.h"
-#include "smart_holder_sfinae_hooks_only.h"
+#include "smart_holder_poc.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
@@ -156,9 +156,10 @@ 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,
-          detail::enable_if_t<!detail::type_uses_smart_holder_type_caster<Cpp<Class>>::value, int>
-          = 0>
+template <
+    typename Class,
+    detail::enable_if_t<!std::is_same<Holder<Class>, pybindit::memory::smart_holder>::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);
@@ -200,10 +201,11 @@ 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>
+template <
+    typename Class,
+    typename D = std::default_delete<Cpp<Class>>,
+    detail::enable_if_t<std::is_same<Holder<Class>, pybindit::memory::smart_holder>::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();
@@ -223,10 +225,11 @@ void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_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>
+template <
+    typename Class,
+    typename D = std::default_delete<Alias<Class>>,
+    detail::enable_if_t<std::is_same<Holder<Class>, pybindit::memory::smart_holder>::value, int>
+    = 0>
 void construct(value_and_holder &v_h,
                std::unique_ptr<Alias<Class>, D> &&unq_ptr,
                bool /*need_alias*/) {
@@ -238,9 +241,10 @@ void construct(value_and_holder &v_h,
     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>
+template <
+    typename Class,
+    detail::enable_if_t<std::is_same<Holder<Class>, pybindit::memory::smart_holder>::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();
@@ -254,9 +258,10 @@ void construct(value_and_holder &v_h, std::shared_ptr<Cpp<Class>> &&shd_ptr, boo
     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>
+template <
+    typename Class,
+    detail::enable_if_t<std::is_same<Holder<Class>, pybindit::memory::smart_holder>::value, int>
+    = 0>
 void construct(value_and_holder &v_h,
                std::shared_ptr<Alias<Class>> &&shd_ptr,
                bool /*need_alias*/) {