diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index 75d25c119..a2cfd8e61 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -156,8 +156,7 @@ 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<!std::is_same<Holder<Class>, smart_holder>::value, int> = 0>
+template <typename Class, detail::enable_if_t<!is_smart_holder<Holder<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);
@@ -199,6 +198,8 @@ void construct(value_and_holder &v_h, Alias<Class> &&result, bool) {
     v_h.value_ptr() = new Alias<Class>(std::move(result));
 }
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 namespace originally_smart_holder_type_casters_h {
 template <typename T, typename D>
 smart_holder smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&unq_ptr,
@@ -215,7 +216,7 @@ smart_holder smart_holder_from_shared_ptr(std::shared_ptr<T> shd_ptr) {
 
 template <typename Class,
           typename D = std::default_delete<Cpp<Class>>,
-          detail::enable_if_t<std::is_same<Holder<Class>, smart_holder>::value, int> = 0>
+          detail::enable_if_t<is_smart_holder<Holder<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();
@@ -237,7 +238,7 @@ void construct(value_and_holder &v_h, std::unique_ptr<Cpp<Class>, D> &&unq_ptr,
 
 template <typename Class,
           typename D = std::default_delete<Alias<Class>>,
-          detail::enable_if_t<std::is_same<Holder<Class>, smart_holder>::value, int> = 0>
+          detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
 void construct(value_and_holder &v_h,
                std::unique_ptr<Alias<Class>, D> &&unq_ptr,
                bool /*need_alias*/) {
@@ -249,8 +250,7 @@ void construct(value_and_holder &v_h,
     v_h.type->init_instance(v_h.inst, &smhldr);
 }
 
-template <typename Class,
-          detail::enable_if_t<std::is_same<Holder<Class>, smart_holder>::value, int> = 0>
+template <typename Class, detail::enable_if_t<is_smart_holder<Holder<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();
@@ -264,8 +264,7 @@ 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<std::is_same<Holder<Class>, smart_holder>::value, int> = 0>
+template <typename Class, detail::enable_if_t<is_smart_holder<Holder<Class>>::value, int> = 0>
 void construct(value_and_holder &v_h,
                std::shared_ptr<Alias<Class>> &&shd_ptr,
                bool /*need_alias*/) {
@@ -276,6 +275,8 @@ void construct(value_and_holder &v_h,
     v_h.type->init_instance(v_h.inst, &smhldr);
 }
 
+#endif // PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 // Implementing class for py::init<...>()
 template <typename... Args>
 struct constructor {
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index 1c6a82526..82a9860e0 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -472,6 +472,8 @@ inline PyThreadState *get_thread_state_unchecked() {
 void keep_alive_impl(handle nurse, handle patient);
 inline PyObject *make_new_instance(PyTypeObject *type);
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 // SMART_HOLDER_WIP: Needs refactoring of existing pybind11 code.
 inline bool deregister_instance(instance *self, void *valptr, const type_info *tinfo);
 
@@ -828,6 +830,8 @@ struct load_helper : value_and_holder_helper {
 
 PYBIND11_NAMESPACE_END(smart_holder_type_caster_support)
 
+#endif // PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 class type_caster_generic {
 public:
     PYBIND11_NOINLINE explicit type_caster_generic(const std::type_info &type_info)
diff --git a/include/pybind11/detail/using_smart_holder.h b/include/pybind11/detail/using_smart_holder.h
index ef95ea7fc..c47b691d4 100644
--- a/include/pybind11/detail/using_smart_holder.h
+++ b/include/pybind11/detail/using_smart_holder.h
@@ -4,17 +4,30 @@
 
 #pragma once
 
+#include "common.h"
 #include "internals.h"
 
-#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+#include <type_traits>
 
-#    include "common.h"
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 #    include "smart_holder_poc.h"
+#endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 using pybindit::memory::smart_holder;
-
-PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
-
 #endif
+
+PYBIND11_NAMESPACE_BEGIN(detail)
+
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+template <typename H>
+using is_smart_holder = std::is_same<H, smart_holder>;
+#else
+template <typename>
+struct is_smart_holder : std::false_type {};
+#endif
+
+PYBIND11_NAMESPACE_END(detail)
+PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 6fb8ca6f8..d79b36b02 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -2194,7 +2194,7 @@ private:
     /// an optional pointer to an existing holder to use; if not specified and the instance is
     /// `.owned`, a new holder will be constructed to manage the value pointer.
     template <typename H = holder_type,
-              detail::enable_if_t<!std::is_same<H, smart_holder>::value, int> = 0>
+              detail::enable_if_t<!detail::is_smart_holder<H>::value, int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         auto v_h = inst->get_value_and_holder(detail::get_type_info(typeid(type)));
         if (!v_h.instance_registered()) {
@@ -2229,8 +2229,9 @@ private:
         return true;
     }
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
     template <typename H = holder_type,
-              detail::enable_if_t<std::is_same<H, smart_holder>::value, int> = 0>
+              detail::enable_if_t<detail::is_smart_holder<H>::value, int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_const_void_ptr) {
         // Need for const_cast is a consequence of the type_info::init_instance type:
         // void (*init_instance)(instance *, const void *);
@@ -2263,6 +2264,7 @@ private:
             = pointee_depends_on_holder_owner;
         v_h.set_holder_constructed();
     }
+#endif // PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 
     /// Deallocates an instance; via holder, if constructed; otherwise via operator delete.
     static void dealloc(detail::value_and_holder &v_h) {
diff --git a/include/pybind11/trampoline_self_life_support.h b/include/pybind11/trampoline_self_life_support.h
index eac509438..068835726 100644
--- a/include/pybind11/trampoline_self_life_support.h
+++ b/include/pybind11/trampoline_self_life_support.h
@@ -4,9 +4,13 @@
 
 #pragma once
 
-#include "detail/common.h"
-#include "detail/using_smart_holder.h"
-#include "detail/value_and_holder.h"
+#include "detail/internals.h"
+
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
+#    include "detail/common.h"
+#    include "detail/using_smart_holder.h"
+#    include "detail/value_and_holder.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
@@ -59,3 +63,5 @@ struct trampoline_self_life_support {
 };
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
+
+#endif // PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
diff --git a/tests/test_class_sh_trampoline_basic.cpp b/tests/test_class_sh_trampoline_basic.cpp
index 856f274c2..faadb4024 100644
--- a/tests/test_class_sh_trampoline_basic.cpp
+++ b/tests/test_class_sh_trampoline_basic.cpp
@@ -34,6 +34,7 @@ struct AbaseAlias : Abase<SerNo> {
     }
 };
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 template <>
 struct AbaseAlias<1> : Abase<1>, py::trampoline_self_life_support {
     using Abase<1>::Abase;
@@ -45,6 +46,7 @@ struct AbaseAlias<1> : Abase<1>, py::trampoline_self_life_support {
                                other_val);
     }
 };
+#endif // PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 
 template <int SerNo>
 int AddInCppRawPtr(const Abase<SerNo> *obj, int other_val) {
diff --git a/tests/test_class_sh_trampoline_self_life_support.cpp b/tests/test_class_sh_trampoline_self_life_support.cpp
index 3b202deb3..9b67323f3 100644
--- a/tests/test_class_sh_trampoline_self_life_support.cpp
+++ b/tests/test_class_sh_trampoline_self_life_support.cpp
@@ -37,9 +37,11 @@ protected:
     Big5() : history{"DefaultConstructor"} {}
 };
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 struct Big5Trampoline : Big5, py::trampoline_self_life_support {
     using Big5::Big5;
 };
+#endif
 
 } // namespace
 
diff --git a/tests/test_class_sh_trampoline_shared_from_this.cpp b/tests/test_class_sh_trampoline_shared_from_this.cpp
index 38b36e1b0..aaf5a8750 100644
--- a/tests/test_class_sh_trampoline_shared_from_this.cpp
+++ b/tests/test_class_sh_trampoline_shared_from_this.cpp
@@ -71,9 +71,11 @@ struct SftSharedPtrStash {
     }
 };
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 struct SftTrampoline : Sft, py::trampoline_self_life_support {
     using Sft::Sft;
 };
+#endif
 
 long use_count(const std::shared_ptr<Sft> &obj) { return obj.use_count(); }
 
diff --git a/tests/test_class_sh_trampoline_unique_ptr.cpp b/tests/test_class_sh_trampoline_unique_ptr.cpp
index 26ddf7750..af0fb16ef 100644
--- a/tests/test_class_sh_trampoline_unique_ptr.cpp
+++ b/tests/test_class_sh_trampoline_unique_ptr.cpp
@@ -32,7 +32,6 @@ private:
 };
 
 #ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
-
 class PyClass : public Class, public py::trampoline_self_life_support {
 public:
     std::unique_ptr<Class> clone() const override {
@@ -41,7 +40,6 @@ public:
 
     int foo() const override { PYBIND11_OVERRIDE_PURE(int, Class, foo); }
 };
-
 #endif
 
 } // namespace class_sh_trampoline_basic
diff --git a/tests/test_class_sh_virtual_py_cpp_mix.cpp b/tests/test_class_sh_virtual_py_cpp_mix.cpp
index c8477d483..f85c87b27 100644
--- a/tests/test_class_sh_virtual_py_cpp_mix.cpp
+++ b/tests/test_class_sh_virtual_py_cpp_mix.cpp
@@ -31,6 +31,8 @@ int get_from_cpp_plainc_ptr(const Base *b) { return b->get() + 4000; }
 
 int get_from_cpp_unique_ptr(std::unique_ptr<Base> b) { return b->get() + 5000; }
 
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 struct BaseVirtualOverrider : Base, py::trampoline_self_life_support {
     using Base::Base;
 
@@ -43,6 +45,8 @@ struct CppDerivedVirtualOverrider : CppDerived, py::trampoline_self_life_support
     int get() const override { PYBIND11_OVERRIDE(int, CppDerived, get); }
 };
 
+#endif
+
 } // namespace class_sh_virtual_py_cpp_mix
 } // namespace pybind11_tests