diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index f182e2499..6c8a21d78 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -581,6 +581,9 @@ add_custom_command(
     ${CMAKE_CURRENT_BINARY_DIR}/sosize-$<TARGET_FILE_NAME:pybind11_tests>.txt)
 
 if(NOT PYBIND11_CUDA_TESTS)
+  # Test pure C++ code (not depending on Python). Provides the `test_pure_cpp` target.
+  add_subdirectory(pure_cpp)
+
   # Test embedding the interpreter. Provides the `cpptest` target.
   add_subdirectory(test_embed)
 
diff --git a/tests/pure_cpp/CMakeLists.txt b/tests/pure_cpp/CMakeLists.txt
new file mode 100644
index 000000000..17be74b7f
--- /dev/null
+++ b/tests/pure_cpp/CMakeLists.txt
@@ -0,0 +1,20 @@
+find_package(Catch 2.13.2)
+
+if(CATCH_FOUND)
+  message(STATUS "Building pure C++ tests (not depending on Python) using Catch v${CATCH_VERSION}")
+else()
+  message(STATUS "Catch not detected. Interpreter tests will be skipped. Install Catch headers"
+                 " manually or use `cmake -DDOWNLOAD_CATCH=ON` to fetch them automatically.")
+  return()
+endif()
+
+add_executable(smart_holder_poc_test smart_holder_poc_test.cpp)
+pybind11_enable_warnings(smart_holder_poc_test)
+target_link_libraries(smart_holder_poc_test PRIVATE pybind11::headers Catch2::Catch2)
+
+add_custom_target(
+  test_pure_cpp
+  COMMAND "$<TARGET_FILE:smart_holder_poc_test>"
+  WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}")
+
+add_dependencies(check test_pure_cpp)
diff --git a/tests/pure_cpp/smart_holder_poc_test.cpp b/tests/pure_cpp/smart_holder_poc_test.cpp
new file mode 100644
index 000000000..f820b9bf6
--- /dev/null
+++ b/tests/pure_cpp/smart_holder_poc_test.cpp
@@ -0,0 +1,413 @@
+#define PYBIND11_TESTS_PURE_CPP_SMART_HOLDER_POC_TEST_CPP
+
+#include "pybind11/detail/smart_holder_poc.h"
+
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+// Catch uses _ internally, which breaks gettext style defines
+#ifdef _
+#    undef _
+#endif
+
+#define CATCH_CONFIG_MAIN
+#include "catch.hpp"
+
+using pybindit::memory::smart_holder;
+
+namespace helpers {
+
+struct movable_int {
+    int valu;
+    explicit movable_int(int v) : valu{v} {}
+    movable_int(movable_int &&other) noexcept : valu(other.valu) { other.valu = 91; }
+};
+
+template <typename T>
+struct functor_builtin_delete {
+    void operator()(T *ptr) { delete ptr; }
+#if (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ == 8)                                   \
+    || (defined(__clang_major__) && __clang_major__ == 3 && __clang_minor__ == 6)
+    // Workaround for these errors:
+    // gcc 4.8.5: too many initializers for 'helpers::functor_builtin_delete<int>'
+    // clang 3.6: excess elements in struct initializer
+    functor_builtin_delete() = default;
+    functor_builtin_delete(const functor_builtin_delete &) {}
+    functor_builtin_delete(functor_builtin_delete &&) {}
+#endif
+};
+
+template <typename T>
+struct functor_other_delete : functor_builtin_delete<T> {};
+
+struct indestructible_int {
+    int valu;
+    explicit indestructible_int(int v) : valu{v} {}
+
+private:
+    ~indestructible_int() = default;
+};
+
+struct base {
+    virtual int get() { return 10; }
+    virtual ~base() = default;
+};
+
+struct derived : public base {
+    int get() override { return 100; }
+};
+
+} // namespace helpers
+
+TEST_CASE("from_raw_ptr_unowned+as_raw_ptr_unowned", "[S]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE(*hld.as_raw_ptr_unowned<int>() == 19);
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_lvalue_ref", "[S]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_rvalue_ref", "[S]") {
+    helpers::movable_int orig(19);
+    {
+        auto hld = smart_holder::from_raw_ptr_unowned(&orig);
+        helpers::movable_int othr(hld.as_rvalue_ref<helpers::movable_int>());
+        REQUIRE(othr.valu == 19);
+        REQUIRE(orig.valu == 91);
+    }
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_raw_ptr_release_ownership", "[E]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE_THROWS_WITH(hld.as_raw_ptr_release_ownership<int>(),
+                        "Cannot disown non-owning holder (as_raw_ptr_release_ownership).");
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_unique_ptr", "[E]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(),
+                        "Cannot disown non-owning holder (as_unique_ptr).");
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_unique_ptr_with_deleter", "[E]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE_THROWS_WITH((hld.as_unique_ptr<int, helpers::functor_builtin_delete<int>>()),
+                        "Missing unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_raw_ptr_unowned+as_shared_ptr", "[S]") {
+    static int value = 19;
+    auto hld = smart_holder::from_raw_ptr_unowned(&value);
+    REQUIRE(*hld.as_shared_ptr<int>() == 19);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_lvalue_ref", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    REQUIRE(hld.has_pointee());
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_raw_ptr_release_ownership1", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    auto new_owner = std::unique_ptr<int>(hld.as_raw_ptr_release_ownership<int>());
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_raw_ptr_release_ownership2", "[E]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    auto shd_ptr = hld.as_shared_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_raw_ptr_release_ownership<int>(),
+                        "Cannot disown use_count != 1 (as_raw_ptr_release_ownership).");
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_unique_ptr1", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    std::unique_ptr<int> new_owner = hld.as_unique_ptr<int>();
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_unique_ptr2", "[E]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    auto shd_ptr = hld.as_shared_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(), "Cannot disown use_count != 1 (as_unique_ptr).");
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_unique_ptr_with_deleter", "[E]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    REQUIRE_THROWS_WITH((hld.as_unique_ptr<int, helpers::functor_builtin_delete<int>>()),
+                        "Missing unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_shared_ptr", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    std::shared_ptr<int> new_owner = hld.as_shared_ptr<int>();
+    REQUIRE(hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+disown+reclaim_disowned", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
+    hld.disown();
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+    REQUIRE(*new_owner == 19);
+    hld.reclaim_disowned(); // Manually veriified: without this, clang++ -fsanitize=address reports
+                            // "detected memory leaks".
+    (void) new_owner.release(); // Manually verified: without this, clang++ -fsanitize=address
+                                // reports "attempting double-free".
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+    REQUIRE(new_owner.get() == nullptr);
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+disown+release_disowned", "[S]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
+    hld.disown();
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+    REQUIRE(*new_owner == 19);
+    hld.release_disowned();
+    REQUIRE(!hld.has_pointee());
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+disown+ensure_is_not_disowned", "[E]") {
+    const char *context = "test_case";
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    hld.ensure_is_not_disowned(context); // Does not throw.
+    std::unique_ptr<int> new_owner(hld.as_raw_ptr_unowned<int>());
+    hld.disown();
+    REQUIRE_THROWS_WITH(hld.ensure_is_not_disowned(context),
+                        "Holder was disowned already (test_case).");
+}
+
+TEST_CASE("from_unique_ptr+as_lvalue_ref", "[S]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_unique_ptr+as_raw_ptr_release_ownership1", "[S]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    auto new_owner = std::unique_ptr<int>(hld.as_raw_ptr_release_ownership<int>());
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_unique_ptr+as_raw_ptr_release_ownership2", "[E]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    auto shd_ptr = hld.as_shared_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_raw_ptr_release_ownership<int>(),
+                        "Cannot disown use_count != 1 (as_raw_ptr_release_ownership).");
+}
+
+TEST_CASE("from_unique_ptr+as_unique_ptr1", "[S]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::unique_ptr<int> new_owner = hld.as_unique_ptr<int>();
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_unique_ptr+as_unique_ptr2", "[E]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    auto shd_ptr = hld.as_shared_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(), "Cannot disown use_count != 1 (as_unique_ptr).");
+}
+
+TEST_CASE("from_unique_ptr+as_unique_ptr_with_deleter", "[E]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH((hld.as_unique_ptr<int, helpers::functor_builtin_delete<int>>()),
+                        "Incompatible unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_unique_ptr+as_shared_ptr", "[S]") {
+    std::unique_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::shared_ptr<int> new_owner = hld.as_shared_ptr<int>();
+    REQUIRE(hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_unique_ptr_derived+as_unique_ptr_base", "[S]") {
+    std::unique_ptr<helpers::derived> orig_owner(new helpers::derived());
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::unique_ptr<helpers::base> new_owner = hld.as_unique_ptr<helpers::base>();
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(new_owner->get() == 100);
+}
+
+TEST_CASE("from_unique_ptr_derived+as_unique_ptr_base2", "[E]") {
+    std::unique_ptr<helpers::derived, helpers::functor_other_delete<helpers::derived>> orig_owner(
+        new helpers::derived());
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH(
+        (hld.as_unique_ptr<helpers::base, helpers::functor_builtin_delete<helpers::base>>()),
+        "Incompatible unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_lvalue_ref", "[S]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_unique_ptr_with_std_function_deleter+as_lvalue_ref", "[S]") {
+    std::unique_ptr<int, std::function<void(const int *)>> orig_owner(
+        new int(19), [](const int *raw_ptr) { delete raw_ptr; });
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_raw_ptr_release_ownership", "[E]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH(hld.as_raw_ptr_release_ownership<int>(),
+                        "Cannot disown custom deleter (as_raw_ptr_release_ownership).");
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_unique_ptr", "[E]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(),
+                        "Incompatible unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_unique_ptr_with_deleter1", "[S]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> new_owner
+        = hld.as_unique_ptr<int, helpers::functor_builtin_delete<int>>();
+    REQUIRE(!hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_unique_ptr_with_deleter2", "[E]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    REQUIRE_THROWS_WITH((hld.as_unique_ptr<int, helpers::functor_other_delete<int>>()),
+                        "Incompatible unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_shared_ptr", "[S]") {
+    std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+    auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+    REQUIRE(orig_owner.get() == nullptr);
+    std::shared_ptr<int> new_owner = hld.as_shared_ptr<int>();
+    REQUIRE(hld.has_pointee());
+    REQUIRE(*new_owner == 19);
+}
+
+TEST_CASE("from_shared_ptr+as_lvalue_ref", "[S]") {
+    std::shared_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_shared_ptr(orig_owner);
+    REQUIRE(hld.as_lvalue_ref<int>() == 19);
+}
+
+TEST_CASE("from_shared_ptr+as_raw_ptr_release_ownership", "[E]") {
+    std::shared_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_shared_ptr(orig_owner);
+    REQUIRE_THROWS_WITH(hld.as_raw_ptr_release_ownership<int>(),
+                        "Cannot disown external shared_ptr (as_raw_ptr_release_ownership).");
+}
+
+TEST_CASE("from_shared_ptr+as_unique_ptr", "[E]") {
+    std::shared_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_shared_ptr(orig_owner);
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(),
+                        "Cannot disown external shared_ptr (as_unique_ptr).");
+}
+
+TEST_CASE("from_shared_ptr+as_unique_ptr_with_deleter", "[E]") {
+    std::shared_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_shared_ptr(orig_owner);
+    REQUIRE_THROWS_WITH((hld.as_unique_ptr<int, helpers::functor_builtin_delete<int>>()),
+                        "Missing unique_ptr deleter (as_unique_ptr).");
+}
+
+TEST_CASE("from_shared_ptr+as_shared_ptr", "[S]") {
+    std::shared_ptr<int> orig_owner(new int(19));
+    auto hld = smart_holder::from_shared_ptr(orig_owner);
+    REQUIRE(*hld.as_shared_ptr<int>() == 19);
+}
+
+TEST_CASE("error_unpopulated_holder", "[E]") {
+    smart_holder hld;
+    REQUIRE_THROWS_WITH(hld.as_lvalue_ref<int>(), "Unpopulated holder (as_lvalue_ref).");
+}
+
+TEST_CASE("error_disowned_holder", "[E]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    hld.as_unique_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_lvalue_ref<int>(), "Disowned holder (as_lvalue_ref).");
+}
+
+TEST_CASE("error_cannot_disown_nullptr", "[E]") {
+    auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+    hld.as_unique_ptr<int>();
+    REQUIRE_THROWS_WITH(hld.as_unique_ptr<int>(), "Cannot disown nullptr (as_unique_ptr).");
+}
+
+TEST_CASE("indestructible_int-from_raw_ptr_unowned+as_raw_ptr_unowned", "[S]") {
+    using zombie = helpers::indestructible_int;
+    // Using placement new instead of plain new, to not trigger leak sanitizer errors.
+    static std::aligned_storage<sizeof(zombie), alignof(zombie)>::type memory_block[1];
+    auto *value = new (memory_block) zombie(19);
+    auto hld = smart_holder::from_raw_ptr_unowned(value);
+    REQUIRE(hld.as_raw_ptr_unowned<zombie>()->valu == 19);
+}
+
+TEST_CASE("indestructible_int-from_raw_ptr_take_ownership", "[E]") {
+    helpers::indestructible_int *value = nullptr;
+    REQUIRE_THROWS_WITH(smart_holder::from_raw_ptr_take_ownership(value),
+                        "Pointee is not destructible (from_raw_ptr_take_ownership).");
+}
+
+TEST_CASE("from_raw_ptr_take_ownership+as_shared_ptr-outliving_smart_holder", "[S]") {
+    // Exercises guarded_builtin_delete flag_ptr validity past destruction of smart_holder.
+    std::shared_ptr<int> longer_living;
+    {
+        auto hld = smart_holder::from_raw_ptr_take_ownership(new int(19));
+        longer_living = hld.as_shared_ptr<int>();
+    }
+    REQUIRE(*longer_living == 19);
+}
+
+TEST_CASE("from_unique_ptr_with_deleter+as_shared_ptr-outliving_smart_holder", "[S]") {
+    // Exercises guarded_custom_deleter flag_ptr validity past destruction of smart_holder.
+    std::shared_ptr<int> longer_living;
+    {
+        std::unique_ptr<int, helpers::functor_builtin_delete<int>> orig_owner(new int(19));
+        auto hld = smart_holder::from_unique_ptr(std::move(orig_owner));
+        longer_living = hld.as_shared_ptr<int>();
+    }
+    REQUIRE(*longer_living == 19);
+}