From 58a1b75e437c3cff526c6d6c170903cfcd862953 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 18 Jul 2024 22:31:21 -0700
Subject: [PATCH] Reinterpret `detail::type_info::default_holder` as "uses
 `std::unique_ptr` holder" (which is the original meaning, and compatible with
 the original smart_holder branch).

---
 include/pybind11/pybind11.h | 5 ++++-
 tests/test_class.cpp        | 5 +++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index cad97eaa2..82c5423de 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1849,7 +1849,10 @@ public:
         record.holder_size = sizeof(holder_type);
         record.init_instance = init_instance;
         record.dealloc = dealloc;
-        record.default_holder = std::is_same<holder_type, pybindit::memory::smart_holder>::value;
+
+        // A more fitting name would be uses_unique_ptr_holder.
+        record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
+
 #ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
         if (detail::is_instantiation<std::unique_ptr, holder_type>::value) {
             record.holder_enum_v = detail::holder_enum_t::std_unique_ptr;
diff --git a/tests/test_class.cpp b/tests/test_class.cpp
index e2981aca4..ba462fa85 100644
--- a/tests/test_class.cpp
+++ b/tests/test_class.cpp
@@ -211,11 +211,12 @@ TEST_SUBMODULE(class_, m) {
     m.def("mismatched_holder_1", []() {
         auto mod = py::module_::import("__main__");
         py::class_<MismatchBase1, std::shared_ptr<MismatchBase1>>(mod, "MismatchBase1");
-        py::class_<MismatchDerived1, MismatchBase1>(mod, "MismatchDerived1");
+        py::class_<MismatchDerived1, std::unique_ptr<MismatchDerived1>, MismatchBase1>(
+            mod, "MismatchDerived1");
     });
     m.def("mismatched_holder_2", []() {
         auto mod = py::module_::import("__main__");
-        py::class_<MismatchBase2>(mod, "MismatchBase2");
+        py::class_<MismatchBase2, std::unique_ptr<MismatchBase2>>(mod, "MismatchBase2");
         py::class_<MismatchDerived2, std::shared_ptr<MismatchDerived2>, MismatchBase2>(
             mod, "MismatchDerived2");
     });