From c9d73aaa43d4f081109c7365c9db93e81a910636 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 28 Jan 2021 21:01:37 -0800
Subject: [PATCH] Giving up on idea to use legacy init_instance only if
 is_base_of<type_caster_generic, type_caster<T>. There are use cases in the
 wild that define both a custom type_caster and class_.

---
 include/pybind11/pybind11.h | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index efb460b1f..9d3fd5def 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -1506,17 +1506,16 @@ public:
     }
 
 private:
-    template <typename T = type,
-              detail::enable_if_t<
-                  std::is_base_of<detail::type_caster_generic, detail::type_caster<T>>::value,
-                  int> = 0>
+    template <
+        typename T = type,
+        detail::enable_if_t<!detail::is_smart_holder_type_caster<T>::value, int> = 0>
     void generic_type_initialize(const detail::type_record &record) {
         generic_type::initialize(record, &detail::type_caster_generic::local_load);
     }
 
     template <
         typename T = type,
-        detail::enable_if_t<detail::type_caster<T>::is_smart_holder_type_caster::value, int> = 0>
+        detail::enable_if_t<detail::is_smart_holder_type_caster<T>::value, int> = 0>
     void generic_type_initialize(const detail::type_record &record) {
         generic_type::initialize(record, detail::type_caster<T>::get_local_load_function_ptr());
     }
@@ -1565,10 +1564,9 @@ private:
     /// instance.  Should be called as soon as the `type` value_ptr is set for an instance.  Takes 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 T = type,
-              detail::enable_if_t<
-                  std::is_base_of<detail::type_caster_generic, detail::type_caster<T>>::value,
-                  int> = 0>
+    template <
+        typename T = type,
+        detail::enable_if_t<!detail::is_smart_holder_type_caster<T>::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()) {
@@ -1580,7 +1578,7 @@ private:
 
     template <
         typename T = type,
-        detail::enable_if_t<detail::type_caster<T>::is_smart_holder_type_caster::value, int> = 0>
+        detail::enable_if_t<detail::is_smart_holder_type_caster<T>::value, int> = 0>
     static void init_instance(detail::instance *inst, const void *holder_ptr) {
         detail::type_caster<T>::template init_instance_for_type<type>(inst, holder_ptr);
     }