Introducing 1. type_caster_for_class_, used in PYBIND11_MAKE_OPAQUE, and 2. default_holder_type, used in stl_bind.h.

include/pybind11/cast.h

@@ -1591,7 +1591,7 @@ struct smart_holder_type_caster<std::unique_ptr<T const, D>>
 
 #define PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS(T, H)
 
-template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
+template <typename T> class type_caster_for_class_ : public type_caster_base<T> {};
 
 #else
 
@@ -1605,27 +1605,29 @@ template <typename type, typename SFINAE = void> class type_caster : public type
     }                                                                                             \
     }
 
-template <typename type, typename SFINAE = void> class type_caster : public smart_holder_type_caster<type> {};
+template <typename T> class type_caster_for_class_ : public smart_holder_type_caster<T> {};
 
 template <typename T>
-class type_caster<std::shared_ptr<T>> : public smart_holder_type_caster<std::shared_ptr<T>> {};
+class type_caster_for_class_<std::shared_ptr<T>> : public smart_holder_type_caster<std::shared_ptr<T>> {};
 
 template <typename T>
-class type_caster<std::shared_ptr<T const>>
+class type_caster_for_class_<std::shared_ptr<T const>>
     : public smart_holder_type_caster<std::shared_ptr<T const>> {};
 
 template <typename T, typename D>
-class type_caster<std::unique_ptr<T, D>>
+class type_caster_for_class_<std::unique_ptr<T, D>>
     : public smart_holder_type_caster<std::unique_ptr<T, D>> {};
 
 template <typename T, typename D>
-class type_caster<std::unique_ptr<T const, D>>
+class type_caster_for_class_<std::unique_ptr<T const, D>>
     : public smart_holder_type_caster<std::unique_ptr<T const, D>> {};
 
 #define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T)
 
 #endif
 
+template <typename type, typename SFINAE = void> class type_caster : public type_caster_for_class_<type> { };
+
 template <typename type> using make_caster = type_caster<intrinsic_t<type>>;
 
 // Shortcut for calling a caster's `cast_op_type` cast operator for casting a type_caster to a T
@@ -2963,7 +2965,7 @@ handle type::handle_of() {
 
 #define PYBIND11_MAKE_OPAQUE(...) \
     namespace pybind11 { namespace detail { \
-        template<> class type_caster<__VA_ARGS__> : public type_caster_base<__VA_ARGS__> { }; \
+        template<> class type_caster<__VA_ARGS__> : public type_caster_for_class_<__VA_ARGS__> { }; \
     }}
 
 /// Lets you pass a type containing a `,` through a macro parameter without needing a separate

include/pybind11/pybind11.h

@@ -1240,6 +1240,13 @@ auto method_adaptor(Return (Class::*pmf)(Args...) const) -> Return (Derived::*)(
     return pmf;
 }
 
+template <typename T>
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+using default_holder_type = std::unique_ptr<T>;
+#else
+using default_holder_type = smart_holder;
+#endif
+
 template <typename type_, typename... options>
 class class_ : public detail::generic_type {
     template <typename T> using is_subtype = detail::is_strict_base_of<type_, T>;
@@ -1257,13 +1264,7 @@ public:
     using type = type_;
     using type_alias = detail::exactly_one_t<is_subtype, void, options...>;
     constexpr static bool has_alias = !std::is_void<type_alias>::value;
-    using holder_type = detail::exactly_one_t<is_holder,
+    using holder_type = detail::exactly_one_t<is_holder, default_holder_type<type>, options...>;
-#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
-        std::unique_ptr<type>
-#else
-        smart_holder
-#endif
-        , options...>;
 
     static_assert(detail::all_of<is_valid_class_option<options>...>::value,
             "Unknown/invalid class_ template parameters provided");
@@ -1297,14 +1298,15 @@ public:
 #ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
         record.default_holder = detail::is_instantiation<std::unique_ptr, holder_type>::value;
 #else
-        record.default_holder = std::is_same<holder_type, smart_holder>::value;
+        static constexpr bool holder_is_smart_holder = std::is_same<holder_type, smart_holder>::value;
+        record.default_holder = holder_is_smart_holder;
 #if 0
-        static_assert(!(detail::is_instantiation<std::unique_ptr, holder_type>::value && detail::is_smart_holder_type_caster<type>::value));
+        static constexpr bool type_caster_type_is_smart_holder_type_caster = detail::is_smart_holder_type_caster<type>::value;
-        static_assert(!(detail::is_instantiation<std::shared_ptr, holder_type>::value && detail::is_smart_holder_type_caster<type>::value));
+        static_assert(!(detail::is_instantiation<std::unique_ptr, holder_type>::value && type_caster_type_is_smart_holder_type_caster));
-        static_assert(detail::is_smart_holder_type_caster<type>::value == std::is_same<holder_type, smart_holder>::value);
+        static_assert(!(detail::is_instantiation<std::shared_ptr, holder_type>::value && type_caster_type_is_smart_holder_type_caster));
+        static_assert(holder_is_smart_holder == type_caster_type_is_smart_holder_type_caster);
 #endif
 #endif
-
         set_operator_new<type>(&record);
 
         /* Register base classes specified via template arguments to class_, if any */

include/pybind11/stl_bind.h

@@ -438,7 +438,7 @@ PYBIND11_NAMESPACE_END(detail)
 //
 // std::vector
 //
-template <typename Vector, typename holder_type = std::unique_ptr<Vector>, typename... Args>
+template <typename Vector, typename holder_type = default_holder_type<Vector>, typename... Args>
 class_<Vector, holder_type> bind_vector(handle scope, std::string const &name, Args&&... args) {
     using Class_ = class_<Vector, holder_type>;
 
@@ -599,7 +599,7 @@ template <typename Map, typename Class_> auto map_if_insertion_operator(Class_ &
 
 PYBIND11_NAMESPACE_END(detail)
 
-template <typename Map, typename holder_type = std::unique_ptr<Map>, typename... Args>
+template <typename Map, typename holder_type = default_holder_type<Map>, typename... Args>
 class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args&&... args) {
     using KeyType = typename Map::key_type;
     using MappedType = typename Map::mapped_type;

tests/test_class_sh_unique_ptr_member.cpp

@@ -41,6 +41,9 @@ private:
 } // namespace pybind11_tests
 
 PYBIND11_SMART_HOLDER_TYPE_CASTERS(pybind11_tests::class_sh_unique_ptr_member::pointee)
+PYBIND11_SMART_POINTER_HOLDER_TYPE_CASTERS(
+    pybind11_tests::class_sh_unique_ptr_member::ptr_owner,
+    std::unique_ptr<pybind11_tests::class_sh_unique_ptr_member::ptr_owner>)
 
 namespace pybind11_tests {
 namespace class_sh_unique_ptr_member {