Introducing PYBIND11_USE_SMART_HOLDER_AS_DEFAULT macro (tested only undefined; there are many errors with the macro defined).

include/pybind11/cast.h

@@ -1560,6 +1560,7 @@ struct smart_holder_type_caster<std::unique_ptr<T const>> : smart_holder_type_ca
     operator std::unique_ptr<T const>() { return this->loaded_as_unique_ptr(); }
 };
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
 #define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T)                                                     \
     namespace pybind11 {                                                                          \
     namespace detail {                                                                            \
@@ -1579,10 +1580,36 @@ struct smart_holder_type_caster<std::unique_ptr<T const>> : smart_holder_type_ca
         : public smart_holder_type_caster<std::unique_ptr<T const>> {};                           \
     }                                                                                             \
     }
+#endif
 
 //DETAIL/SMART_HOLDER_TYPE_CASTERS_H///////////////////////////////////////////////////////////////
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+
 template <typename type, typename SFINAE = void> class type_caster : public type_caster_base<type> { };
+
+#else
+
+template <typename type, typename SFINAE = void> class type_caster : public smart_holder_type_caster<type> {};
+
+template <typename T>
+class type_caster<std::shared_ptr<T>> : public smart_holder_type_caster<std::shared_ptr<T>> {};
+
+template <typename T>
+class type_caster<std::shared_ptr<T const>>
+    : public smart_holder_type_caster<std::shared_ptr<T const>> {};
+
+template <typename T>
+class type_caster<std::unique_ptr<T>> : public smart_holder_type_caster<std::unique_ptr<T>> {};
+
+template <typename T>
+class type_caster<std::unique_ptr<T const>>
+    : public smart_holder_type_caster<std::unique_ptr<T const>> {};
+
+#define PYBIND11_SMART_HOLDER_TYPE_CASTERS(T)
+
+#endif
+
 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
@@ -2227,9 +2254,11 @@ protected:
     holder_type holder;
 };
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
 /// Specialize for the common std::shared_ptr, so users don't need to
 template <typename T>
 class type_caster<std::shared_ptr<T>> : public copyable_holder_caster<T, std::shared_ptr<T>> { };
+#endif
 
 template <typename type, typename holder_type>
 struct move_only_holder_caster {
@@ -2243,9 +2272,11 @@ struct move_only_holder_caster {
     static constexpr auto name = type_caster_base<type>::name;
 };
 
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
 template <typename type, typename deleter>
 class type_caster<std::unique_ptr<type, deleter>>
     : public move_only_holder_caster<type, std::unique_ptr<type, deleter>> { };
+#endif
 
 template <typename type, typename holder_type>
 using type_caster_holder = conditional_t<is_copy_constructible<holder_type>::value,

include/pybind11/pybind11.h

@@ -1257,7 +1257,13 @@ 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, std::unique_ptr<type>, options...>;
+    using holder_type = detail::exactly_one_t<is_holder,
+#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");

tests/test_class.cpp

@@ -505,7 +505,14 @@ CHECK_BASE(1); CHECK_BASE(2); CHECK_BASE(3); CHECK_BASE(4); CHECK_BASE(5); CHECK
 CHECK_ALIAS(1); CHECK_ALIAS(2); CHECK_NOALIAS(3); CHECK_ALIAS(4); CHECK_NOALIAS(5); CHECK_ALIAS(6); CHECK_ALIAS(7); CHECK_NOALIAS(8);
 #define CHECK_HOLDER(N, TYPE) static_assert(std::is_same<typename DoesntBreak##N::holder_type, std::TYPE##_ptr<BreaksBase<N>>>::value, \
         "DoesntBreak" #N " has wrong holder_type!")
-CHECK_HOLDER(1, unique); CHECK_HOLDER(2, unique); CHECK_HOLDER(3, unique); CHECK_HOLDER(4, unique); CHECK_HOLDER(5, unique);
+#define CHECK_SMART_HOLDER(N) static_assert(std::is_same<typename DoesntBreak##N::holder_type, smart_holder, \
+        "DoesntBreak" #N " has wrong holder_type!")
+CHECK_HOLDER(1, unique); CHECK_HOLDER(2, unique); CHECK_HOLDER(3, unique);
+#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+CHECK_HOLDER(4, unique); CHECK_HOLDER(5, unique);
+#else
+CHECK_SMART_HOLDER(4); CHECK_SMART_HOLDER(5);
+#endif
 CHECK_HOLDER(6, shared); CHECK_HOLDER(7, shared); CHECK_HOLDER(8, shared);
 
 // There's no nice way to test that these fail because they fail to compile; leave them here,