Introducing PYBIND11_USE_SMART_HOLDER_AS_DEFAULT macro (tested only undefined;...

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

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

@@ -504,7 +504,14 @@ 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,