hpb: Establish plumbing for returning a default msg for extensions

Not invoked just yet, as extant users may expect the old `GetOrPromote` pathway.

An incremental Δ for getting message extensions up to parity with proto2.

PiperOrigin-RevId: 695723050

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-//
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file or at
-// https://developers.google.com/open-source/licenses/bsd
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}

hpb/extension.h

@@ -72,9 +72,8 @@ struct UpbExtensionTrait<int64_t> {
 // not return an error if missing but the default msg
 template <typename T>
 struct UpbExtensionTrait<T> {
-  using DefaultType = int;
-  using ReturnType = int;
-  using DefaultFuncType = void (*)();
+  using DefaultType = std::false_type;
+  using ReturnType = Ptr<const T>;
 };
 
 // -------------------------------------------------------------------
@@ -109,7 +108,11 @@ class ExtensionIdentifier {
   const upb_MiniTableExtension* mini_table_ext_;
 
   typename UpbExtensionTrait<ExtensionType>::ReturnType default_value() const {
-    return default_val_;
+    if constexpr (IsHpbClass<ExtensionType>) {
+      return ExtensionType::default_instance();
+    } else {
+      return default_val_;
+    }
   }
 
   typename UpbExtensionTrait<ExtensionType>::DefaultType default_val_;
@@ -266,26 +269,10 @@ absl::Status SetExtension(
 
 template <typename T, typename Extendee, typename Extension,
           typename = hpb::internal::EnableIfHpbClassThatHasExtensions<T>>
-absl::StatusOr<Ptr<const Extension>> GetExtension(
+absl::StatusOr<typename internal::UpbExtensionTrait<Extension>::ReturnType>
+GetExtension(
     Ptr<T> message,
     const ::hpb::internal::ExtensionIdentifier<Extendee, Extension>& id) {
-  upb_MessageValue value;
-  const bool ok = ::hpb::internal::GetOrPromoteExtension(
-      hpb::interop::upb::GetMessage(message), id.mini_table_ext(),
-      hpb::interop::upb::GetArena(message), &value);
-  if (!ok) {
-    return ExtensionNotFoundError(
-        upb_MiniTableExtension_Number(id.mini_table_ext()));
-  }
-  return Ptr<const Extension>(::hpb::interop::upb::MakeCHandle<Extension>(
-      (upb_Message*)value.msg_val, hpb::interop::upb::GetArena(message)));
-}
-
-template <typename T, typename Extendee, typename Extension,
-          typename = hpb::internal::EnableIfHpbClassThatHasExtensions<T>>
-decltype(auto) GetExtension(
-    const T* message,
-    const hpb::internal::ExtensionIdentifier<Extendee, Extension>& id) {
   if constexpr (std::is_integral_v<Extension>) {
     auto default_val = hpb::internal::PrivateAccess::GetDefaultValue(id);
     absl::StatusOr<Extension> res =
@@ -294,10 +281,28 @@ decltype(auto) GetExtension(
             default_val);
     return res;
   } else {
-    return GetExtension(Ptr(message), id);
+    upb_MessageValue value;
+    const bool ok = ::hpb::internal::GetOrPromoteExtension(
+        hpb::interop::upb::GetMessage(message), id.mini_table_ext(),
+        hpb::interop::upb::GetArena(message), &value);
+    if (!ok) {
+      return ExtensionNotFoundError(
+          upb_MiniTableExtension_Number(id.mini_table_ext()));
+    }
+    return Ptr<const Extension>(::hpb::interop::upb::MakeCHandle<Extension>(
+        (upb_Message*)value.msg_val, hpb::interop::upb::GetArena(message)));
   }
 }
 
+template <typename T, typename Extendee, typename Extension,
+          typename = hpb::internal::EnableIfHpbClassThatHasExtensions<T>>
+absl::StatusOr<typename internal::UpbExtensionTrait<Extension>::ReturnType>
+GetExtension(
+    const T* message,
+    const hpb::internal::ExtensionIdentifier<Extendee, Extension>& id) {
+  return GetExtension(Ptr(message), id);
+}
+
 template <typename T, typename Extension>
 constexpr uint32_t ExtensionNumber(
     const internal::ExtensionIdentifier<T, Extension>& id) {

hpb_generator/gen_utils.cc

@@ -136,12 +136,14 @@ std::string DefaultValue(const FieldDescriptor* field) {
       return absl::StrCat(field->default_value_int32());
     case FieldDescriptor::CPPTYPE_INT64:
       return absl::StrCat(field->default_value_int64());
+    case FieldDescriptor::CPPTYPE_MESSAGE:
+      return "::std::false_type()";
     default:
-      // TODO: b/375460289 - implement rest of scalars + msg
+      // TODO: b/375460289 - implement rest of scalars
       ABSL_LOG(WARNING) << "Unsupported default value type (in-progress): <"
                         << field->cpp_type_name()
                         << "> For field: " << field->full_name();
-      return "-1";
+      return "::std::false_type()";
   }
 }