Add a comment that `is_cleared` is always false for repeated fields.

Also added a test to verify that no uninitialized access happens in NumExtensions.

Creation `Extension()` does zero initialization. Illustration: https://gcc.godbolt.org/z/37j8hrb9T

PiperOrigin-RevId: 698094874

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";
-}

src/google/protobuf/extension_set.cc

@@ -385,6 +385,7 @@ void* ExtensionSet::MutableRawRepeatedField(int number, FieldType field_type,
     extension->is_pointer = true;
     extension->type = field_type;
     extension->is_packed = packed;
+    ABSL_DCHECK(!extension->is_cleared);
 
     switch (WireFormatLite::FieldTypeToCppType(
         static_cast<WireFormatLite::FieldType>(field_type))) {

src/google/protobuf/extension_set.h

@@ -762,9 +762,10 @@ class PROTOBUF_EXPORT ExtensionSet {
     // For singular types, indicates if the extension is "cleared".  This
     // happens when an extension is set and then later cleared by the caller.
     // We want to keep the Extension object around for reuse, so instead of
-    // removing it from the map, we just set is_cleared = true.  This has no
+    // removing it from the map, we just set is_cleared = true.
-    // meaning for repeated types; for those, the size of the RepeatedField
+    //
-    // simply becomes zero when cleared.
+    // This is always set to false for repeated types.
+    // The size of the RepeatedField simply becomes zero when cleared.
     bool is_cleared : 1;
 
     // For singular message types, indicates whether lazy parsing is enabled

src/google/protobuf/extension_set_unittest.cc

@@ -1422,6 +1422,19 @@ TEST(ExtensionSetTest, ConstInit) {
   EXPECT_EQ(set.NumExtensions(), 0);
 }
 
+// Make sure that is_cleared is set correctly for repeated fields.
+TEST(ExtensionSetTest, NumExtensionsWithRepeatedFields) {
+  unittest::TestAllExtensions msg;
+  ExtensionSet set;
+  const auto* desc =
+      unittest::TestAllExtensions::descriptor()->file()->FindExtensionByName(
+          "repeated_int32_extension");
+  ASSERT_NE(desc, nullptr);
+  set.MutableRawRepeatedField(desc->number(), WireFormatLite::TYPE_INT32, false,
+                              desc);
+  EXPECT_EQ(set.NumExtensions(), 1);
+}
+
 TEST(ExtensionSetTest, ExtensionSetSpaceUsed) {
   unittest::TestAllExtensions msg;
   size_t l = msg.SpaceUsedLong();