Add a unittest locking down presence behavior for proto3 extensions

PiperOrigin-RevId: 613016814

src/google/protobuf/BUILD.bazel

@@ -828,6 +828,7 @@ filegroup(
         "unittest_proto3_arena.proto",
         "unittest_proto3_arena_lite.proto",
         "unittest_proto3_bad_macros.proto",
+        "unittest_proto3_extensions.proto",
         "unittest_proto3_lite.proto",
         "unittest_proto3_optional.proto",
         "unittest_retention.proto",

src/google/protobuf/extension_set_unittest.cc

@@ -32,6 +32,7 @@
 #include "google/protobuf/unittest.pb.h"
 #include "google/protobuf/unittest.pb.h"
 #include "google/protobuf/unittest_mset.pb.h"
+#include "google/protobuf/unittest_proto3_extensions.pb.h"
 #include "google/protobuf/wire_format.h"
 #include "google/protobuf/wire_format_lite.h"
 
@@ -1393,6 +1394,19 @@ TEST(ExtensionSetTest, Proto3PackedDynamicExtensions) {
   EXPECT_EQ(reserialized_options, "\xca\xb5\x18\x01\x01");
 }
 
+TEST(ExtensionSetTest, Proto3ExtensionPresenceSingular) {
+  using protobuf_unittest::Proto3FileExtensions;
+  FileDescriptorProto file;
+
+  EXPECT_FALSE(file.options().HasExtension(Proto3FileExtensions::singular_int));
+  EXPECT_EQ(file.options().GetExtension(Proto3FileExtensions::singular_int), 0);
+
+  file.mutable_options()->SetExtension(Proto3FileExtensions::singular_int, 1);
+
+  EXPECT_TRUE(file.options().HasExtension(Proto3FileExtensions::singular_int));
+  EXPECT_EQ(file.options().GetExtension(Proto3FileExtensions::singular_int), 1);
+}
+
 TEST(ExtensionSetTest, BoolExtension) {
   unittest::TestAllExtensions msg;
   uint8_t wire_bytes[2] = {13 * 8, 42 /* out of bounds payload for bool */};

src/google/protobuf/unittest_proto3_extensions.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+package protobuf_unittest;
+
+import "google/protobuf/descriptor.proto";
+
+// For testing proto3 extension behaviors.
+message Proto3FileExtensions {
+  extend google.protobuf.FileOptions {
+    int32 singular_int = 1001;
+    repeated int32 repeated_int = 1002;
+  }
+}