Enable new JSON field name conflict handling.

This will apply uniformly in both proto2 and proto3, taking into account `json_name` options.  See https://github.com/protocolbuffers/protobuf/pull/10750 for more details.

PiperOrigin-RevId: 502972769

java/core/src/test/proto/com/google/protobuf/test_bad_identifiers.proto

@@ -177,6 +177,9 @@ service TestConflictingMethodNames {
 }
 
 message TestConflictingFieldNames {
+  // TODO(b/261750190) Remove these tests once this behavior is removed.
+  option deprecated_legacy_json_field_conflicts = true;
+
   enum TestEnum {
     UNKNOWN = 0;
     FOO = 1;

python/google/protobuf/internal/descriptor_test.py

@@ -1186,6 +1186,7 @@ class MakeDescriptorTest(unittest.TestCase):
 
   def testCamelcaseName(self):
     descriptor_proto = descriptor_pb2.DescriptorProto()
+    descriptor_proto.options.deprecated_legacy_json_field_conflicts = True
     descriptor_proto.name = 'Bar'
     names = ['foo_foo', 'FooBar', 'fooBaz', 'fooFoo', 'foobar']
     camelcase_names = ['fooFoo', 'fooBar', 'fooBaz', 'fooFoo', 'foobar']
@@ -1200,6 +1201,7 @@ class MakeDescriptorTest(unittest.TestCase):
 
   def testJsonName(self):
     descriptor_proto = descriptor_pb2.DescriptorProto()
+    descriptor_proto.options.deprecated_legacy_json_field_conflicts = True
     descriptor_proto.name = 'TestJsonName'
     names = ['field_name', 'fieldName', 'FieldName',
              '_field_name', 'FIELD_NAME', 'json_name']

src/google/protobuf/descriptor.cc

@@ -5617,18 +5617,10 @@ void DescriptorBuilder::CheckFieldJsonNameUniqueness(
         this_type, field.name(), details.orig_name, existing_type,
         match.field->name(), name_suffix);
 
-    bool involves_default = !details.is_custom || !match.is_custom;
-    if (syntax == FileDescriptor::SYNTAX_PROTO2 && involves_default) {
-      // TODO(b/261750676) Upgrade this to an error once downstream proto2 files
-      // have been fixed.
-      AddWarning(message_name, field, DescriptorPool::ErrorCollector::NAME,
-                 error_message);
-    } else {
     AddError(message_name, field, DescriptorPool::ErrorCollector::NAME,
              error_message);
   }
 }
-}
 
 namespace {
 bool IsAllowedReservedField(const FieldDescriptorProto& field) {

src/google/protobuf/descriptor_unittest.cc

@@ -1277,6 +1277,8 @@ class StylizedFieldNamesTest : public testing::Test {
     AddExtensionRange(AddMessage(&file, "ExtendableMessage"), 1, 1000);
 
     DescriptorProto* message = AddMessage(&file, "TestMessage");
+    message->mutable_options()->set_deprecated_legacy_json_field_conflicts(
+        true);
     AddField(message, "foo_foo", 1, FieldDescriptorProto::LABEL_OPTIONAL,
              FieldDescriptorProto::TYPE_INT32);
     AddField(message, "FooBar", 2, FieldDescriptorProto::LABEL_OPTIONAL,
@@ -7012,7 +7014,7 @@ TEST_F(ValidationErrorTest, ValidateJsonNameConflictProto3) {
 }
 
 TEST_F(ValidationErrorTest, ValidateJsonNameConflictProto2) {
-  BuildFileWithWarnings(
+  BuildFileWithErrors(
       "name: 'foo.proto' "
       "syntax: 'proto2' "
       "message_type {"