difference to protoc: tag number validation overly strict

[stapelberg]

The following .proto file fails parsing with protoreflect, but not with protoc:

syntax = "proto2";

message Foo {
  extend proto2.bridge.MessageSet {
    optional string message_set_extension = 999999999;
  }
}

I’m getting this error message from protoreflect:

msgsetrepro.proto:5:45: tag number 999999999 is higher than max allowed tag number (536870911)

I can’t find corresponding tag validation code in protoc itself, but this code seems related:

https://github.com/protocolbuffers/protobuf/blob/7f520092d9050d96fb4b707ad11a51701af4ce49/src/google/protobuf/compiler/parser.cc#L806

To me, it looks like for message sets, the maximum tag number is kint32max, not FieldDescriptor::kMaxNumber.