How can I mention the underlying datatype for an enum (c++) in protocol buffers?

Question

I want to declare an enum, but I want it to be of size "int8_t" explicitly. As per protbuf docs[1], the generated enum in c++ is a standard c++ enum without explicitly mentioning the underlying type.

i.e. A declaration like this:

enum Foo {
  VALUE_A = 0;
  VALUE_B = 5;
  VALUE_C = 1234;
}

Would generate something like this:

enum Foo {
      VALUE_A = 0;
      VALUE_B = 5;
      VALUE_C = 1234;
      Foo_INT_MIN_SENTINEL_DO_NOT_USE_ = ::google::protobuf::kint32min,
      Foo_INT_MIN_SENTINEL_DO_NOT_USE_ = ::google::protobuf::kint32max
    }

Is there anyway I can force protobuf to generate the enum with the type explicitly mentioned in the definition? Something like this:

enum Foo : int8_t {
      VALUE_A = 0;
      VALUE_B = 5;
      VALUE_C = 1234;
      Foo_INT_MIN_SENTINEL_DO_NOT_USE_ = ::google::protobuf::kint32min,
      Foo_INT_MIN_SENTINEL_DO_NOT_USE_ = ::google::protobuf::kint32max
    }

Why do I need this?

I want to forward declare my proto enum, and forward declaration of enums need you to:

1. Mention the datatype (so that compiler can assume the size used by the object)

2. The declaration of the enum must mention the exact same type as the definition [2]

Any help would be much appreciated.

[1] https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#enum

[2] https://stackoverflow.com/a/42768812/3069919