Error when using oneof name in response body selector

As asked from my question in #585 I'm filing a new bug here.

I'm trying to have the response_body in the HTTP options use the oneof field, so that we can have alternate schemas in the response. However, this causes an error. Here is a minimal proto:

syntax = "proto3";

package services;

option go_package = "services;services";

import "google/api/annotations.proto";

service HostService {
  // This retrieves the host specified in the request using the basic view.
  rpc GetHost(GetHostRequest) returns (GetHostResponse) {
    option (google.api.http) = {
      get: "/hosts/{id}"
      response_body: "item"
    };
  }
}

message HostA {
  string foo = 1;
}

message HostB {
  string bar = 1;
}

message GetHostRequest {
  string id = 1;
}

message GetHostResponse {
  oneof item {
    HostA a = 1;
    HostB b = 2;
  }
}

Here is tree output:

.
├── gen
│   └── service
└── service
    ├── service.proto
    └── third_party
        ├── google
        │   └── api
        │       ├── annotations.proto
        │       ├── http.proto
        │       └── httpbody.proto
        └── protoc-gen-swagger
            └── options
                ├── annotations.proto
                └── openapiv2.proto

And here is my invocation: protoc -I. -I service/third_party --go_out=plugins=grpc,paths=source_relative:./gen --grpc-gateway_out=logtostderr=true,paths=source_relative:./gen service/service.proto

When I run the command I get: --grpc-gateway_out: no field "item" found in GetHostResponse

Note that if I change the response body def to select either "a" or "b" -- not actually what I want, but just to try it out -- I get a different error (which I think is already captured in a different ticket): --grpc-gateway_out: 224:9: expected operand, found 'if'