"Map fields are not allowed in oneofs"

[jkopczyn]

What version of protobuf and what language are you using?
Version: libprotoc 3.6.1 (proto3 syntax explicitly set in the source)
Language: Go

What operating system (Linux, Windows, ...) and version?
Linux, specifically gLinux Rodete

What runtime / compiler are you using (e.g., python version or gcc version)
I don't think that's a meaningful question here.

What did you do?
Steps to reproduce the behavior:

syntax = "proto3";

package foo;

message Step {

  oneof args_spec {
    map<string, string> arbitrary_args = 3;

    Empty empty_args = 4;
  }
}

message Empty {}

$ protoc foo.proto --go_out=.

What did you expect to see
"You can add fields of any type, but cannot use repeated fields.", so I expected to get a foo.pb.go file.

What did you see instead?
foo.proto:8:8: Map fields are not allowed in oneofs.

Anything else we should know about your project / environment
I'm using the Chrome infrastructure package manager, CIPD, to install protoc. That's probably not relevant, but might be.

[jkopczyn]

It has been brought to my attention elsewhere that this is because maps are represented as repeated fields internally. Recommendation: change the error message to

Maps are considered repeated fields, which are not allowed in oneofs.

I am also filing a PR with the doc maintainers to make this clearer in the docs. Belt and suspenders.

[znkr]

Removing the go label, as this is not specific to go but a general limitation of the protobuf language.