What version of protobuf and what language are you using?
Version: v3.6.1
Language: N/A

What operating system (Linux, Windows, ...) and version?
Linux (Manjaro)

What runtime / compiler are you using (e.g., python version or gcc version)
N/A

What did you do?
Steps to reproduce the behavior:

1. Copy the example code for enums with aliases from this documentation to a .proto file, resulting the following contents:

syntax = "proto3";

enum EnumAllowingAlias {
  option allow_alias = true;
  UNKNOWN = 0;
  STARTED = 1;
  RUNNING = 1;
}

enum EnumNotAllowingAlias {
  UNKNOWN = 0;
  STARTED = 1;
  // RUNNING = 1;  // Uncommenting this line will cause a compile error inside Google and a warning message outside.
}

2. Attempt to compile the proto file with `protoc -oout testfile.proto

What did you expect to see
Code copied directly from the official documentation should compile.

What did you see instead?
Code copied directly from the official documentation fails to compile with the following errors:

test.proto:11:3: "UNKNOWN" is already defined.
test.proto:11:3: Note that enum values use C++ scoping rules, meaning that enum values are siblings of their type, not children of it.  Therefore, "UNKNOWN" must be unique within the global scope, not just within "EnumNotAllowingAlias".
test.proto:12:3: "STARTED" is already defined.
test.proto:12:3: Note that enum values use C++ scoping rules, meaning that enum values are siblings of their type, not children of it.  Therefore, "STARTED" must be unique within the global scope, not just within "EnumNotAllowingAlias".

Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).

Anything else we should know about your project / environment