> Your guess is as good as mine for why an enum can’t be used as a map key.

I filed an issue requesting this and it was denied with an explanation:

https://github.com/protocolbuffers/protobuf/issues/7791#issu...

> Contrast this behavior against message types. While scalar fields are dumb, the behavior for message fields is outright insane.

The reason messages are initialized is that you can easily set a deep property path:

```

message SomeY { string example = 1; }

message SomeX { SomeY y = 1; }

```

later, in java:

```

SomeX some = SomeX.newBuilder();

some.getY().setExample("hello"); // does not produce npe

```

in kotlin this syntax makes even more sense:

```

some {

  y.example = "hello". // does not produce npe

```

> It’s impossible to differentiate a field that was missing in a protobuffer from one that was assigned to the default value.

This is purportedly fixed in proto3 and latest SDK copies (IIRC)