The behaviour of `prop:object-name` does not play well with `#:auto` fields.

What version of Racket are you using?
Racket 8.12 [cs]

What program did you run?

#lang racket/base

(struct thing-1 (a [b #:auto])
  #:auto-value 'b
  #:property prop:object-name (struct-field-index a))
(struct thing-2 (a [b #:auto])
  #:auto-value 'b
  #:property prop:object-name (struct-field-index b))

(object-name (thing-1 'a)) ; => 'a
(object-name (thing-2 'a)) ; => 'b (except it isn't)

What should have happened?
One would except to get back the values 'a and 'b.

If you got an error message, please include it here.
We get 'a as expected but instead of 'b we get the error:

guard-for-prop:object-name: field index >= initialized-field count for structure type
  field index: 1
  initialized-field count: 1

Please include any other relevant details
Looking at the racket source, It's not clear why this error happens but it does seem to be intentional. If that is the case then the correct course of action would probably be to alter the documentation of object-name to specify what kinds of fields can and cannot be used as names.

Reproduce the issue with the supplied struct definitions and calls to object-name. Start by tracing the implementation of prop:object-name and its handling of initialized versus auto fields; determine whether auto fields should be supported, then update the behavior or the object-name documentation accordingly and add or run a regression test for both examples.