[racket] Understanding local-expand with definition contexts

The error message is intended for "end users" and turns out to be
misleading for the implementor of an internal-definition context. The
documentation for `define-values` has essentially the same problem: it
describes how `define-values` should work in an internal-definition
context, but it doesn't say how the form interacts with `local-expand`.

A `define-values` form will only expand in a module or top-level
context. To implement an internal-definition context, you must expand
only far enough to see `define-values` form; in other words, supply
`#'define-values` in the stop list. Then, a partially expanded
`define-values` form must be recognized and handled explicitly, with
tools like `syntax-local-bind-syntaxes` or re-writing to
`letrec-values`, as appropriate for the definition context.

At Mon, 05 Jan 2015 14:49:03 +0000, Spencer Florence wrote:
> Hey all,
> 
> I'm trying to use 'local-expand', however it seems to think its never in a
> definition context. For example:
> 
> (require (for-syntax syntax/parse))
> (define-syntax (test stx)
>   (syntax-parse stx
>     [(_ e)
>      (define ctx
>        (if (list? (syntax-local-context))
>            (cons (gensym) (syntax-local-context))
>            (list (gensym))))
>      (local-expand
>       #'e ctx null
>       ;; result is the same with this uncommented
>       #;(syntax-local-make-definition-context))]))
> (let ()
>   (test (define x 1))
>   x)
> 
> errors with a "define-values: not in a definition context in:
> (define-values (x) 1)"
> 
> Can anyone provide any insight into what is going on?
> 
> --spf
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