redirect-generics etc. only work on (some) structs

[LiberalArtist]

This program:

#lang racket
(require racket/generic)
(chaperone-generics gen:dict #hash()
  [dict-ref (λ (super)
              super)])

produces the following error:

901a31301a/collects/racket/generic.rkt:202:9: gen:dict: contract violation
  expected: gen:dict?
  given: '#hash()

The error message seems to be coming from the internal struct-type property: in particular, note that (object-name dict?) is 'dict (there is no gen:dict?) and #hash() satisfies dict?.

Strictly speaking, getting some error here is consistent with the docs for impersonate-generics, which state that the supplied value "must be a structure that implements the generic interface", but I did not realize that meant the value "must be a structure" specifically. In fact, AFAICT from the implementation, "must be a structure" even more specifically means that the structure type must be created with the internal property attached, either via #:methods or make-struct-type-property/generic/make-generic-struct-type-property: structures handled via #:defaults or #:fast-defaults are not supported. There doesn't seem to be any public API to test for this requirement short of catching and trying to recognize exceptions.

At a minimum, the error message should be fixed, and I think the docs should be clarified.

I can also see at least two reasons to consider changing the behavior:

1. In Racket in general, conceptually "everything is a struct" under some sufficiently-powerful inspector. Treating structs differently than non-structs is unusual.
2. A programmer who wants to interpose on generic methods and doesn't want to hard-code special cases for the currently-supported non-structs can currently do so by creating some wrapper struct that implements the interface by simply bouncing each method through the generic version with define/generic. Even if it used essentially that implementation strategy, an implementation in racket/generic would be much more ergonomic, since racket/generic already knows the complete list of methods (including optional methods that may be added over time), the arity of each method, and which is the "this" argument to each.