A small presentation of the generics scoping weirdness, but doesn't exhibit the bug.

bug-but-not.rkt

#lang racket/load
(module A racket
  (provide gen:foo (rename-out [*a a]))
  (define *a #t)
  (define a #f)
  (define-syntax gen:foo (list #'a)))

(module B racket
  (require 'A) (provide ga)
  (define-syntax (bar stx)
    (define (find-generic-method ctx gen-id delta info method-id)
      (define originals
        (let loop ([original-ids info]
                   [rev-originals '()])
          (cond
           [(null? original-ids) (reverse rev-originals)]
           [else
            (define original-id (car original-ids))
            (define context-id (syntax-local-get-shadower (delta original-id)))
            (cond
             [(free-identifier=? context-id method-id)
              (loop (cdr original-ids) (cons original-id rev-originals))]
             [else
              (loop (cdr original-ids) rev-originals)])])))
      (when (null? originals)
        (raise-syntax-error
         #f
         (format "~.s is not a method of generic interfaces ~.s"
                 (syntax-e method-id)
                 (syntax-e gen-id))
         ctx
         method-id))
      (unless (null? (cdr originals))
        (raise-syntax-error
         #f
         (format "multiple methods match ~.s in generic interface ~.s: ~.s"
                 (syntax-e method-id)
                 (syntax-e gen-id)
                 (map syntax-e originals))
         ctx
         method-id))
      (car originals))
    (syntax-case stx ()
        [(_ gen def derp)
         (let ()
           (define info (syntax-local-value #'gen (λ () #f)))
           (define delta (syntax-local-make-delta-introducer #'gen))
           (define methods (map delta info))
           (define method-id (find-generic-method stx #'gen delta info #'def))
           (with-syntax ([method method-id])
             #'(define derp method)))]))

  (bar gen:foo a ga))
(require 'B)
(display ga)