[racket] `require/typed` with a parameterized struct

From: Benjamin Greenman (blg59 at cornell.edu)
Date: Sat Dec 6 17:00:41 EST 2014

The typed racket reference shows how to require/typed a struct from an
untyped program [1]. I'd like to generalize this example to a struct with a
type parameter.

Here is the code I would like to write. How can I get this to type check?

#lang racket/base

(module UNTYPED racket/base
  (struct Tree (elem left right))
  (provide (struct-out Tree)))

(module TYPED typed/racket/base
  (require/typed 'UNTYPED
                 [#:struct (A) Tree
                   ([elem : A]
                    [left : Tree A]
                    [right : Tree A])]))

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