[racket-users] limitation of TR contract translation?

['John Clements' via Racket Users]

It seems that the conversion of TR types to contracts can result in contracts 
like (or/c (cons/c any/c any/c) (cons/c any/c any/c)) that apparently cause 
errors when run. 

I’m too tired to do a good job of providing a full example… no, actually, it’s 
pretty easy. Run this program:

#lang racket


(module foo typed/racket

  (provide val->autorun-table)
  
  (define-type Autorun-Result
    (U (Pair 'success Any)
       (Pair 'fail Any)))

  (define-type Autorun-Entry
    (List String Autorun-Result))
  (define-predicate autorun-entry? Autorun-Entry)

  (define-type Autorun-Table (Listof Autorun-Entry))
  (define-predicate autorun-table? Autorun-Table)

  (define (val->autorun-table [val : Any])
    : (U False Autorun-Table)
    (cond
      [(list? val)
       (define first-bad (findf (compose not autorun-entry?) val))
       (when first-bad
         (fprintf (current-error-port)
                  "not an expected autorun entry:\n~e\n"
                  first-bad))
       (cast val Autorun-Table)]
      [else
       (error 'maybe-fetch-autorun-results
              "result of read wasn't a list. moreinfo.")])))

(require 'foo)

(val->autorun-table '(("1234" (fail (zz 1234) (d 9)))))


… see this error:

val->autorun-table: broke its own contract
  two of the clauses in the or/c might both match: (cons/c any/c Any) and 
(cons/c any/c Any)
  produced: '(fail (zz 1234) (d 9))
  in: the 2nd element of
      an element of
      a part of the or/c of
      the range of
      (->
       any/c
       (or/c
        #f
        (listof
         (list/c
          any/c
          (or/c
           (cons/c any/c Any)
           (cons/c any/c Any))))))
  contract from: (anonymous-module foo)
  blaming: (anonymous-module foo)
   (assuming the contract is correct)
  at: 38-unsaved-editor:20.11


Basically, it looks like the type is getting simplified a bit as it’s being 
translated into a contract, and unfortunately, the resulting contract is 
unsatisfiable?

Is this a bug, or a known limitation?

John


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