From d664ee1430302e225e77b9cfc546e819a7983c3f Mon Sep 17 00:00:00 2001
From: Robby Findler <robby@racket-lang.org>
Date: Thu, 12 Jun 2014 23:50:10 -0500
Subject: [PATCH] change recursive-contract to reject cyclic values

For a few reasons:

 - this seems to fit better with how TR already works

 - cyclic values are something that, at least in my experience,
   abstractions are not generally equipped to handle (and,
   perhaps worse, don't seem all that useful when weighed against
   the non-termination problems that can come up)

 - there was a suspicious case in the projection where, when a
   cycle was detected the projection just returned its argument
   (the place in the diff for this commit where there is now a
   call to raise-blame-error). I couldn't get this to cause problems,
   but this might just be because I'm not smart enough

related to PR 14559
---
 .../racket-test/tests/racket/contract/recursive-contract.rkt | 2 +-
 racket/collects/racket/contract/private/base.rkt             | 5 +++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/pkgs/racket-pkgs/racket-test/tests/racket/contract/recursive-contract.rkt b/pkgs/racket-pkgs/racket-test/tests/racket/contract/recursive-contract.rkt
index 437dcc087c..7cd12ca654 100644
--- a/pkgs/racket-pkgs/racket-test/tests/racket/contract/recursive-contract.rkt
+++ b/pkgs/racket-pkgs/racket-test/tests/racket/contract/recursive-contract.rkt
@@ -74,7 +74,7 @@
       (letrec ([doll-ctc2 (or/c 'center (struct/c doll (recursive-contract doll-ctc2 #:flat)))])
         (contract doll-ctc2 (doll 4) 'pos 'neg))))
 
-  (test/spec-passed
+  (test/pos-blame
    'recursive-contract12
    '(let ()
       (define c
diff --git a/racket/collects/racket/contract/private/base.rkt b/racket/collects/racket/contract/private/base.rkt
index 954eb82cf8..40427fd47f 100644
--- a/racket/collects/racket/contract/private/base.rkt
+++ b/racket/collects/racket/contract/private/base.rkt
@@ -144,7 +144,8 @@
          =>
          (λ (ht)
            (cond
-             [(hash-ref ht val #f) val]
+             [(hash-ref ht val #f)
+              (raise-blame-error blame val '(given: "a value with a cycle"))]
              [else
               (hash-set! ht val #t)
               ((orig-projection blame) val)]))]
@@ -158,7 +159,7 @@
          =>
          (λ (ht)
            (cond
-             [(hash-ref ht val #f) #t]
+             [(hash-ref ht val #f) #f]
              [else
               (hash-set! ht val #t)
               (orig-first-order val)]))]