fix the contract context messages for the case of ->*'s #:rest arguments

collects/racket/contract/private/arrow.rkt

@@ -476,8 +476,11 @@ v4 todo:
         (for/list ([dom (in-list doms-proj)]
                    [n (in-naturals 1)])
           (dom (blame-add-context orig-blame 
+                                  (if (and has-rest?
+                                           (n . > . dom-length))
+                                      "the rest argument of"
                                       (format "the ~a argument of"
-                                          (n->th n))
+                                              (n->th n)))
                                   #:swap? #t))))
       (define partial-optional-doms 
         (for/list ([dom (in-list doms-optional-proj)]

collects/tests/racket/contract-test.rktl

@@ -12837,6 +12837,31 @@ so that propagation occurs.
                  0)
                 1)))
   
+  (ctest '("an element of" "the rest argument of")
+         extract-context-lines
+         (λ ()
+           ((contract (->* () #:rest (listof number?) number?)
+                      +
+                      'pos 'neg)
+            1 "a")))
+
+  (ctest '("the 2nd argument of")
+         extract-context-lines
+         (λ ()
+           ((contract (->* (number? number?) #:rest (listof number?) number?)
+                      +
+                      'pos 'neg)
+            1 "a")))
+
+  (ctest '("an element of" "the rest argument of")
+         extract-context-lines
+         (λ ()
+           ((contract (->* (number?) #:rest (listof number?) number?)
+                      +
+                      'pos 'neg)
+            1 "a")))
+
+  
   (let* ([blame-pos (contract-eval '(make-blame #'here #f (λ () 'integer?) 'positive 'negative #t))]
          [blame-neg (contract-eval `(blame-swap ,blame-pos))])
     (ctest "something ~a" blame-fmt->-string ,blame-neg "something ~a")