allow 'contract' to use #f for the positive

name, but still disallow #f at the blame record
level

closes PR 14387

pkgs/racket-pkgs/racket-test/tests/racket/contract/blame.rkt

@@ -48,6 +48,9 @@
         [rename five chaperone-five chaperone-blame-ok/c]
         [rename five flat-five flat-blame-ok/c]))))
   
+  (test/no-error
+   '(contract string? "string" #f 'neg))
+  
   (begin
     (test/no-error
      '(dynamic-require ''blame-ok-dynamic 'impersonator-five))

racket/collects/racket/contract/private/base.rkt

@@ -53,7 +53,15 @@
       (make-blame (build-source-location loc)
                   name
                   (λ () (contract-name c))
-                  pos
+                  
+                  ;; hack! We need to allow pos = #f for backwards
+                  ;; compatibility, but we cannot put #f into the
+                  ;; blame struct now because #f means that the 
+                  ;; name is not known. Since #f is not a very good
+                  ;; name, we'll just put something stupid here 
+                  ;; instead of changing the library around.
+                  (or pos "false")
+                                  
                   (if cvfp #f neg)
                   #t))
     (define new-val