fix bug in hashing shortcut for transparency

collects/tests/racket/basic.rktl

@@ -2562,6 +2562,16 @@
   (equal-hash-code ht)
   (equal-secondary-hash-code ht))
 
+;; Check that an equal hash code on an
+;;  mutable, opaque structure does not
+;;  see mutation.
+(let ()
+  (struct a (x [y #:mutable]))
+  (define an-a (a 1 2))
+  (define v (equal-hash-code an-a))
+  (set-a-y! an-a 8)
+  (test v equal-hash-code an-a))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Misc
 

src/racket/src/struct.c

@@ -3061,7 +3061,7 @@ int scheme_struct_is_transparent(Scheme_Object *s)
 
   stype = ((Scheme_Structure *)s)->stype;
 
-  for (p = stype->name_pos; p--; ) {
+  for (p = stype->name_pos + 1; p--; ) {
     if (SCHEME_TRUEP(stype->parent_types[p]->inspector))
       return 0;
   }