   .code {
   var s = "abc"
   var i = s.startIndex
-  i = i.successor()
-  i = i.successor()
-  i = i.successor()
+  i = i.advancedBy(3)
   expectCrashLater()
   i = i.successor()
 }

  .code {
  var s = "abc"
  var i = s.startIndex
  i = i.successor()
  i = i.successor()
  i = i.successor()
  expectCrashLater()
  s[i]
}

  .code {
  var s = "abc"
  var i = s.utf8.startIndex
  i = i.advancedBy(3)
  expectCrashLater()
  i = i.successor()
}
