fixes #481

tests/reject/tnotnil1.nim

@@ -1,6 +1,6 @@
 discard """
   errormsg: "'y' is provably nil"
-  line:25
+  line:38
 """
 
 import strutils
@@ -10,6 +10,19 @@ type
   TObj = object
     x, y: int
 
+type
+  superstring = string not nil
+
+
+proc q(s: superstring) =
+  echo s
+
+proc p2() =
+  var  a: string = "I am not nil" 
+  q(a) # but this should and does not
+
+p2()
+
 proc q(x: pointer not nil) =
   nil