bugfix: 'not nil' and 'shared' types

tests/reject/tnotnil.nim

@@ -1,5 +1,5 @@
 discard """
-  line: 11
+  line: 22
   errormgs: "type mismatch"
 """
 
@@ -7,9 +7,17 @@ type
   PObj = ref TObj not nil
   TObj = object
     x: int
+  
+  MyString = string not nil
 
-var x: PObj = nil
+#var x: PObj = nil
 
 proc p(x: string not nil): int =
   result = 45
 
+proc q(x: MyString) = nil
+proc q2(x: string) = nil
+
+q2(nil)
+q(nil)
+