test that box-immutable doesn't break programs that use Boxof

Author: AlexKnauth

typed-racket-test/succeed/pr225-box-immutable.rkt

@@ -0,0 +1,12 @@
+#lang typed/racket
+
+;; box-immutable should not break programs that use Boxof
+
+(define b (box-immutable 5))
+
+(: f : (Boxof Integer) -> Integer)
+(define (f b)
+  (unbox b))
+
+(f b)
+