implement unions of box types

Author: AlexKnauth

typed-racket-lib/typed-racket/types/union.rkt

@@ -37,6 +37,14 @@
     [(_ _) #:when (currently-subtyping?) (cons a b)]
     [((? (λ _ (subtype a b*))) _) b]
     [((? (λ _ (subtype b* a))) _) (list a)]
+    ;; the union of two box types is a box type where the write type has to satisfy both write types,
+    ;; and the read type can satisfy either of the two read types
+    [((Box: a-w a-r) (list-no-order (Box: b-w b-r) bs ...))
+     (define w
+       (cond [(subtype a-w b-w) a-w]
+             [(subtype b-w a-w) b-w]
+             [else -Bottom]))
+     (cons (make-Box w (Un a-r b-r)) bs)]
     [(_ _) (cons a (filter-not (λ (b-elem) (subtype b-elem a)) b))]))
 
 ;; Type -> List[Type]