Add test for PR 10318.

Closes PR 10318.

collects/tests/typed-scheme/succeed/pr10318.rkt

@@ -0,0 +1,35 @@
+#lang typed-scheme
+
+(define-struct: rect ((nw : Symbol) (width : Number) (height : Number)))
+(define-struct: circ ((cntr : Symbol)  (radius : Number)))
+(define-struct: over ((top : Shape) (bot : Shape)))
+
+(define-type-alias Shape (Rec Shape (U Plain over [Listof Plain])))
+(define-type-alias Plain (U rect circ))
+;; (define-type-alias Rect (U (make-rect Posn Number Number)))
+;; Circ  = (make-circ Posn Number)
+
+(: area (Shape -> Number))
+;; the area of all rectangles in this s
+(define (area s)
+  (cond
+    [(plain? s) (plain-area s)]
+    [(over? s) (+ (area (over-top s)) (area (over-bot s)))]
+    [else (apply + (map rect-area (filter rect? s)))]))
+
+(: plain? (Any -> Boolean : Plain))
+;; is this p a plain shape?
+(define (plain? p)
+  (or (rect? p) (circ? p)))
+
+(: plain-area (Plain -> Number))
+;; the area of this plain shape s
+(define (plain-area s)
+  (cond
+    [(rect? s) (rect-area s)]
+    [(circ? s) 0]))
+
+(: rect-area (rect -> Number))
+;; the area of this rectangle r
+(define (rect-area s)
+  (* (rect-width s) (rect-height s)))