From b338fc6b6465079e2dccadd2687a8798b9d91c67 Mon Sep 17 00:00:00 2001
From: Asumu Takikawa <asumu@ccs.neu.edu>
Date: Wed, 1 Jun 2016 14:22:08 -0400
Subject: [PATCH] Be less conservative about struct overlap

Fixes issue #366
---
 .../typed-racket/types/overlap.rkt            | 23 ++++++++++++++++---
 typed-racket-test/succeed/gh-issue-366.rkt    | 15 ++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)
 create mode 100644 typed-racket-test/succeed/gh-issue-366.rkt

diff --git a/typed-racket-lib/typed-racket/types/overlap.rkt b/typed-racket-lib/typed-racket/types/overlap.rkt
index e9127a22..fa308e71 100644
--- a/typed-racket-lib/typed-racket/types/overlap.rkt
+++ b/typed-racket-lib/typed-racket/types/overlap.rkt
@@ -101,7 +101,24 @@
       [((Struct: n #f flds _ _ _)
         (StructTop: (Struct: n* #f flds* _ _ _)))
        #f]
-      [((and t1 (Struct: _ _ _ _ #f _))
-        (and t2 (Struct: _ _ _ _ #f _)))
-       (or (subtype t1 t2) (subtype t2 t1))]
+      [((and t1 (Struct: _ _ _ _ _ _))
+        (and t2 (Struct: _ _ _ _ _ _)))
+       (or (subtype t1 t2) (subtype t2 t1)
+           (parent-of? t1 t2) (parent-of? t2 t1))]
       [(_ _) #t])]))
+
+;; Type Type -> Boolean
+;; Given two struct types, check if the second is a parent struct
+;; type of the other (though possibly at different type instantiations
+;; if they are polymorphic)
+(define (parent-of? t1 t2)
+  (match* (t1 t2)
+    [((Struct: _ (Struct: pname _ _ _ _ _) _ _ _ _)
+      (Struct: pname _ _ _ _ _))
+     #t]
+    [((Struct: _ #f _ _ _ _)
+      other)
+     #f]
+    [((Struct: _ parent _ _ _ _)
+      other)
+     (parent-of? parent other)]))
diff --git a/typed-racket-test/succeed/gh-issue-366.rkt b/typed-racket-test/succeed/gh-issue-366.rkt
new file mode 100644
index 00000000..d7803dec
--- /dev/null
+++ b/typed-racket-test/succeed/gh-issue-366.rkt
@@ -0,0 +1,15 @@
+#lang typed/racket
+
+;; Test for Github issue #366
+
+(struct Parent () #:transparent)
+(struct (A B) S0 Parent ([a : A] [b : B]) #:transparent)
+(struct (B C) S1 Parent ([b : B] [c : C]) #:transparent)
+
+(define v : (S1 'x Integer) (S1 'x 2))
+(ann (if (S0? v)
+         (S0-b v) ;; ERROR HERE
+         (if (S1? v)
+             (S1-b v)
+             (void)))
+     'x)