From fa36ff85ca78861fd24586c0403eeb4a1bf50305 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 15 Feb 2016 09:42:16 -0800
Subject: [PATCH] Don't widen undefined types in unions

---
 src/compiler/checker.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 96c6642fb26..b17de2a2b74 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6360,6 +6360,10 @@ namespace ts {
                 numberIndexInfo && createIndexInfo(getWidenedType(numberIndexInfo.type), numberIndexInfo.isReadonly));
         }
 
+        function getWidenedConstituentType(type: Type): Type {
+            return type.flags & TypeFlags.Undefined ? type : getWidenedType(type);
+        }
+
         function getWidenedType(type: Type): Type {
             if (type.flags & TypeFlags.RequiresWidening) {
                 if (type.flags & TypeFlags.Undefined) {
@@ -6372,7 +6376,7 @@ namespace ts {
                     return getWidenedTypeOfObjectLiteral(type);
                 }
                 if (type.flags & TypeFlags.Union) {
-                    return getUnionType(map((<UnionType>type).types, getWidenedType), /*noSubtypeReduction*/ true);
+                    return getUnionType(map((<UnionType>type).types, getWidenedConstituentType), /*noSubtypeReduction*/ true);
                 }
                 if (isArrayType(type)) {
                     return createArrayType(getWidenedType((<TypeReference>type).typeArguments[0]));