From 0f90d880b47381698dc66aa6ef97a372e5a2e0ee Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 10 Sep 2016 07:13:57 -0700
Subject: [PATCH] Preserve type parameter types in narrowing

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 79488b7b546..98a26fb0b33 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8682,7 +8682,7 @@ namespace ts {
                 // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
                 // two types.
                 const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
-                return isTypeSubtypeOf(candidate, targetType) ? candidate :
+                return isTypeSubtypeOf(candidate, type) ? candidate :
                     isTypeAssignableTo(type, candidate) ? type :
                     isTypeAssignableTo(candidate, targetType) ? candidate :
                     getIntersectionType([type, candidate]);