From 12b80f3183cd97aacf448c29d609aa4b56be260c Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>
Date: Thu, 11 Jan 2018 16:26:38 -0800
Subject: [PATCH] Keep string index from intersections in base constraint of
 index type

---
 src/compiler/checker.ts | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d402f344d60..a32104c0535 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6468,7 +6468,7 @@ namespace ts {
         }
 
         function getConstraintOfIndexedAccess(type: IndexedAccessType) {
-            const transformed = getSimplifiedIndexedAccessType(type);
+            const transformed = getSubstitutedIndexedMappedType(type);
             if (transformed) {
                 return transformed;
             }
@@ -8383,6 +8383,11 @@ namespace ts {
                     getIntersectionType(stringIndexTypes)
                 ]);
             }
+            return getSubstitutedIndexedMappedType(type);
+        }
+
+        function getSubstitutedIndexedMappedType(type: IndexedAccessType): Type {
+            const objectType = type.objectType;
             // If the object type is a mapped type { [P in K]: E }, where K is generic, instantiate E using a mapper
             // that substitutes the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we
             // construct the type Box<T[X]>.