From 04339d2df7c3483bcbc44a92a8e4840274aaef02 Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <nathansa@microsoft.com>
Date: Mon, 28 Aug 2017 09:05:56 -0700
Subject: [PATCH] Disallow T[K] = T[keyof T] where K extends keyof T

`K = keyof T` was already correctly disallowed.
---
 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 8e34b24cf1b..27b0752f53f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5914,7 +5914,7 @@ namespace ts {
             const keepTypeParameterForMappedType = baseObjectType && getObjectFlags(baseObjectType) & ObjectFlags.Mapped &&
                 type.indexType.flags & TypeFlags.TypeParameter;
             const baseIndexType = !keepTypeParameterForMappedType && getBaseConstraintOfType(type.indexType);
-            if (baseObjectType && baseIndexType === stringType && !getIndexInfoOfType(baseObjectType, IndexKind.String)) {
+            if (baseIndexType === stringType && (!baseObjectType || !getIndexInfoOfType(baseObjectType, IndexKind.String))) {
                 // getIndexedAccessType returns `any` for X[string] where X doesn't have an index signature.
                 // to avoid this, return `undefined`.
                 return undefined;