From 9b3b8e3203b71e24c0b285df598d964da6eaa8a2 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 17 Apr 2019 14:59:28 -0700
Subject: [PATCH] Ignore generic mapped types in isStringIndexSignatureOnlyType

---
 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 0c91e1ba062..b595d77295a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12043,7 +12043,7 @@ namespace ts {
         }
 
         function isStringIndexSignatureOnlyType(type: Type): boolean {
-            return type.flags & TypeFlags.Object && getPropertiesOfType(type).length === 0 && getIndexInfoOfType(type, IndexKind.String) && !getIndexInfoOfType(type, IndexKind.Number) ||
+            return type.flags & TypeFlags.Object && !isGenericMappedType(type) && getPropertiesOfType(type).length === 0 && getIndexInfoOfType(type, IndexKind.String) && !getIndexInfoOfType(type, IndexKind.Number) ||
                 type.flags & TypeFlags.UnionOrIntersection && every((<UnionOrIntersectionType>type).types, isStringIndexSignatureOnlyType) ||
                 false;
         }