From b6ec9512076a30f2c635cb58e59282ea6df6ca5b Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 27 Jul 2017 09:50:57 -0700
Subject: [PATCH] Add missing check in getIndexedAccessForMappedType

---
 src/compiler/checker.ts | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ec9867fa9b7..05b06860ecf 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7573,10 +7573,16 @@ namespace ts {
         }
 
         function getIndexedAccessForMappedType(type: MappedType, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode) {
-            const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? <ElementAccessExpression>accessNode : undefined;
-            if (accessExpression && isAssignmentTarget(accessExpression) && type.declaration.readonlyToken) {
-                error(accessExpression, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(type));
-                return unknownType;
+            if (accessNode) {
+                // Check if the index type is assignable to 'keyof T' for the object type.
+                if (!isTypeAssignableTo(indexType, getIndexType(type))) {
+                    error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(type));
+                    return unknownType;
+                }
+                if (accessNode.kind === SyntaxKind.ElementAccessExpression && isAssignmentTarget(accessNode) && type.declaration.readonlyToken) {
+                    error(accessNode, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(type));
+                    return unknownType;
+                }
             }
             const mapper = createTypeMapper([getTypeParameterFromMappedType(type)], [indexType]);
             const templateMapper = type.mapper ? combineTypeMappers(type.mapper, mapper) : mapper;