From 8e337b5121ec9243c07c014e315cfce9b2be597f Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 24 Jan 2018 13:51:37 -0800
Subject: [PATCH] Fix bug in resolveMappedTypeMembers

---
 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 8798b22bfdf..6fe8f5d2e14 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5788,7 +5788,7 @@ namespace ts {
             // and T as the template type.
             const typeParameter = getTypeParameterFromMappedType(type);
             const constraintType = getConstraintTypeFromMappedType(type);
-            const templateType = getTemplateTypeFromMappedType(type);
+            const templateType = getTemplateTypeFromMappedType(<MappedType>type.target || type);
             const modifiersType = getApparentType(getModifiersTypeFromMappedType(type)); // The 'T' in 'keyof T'
             const templateReadonly = !!type.declaration.readonlyToken;
             const templateOptional = !!type.declaration.questionToken;