From 5bb8d2a590899fc225df2472f41be1b52c41678e Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 18 Jan 2018 17:15:48 -0800
Subject: [PATCH] Properly handle contravariant inferences in
 inferReverseMappedType

---
 src/compiler/checker.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 79430adf777..519a9f4cf40 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11029,7 +11029,9 @@ namespace ts {
             const templateType = getTemplateTypeFromMappedType(target);
             const inference = createInferenceInfo(typeParameter);
             inferTypes([inference], sourceType, templateType);
-            return inference.candidates ? getUnionType(inference.candidates, UnionReduction.Subtype) : emptyObjectType;
+            return inference.candidates ? getUnionType(inference.candidates, UnionReduction.Subtype) :
+                inference.contraCandidates ? getCommonSubtype(inference.contraCandidates) :
+                emptyObjectType;
         }
 
         function getUnmatchedProperty(source: Type, target: Type, requireOptionalProperties: boolean) {