Properly handle contravariant inferences in inferReverseMappedType

ahejlsberg · ahejlsberg · commit 5bb8d2a59089 · 2018-01-18T17:15:48.000-08:00
diff --git 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) {
