Transform 'keyof (A | B)' to 'keyof A & keyof B'

Author: ahejlsberg

src/compiler/checker.ts

@@ -8268,7 +8268,8 @@ namespace ts {
         }
 
         function getIndexType(type: Type, stringsOnly = keyofStringsOnly): Type {
-            return type.flags & TypeFlags.Intersection ? getUnionType(map((<IntersectionType>type).types, t => getIndexType(t, stringsOnly))) :
+            return type.flags & TypeFlags.Union ? getIntersectionType(map((<IntersectionType>type).types, t => getIndexType(t, stringsOnly))) :
+                type.flags & TypeFlags.Intersection ? getUnionType(map((<IntersectionType>type).types, t => getIndexType(t, stringsOnly))) :
                 maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive) ? getIndexTypeForGenericType(<InstantiableType | UnionOrIntersectionType>type, stringsOnly) :
                 getObjectFlags(type) & ObjectFlags.Mapped ? getConstraintTypeFromMappedType(<MappedType>type) :
                 type === wildcardType ? wildcardType :