Consider object literals in unions to have properties of type 'undefined'

Author: ahejlsberg

src/compiler/checker.ts

@@ -8016,10 +8016,13 @@ namespace ts {
                     else if (isUnion) {
                         const indexInfo = !isLateBoundName(name) && (isNumericLiteralName(name) && getIndexInfoOfType(type, IndexKind.Number) || getIndexInfoOfType(type, IndexKind.String));
                         if (indexInfo) {
-                            checkFlags |= indexInfo.isReadonly ? CheckFlags.Readonly : 0;
-                            checkFlags |= CheckFlags.WritePartial;
+                            checkFlags |= CheckFlags.WritePartial | (indexInfo.isReadonly ? CheckFlags.Readonly : 0);
                             indexTypes = append(indexTypes, isTupleType(type) ? getRestTypeOfTupleType(type) || undefinedType : indexInfo.type);
                         }
+                        else if (isObjectLiteralType(type)) {
+                            checkFlags |= CheckFlags.WritePartial;
+                            indexTypes = append(indexTypes, undefinedType);
+                        }
                         else {
                             checkFlags |= CheckFlags.ReadPartial;
                         }
@@ -20181,7 +20184,8 @@ namespace ts {
             let propType: Type;
             const leftType = checkNonNullExpression(left);
             const parentSymbol = getNodeLinks(left).resolvedSymbol;
-            const apparentType = getApparentType(getWidenedType(leftType));
+            // We widen array literals to get type any[] instead of undefined[] in non-strict mode
+            const apparentType = getApparentType(isEmptyArrayLiteralType(leftType) ? getWidenedType(leftType) : leftType);
             if (isTypeAny(apparentType) || apparentType === silentNeverType) {
                 if (isIdentifier(left) && parentSymbol) {
                     markAliasReferenced(parentSymbol, node);