Fixed bug with accessing length tuple property

Fixes #3

Author: timocov

src/transformer.ts

@@ -249,6 +249,14 @@ function createTransformerFactory(program: ts.Program, options?: Partial<RenameO
 
 		// tslint:disable-next-line:cyclomatic-complexity
 		function isTypePropertyExternal(type: ts.Type, typePropertyName: string): boolean {
+			if (type.flags & ts.TypeFlags.Object) {
+				const objectType = type as ts.ObjectType;
+				// treat any tuple property as "external"
+				if (objectType.objectFlags & ts.ObjectFlags.Tuple) {
+					return true;
+				}
+			}
+
 			if (type.isUnionOrIntersection()) {
 				const hasExternalSubType = type.types.some((t: ts.Type) => isTypePropertyExternal(t, typePropertyName));
 				if (hasExternalSubType) {

tests/test-cases/tuple/input.ts

@@ -0,0 +1,7 @@
+type Tuple = [number, string] | [];
+
+declare function getTuple(): Tuple;
+
+export function doSomething(): void {
+	console.log(getTuple().length);
+}

tests/test-cases/tuple/output.js

@@ -0,0 +1,6 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+function doSomething() {
+    console.log(getTuple().length);
+}
+exports.doSomething = doSomething;