interface Foo {
    method(a: number): string;
    optionalMethod?(a: number): string;
    property: string;
    optionalProperty?: string;
}

class Foo {
    additionalProperty: string;

    additionalMethod(a: number): string {
        return this.method(0);
    }
}

class Bar extends Foo {
    method(a: number) {
        return this.optionalProperty;
    }
}


var bar = new Bar();
bar.method(0);
bar.optionalMethod(1);
bar.property;
bar.optionalProperty;
bar.additionalProperty;
bar.additionalMethod(2);

var obj: {
    method(a: number): string;
    property: string;
    additionalProperty: string;
    additionalMethod(a: number): string;
};

bar = obj;
obj = bar;