Is this a bug?

halp.ts

type GeometryUnion = Circle | Rectangle | Polygon | PolyLine;

// This does not compile, the error is:
// error TS2339: Property 'center' does not exist on type 'Circle | Rectangle | Polygon | PolyLine'
class GeometryItem {
	name: string;
	object: GeometryUnion;
	children: GeometryItem[];
	constructor(name : string, object : GeometryUnion) {
		this.name = name;
		this.object = object;
		if (this.object instanceof Circle) {
			console.log(this.object.center);
		}
	}
}

// But if I assign "this.object" to a local and do the type-check on *it*, all is well...
class GeometryItem {
	name: string;
	object: GeometryUnion;
	children: GeometryItem[];
	constructor(name : string, object : GeometryUnion) {
		this.name = name;
		this.object = object;
		var obj = this.object;
		if (obj instanceof Circle) {
			console.log(obj.center);
		}
	}
}

This is by design. Type guards only apply to local variables. I describe some of the reasons here.