// @target: es2015
class IPropertySet {
    [index: string]: any
}

var ps: IPropertySet = null;
var index: any = "hello";
ps[index] = 12;

interface indexErrors {
    [p2?: string];
    [...p3: any[]];
    [p4: string, p5?: string];
    [p6: string, ...p7: any[]];
}