// Derived type indexer must be subtype of base type indexer interface Base { foo: string; } interface Derived extends Base { bar: string; } interface Derived2 extends Derived { baz: string; } var a: A; var b1: { [x: string]: string; } a = b1; // error b1 = a; // error module Generics { class A { [x: string]: T; } function foo() { var a: A; var b: { [x: string]: string; } a = b; // error b = a; // error } }