// index signatures must be compatible in assignments interface Base { foo: string; } interface Derived extends Base { bar: string; } interface Derived2 extends Derived { baz: string; } interface A { [x: string]: Base; } var a: A; var b: { [x: string]: Derived; } a = b; // ok b = a; // error var b2: { [x: string]: Derived2; } a = b2; // ok b2 = a; // error module Generics { interface A { [x: string]: T; } interface B extends A { [x: string]: Derived; // ok } var b1: { [x: string]: Derived; }; var a1: A; a1 = b1; // ok b1 = a1; // error interface B2 extends A { [x: string]: Derived2; // ok } var b2: { [x: string]: Derived2; }; a1 = b2; // ok b2 = a1; // error function foo() { var b3: { [x: string]: Derived; }; var a3: A; a3 = b3; // error b3 = a3; // error var b4: { [x: string]: Derived2; }; a3 = b4; // error b4 = a3; // error } }