// 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; } interface A { [x: number]: Derived; } class B implements A { [x: string]: Base; // error } class B2 implements A { [x: string]: Derived2; // ok } module Generics { interface A { [x: number]: T; } class B implements A { [x: string]: Derived; // ok } class B2 implements A { [x: string]: Derived2; // ok } class B3 implements A { [x: string]: Base; // error } class B4 implements A { [x: string]: Derived; // error } class B5 implements A { [x: string]: Derived2; // error } }