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