=== tests/cases/compiler/baseIndexSignatureResolution.ts ===
class Base { private a: string; }
>Base : Base
>a : string

class Derived extends Base { private b: string; }
>Derived : Derived
>Base : Base
>b : string

// Note - commmenting "extends Foo" prevents the error
interface Foo {
>Foo : Foo

    [i: number]: Base;
>i : number
>Base : Base
}
interface FooOf<TBase extends Base> extends Foo {
>FooOf : FooOf<TBase>
>TBase : TBase
>Base : Base
>Foo : Foo

    [i: number]: TBase;
>i : number
>TBase : TBase
}
var x: FooOf<Derived> = null;
>x : FooOf<Derived>
>FooOf : FooOf<TBase>
>Derived : Derived

var y: Derived = x[0];
>y : Derived
>Derived : Derived
>x[0] : Derived
>x : FooOf<Derived>

/*
// Note - the equivalent for normal interface methods works fine:
interface A {
    foo(): Base;
}
interface B<TBase extends Base> extends A {
    foo(): TBase;
}
var b: B<Derived> = null;
var z: Derived = b.foo();
*/