baseIndexSignatureResolution.ts(1,22): error TS2564: Property 'a' has no initializer and is not definitely assigned in the constructor.
baseIndexSignatureResolution.ts(2,38): error TS2564: Property 'b' has no initializer and is not definitely assigned in the constructor.
baseIndexSignatureResolution.ts(11,5): error TS2322: Type 'null' is not assignable to type 'FooOf<Derived>'.


==== baseIndexSignatureResolution.ts (3 errors) ====
    class Base { private a: string; }
                         ~
!!! error TS2564: Property 'a' has no initializer and is not definitely assigned in the constructor.
    class Derived extends Base { private b: string; }
                                         ~
!!! error TS2564: Property 'b' has no initializer and is not definitely assigned in the constructor.
    
    // Note - commmenting "extends Foo" prevents the error
    interface Foo {
        [i: number]: Base;
    }
    interface FooOf<TBase extends Base> extends Foo {
        [i: number]: TBase;
    }
    var x: FooOf<Derived> = null;
        ~
!!! error TS2322: Type 'null' is not assignable to type 'FooOf<Derived>'.
    var y: Derived = x[0];
    
    /*
    // 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();
    */