hidingIndexSignatures.ts(10,1): error TS2454: Variable 'b' is used before being assigned.
hidingIndexSignatures.ts(12,1): error TS2454: Variable 'a' is used before being assigned.


==== hidingIndexSignatures.ts (2 errors) ====
    interface A {
        [a: string]: {};
    }
    
    interface B extends A {
        [a: string]: number; // Number is not a subtype of string.  Should error.
    }
    
    var b: B;
    b[""]; // Should be number
    ~
!!! error TS2454: Variable 'b' is used before being assigned.
    var a: A;
    a[""]; // Should be {}
    ~
!!! error TS2454: Variable 'a' is used before being assigned.