class Base {
        public a: string;
}

class Derived extends Base {
        public b: string;
}



var a4: { [index: number]: Base };
var b4: { [index: string]: Derived }; 

var r1a1 = a4 < b4;

Actual: no error
Expected: Operator '<' cannot be applied to types '{ [x: number]: Base; }' and '{ [x: string]: Derived; }'.

The current behavior matches the old compiler because fixing this would be a breaking change we aligned them. Worth considering whether to just take the break since it's unlikely someone would take an intentional dependeny on this undefined behavior.