>>59
class Base {
equals(other: this): boolean { return other.name === "Derived"; }
}
class Derived extends Base {
name = "Derived";
}
let derived = new Derived();
console.log(derived.equals(derived));

だと、1.7でも(正確には1.7.0-dev.20151016)
test.ts(3,18): error TS2339: Property 'name' does not exist on type 'Base'.
って、ポリモらないけど…なんでなん？