用更严格的泛型覆盖方法

Question

在typescript中使用d3.js我试图覆盖一个返回SVGElement选择的方法，该方法返回一个更具体的SVGRectElement选择。

class ElementSelector {
    public getSelection(): d3.Selection<SVGElement, any, any, any> {
        return d3.select<SVGElement, any>("");
    }
}

class RectSelector extends ElementSelector {
    public getSelection(): d3.Selection<SVGRectElement, any, any, any> {
        return d3.select<SVGRectElement, any>("");
    }
}

但是我收到了这个错误：

Class 'B' incorrectly extends base class 'A'.
  Types of property 'getSelection' are incompatible.
    Type '() => Selection<SVGRectElement, any, any, any>' is not assignable to type '() => Selection<SVGElement, any, any, any>'.
      Type 'Selection<SVGRectElement, any, any, any>' is not assignable to type 'Selection<SVGElement, any, any, any>'.
        Types of property 'datum' are incompatible.
          Type '{ (): any; (value: null): Selection<SVGRectElement, undefined, any, any>; <NewDatum>(value: Value...' is not assignable to type '{ (): any; (value: null): Selection<SVGElement, undefined, any, any>; <NewDatum>(value: ValueFn<S...'.
            Type 'Selection<SVGRectElement, undefined, any, any>' is not assignable to type 'Selection<SVGElement, undefined, any, any>'.
              Types of property 'on' are incompatible.
                Type '{ (typenames: string): ValueFn<SVGRectElement, undefined, void> | undefined; (typenames: string, ...' is not assignable to type '{ (typenames: string): ValueFn<SVGElement, undefined, void> | undefined; (typenames: string, list...'.
                  Type 'ValueFn<SVGRectElement, undefined, void> | undefined' is not assignable to type 'ValueFn<SVGElement, undefined, void> | undefined'.
                    Type 'ValueFn<SVGRectElement, undefined, void>' is not assignable to type 'ValueFn<SVGElement, undefined, void> | undefined'.
                      Type 'ValueFn<SVGRectElement, undefined, void>' is not assignable to type 'ValueFn<SVGElement, undefined, void>'.
                        The 'this' types of each signature are incompatible.
                          Type 'SVGElement' is not assignable to type 'SVGRectElement'.

显然我无法将SVGRectElement分配给SVGElement，但原因并不清楚。

例如，这有效：

interface Gen<T> { 
  prop: T
}

class A {
    init(param1: number): Gen<SVGElement> {
      return { prop: {} as SVGElement };
    }
}

class B extends A {
    init(param1: number): Gen<SVGRectElement> {
        return { prop: {} as SVGRectElement  };
    }
}

我错过了什么？