TypeScript not enforcing the return type of optional methods defined in interfaces implemented by parents of classes

TypeScript Version: 3.5.0 (Also tried 3.4.1, 3.0.1, and 2.4.1)

Search Terms: Inheritance, interface, return type, parent, child, override, overload, abstract, optional method, optional interface, implements, extends

Code

interface Mx {
  aMethod?(): number;
}

abstract class Base implements Mx {

}

class Child extends Base {
  public aMethod() {
    return 'hello world';
  }
}

Expected behavior:
TypeScript tells me that the return type for Child#aMethod is wrong.

This behaviour is achieved if I add implements Mx on the Child class.

Actual behavior:
TypeScript tells me nothing! It feels that Child#aMethod is well within its rights to return a string.

Playground Link: linky!

Related Issues: I didn't really find any similar looking issues?

The result is this could maybe relate to #22815

まず、リンクされた TypeScript Playground の例を再現し、次に Child が Mx を明示的に実装しているバージョンと比較します。TypeScript が冗長な implements 句を要求せずに、Child#aMethod の互換性のない string 戻り値の型を報告すれば、作業は完了です。