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

Beginne damit, das Beispiel im verlinkten TypeScript Playground zu reproduzieren, und vergleiche es dann mit der Version, in der Child Mx explizit implementiert. Die Arbeit ist abgeschlossen, wenn TypeScript den inkompatiblen Rückgabetyp string für Child#aMethod meldet, ohne die redundante implements-Klausel zu erfordern.