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

[G-Rath]

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

[RyanCavanaugh]

This is the intended behavior, though your code should have an error. implements does not cause optional methods to exist, so your base class is empty, so it's fine for your derived class to add a new method. Note that if you had any properties in your base class, you'd see an error about "implementing" the interface without actually doing anything.

The bug here is that we should have complained about the implements clause even though the class is empty