Argument of type 'this' is not assignable to parameter of type 'NonNullable'?

[checnev]

TypeScript Version: 4.0.3

Search Terms: 'this' is not assignable to 'NonNullable'

Code

abstract class TreeNode {
  parent: this | null = null; // polymorph this
  left: this | null = null;
  right: this | null = null;

  replaceChild(child: this, replacement: this | null = null) { // this - polymorph type
    this.left = child;
    this.right = replacement;
  }

  removeNode() {
    this.parent?.replaceChild(this, null); // ERROR
  }

  removeNode2Way() {
    if (this.parent) {
      this.parent.replaceChild(this, null); // OK
    }
  }
}

Expected behavior:
Conditional call without errors.

Actual behavior:
Error: Argument of type 'this' is not assignable to parameter of type 'NonNullable'

Playground Link: Playground

Related Issues:

[DanielRosenwasser]

Seems like any reference to the left of a . or ?. should in principle be narrowed, but we have concerns around the control flow graph. We would accept an experiment PR, but we are not committed to this, especially if there are perf/memory ramifications.