_attachDom type signature is too restrictive

[e111077]

Description

_attachDom's docs say that it can be overriden to support any type of node that is not just only a shadow root. The return types though seem to be ShadowRoot | null which means that you cannot return this as an acceptable root.

polymer/lib/mixins/element-mixin.js

Line 734 in 76c71e1

* @return {ShadowRoot} node to which the dom has been attached.

polymer/lib/mixins/element-mixin.d.ts

Line 221 in 36fb37e

_attachDom(dom: StampedTemplate|null): ShadowRoot|null;

Steps to Reproduce

1. override _attachDom
2. place the snippet below
3. attempt to compile with closure or TS

_attachDom(dom) {
  this.appendChild(dom);
  return this;
}

Expected Results

no compilation error

Actual Results

compilation error

Browsers Affected

N/A

Versions

• Polymer: v3.4.1

[stale]

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