Type issues with Polymer 3.0.5 and legacy elements

[a-xin]

I am having issues with the types for legacy elements:

Steps to reproduce

Declare a legacy polymer element via TypeScript:

import { Polymer, html } from '@polymer/polymer/polymer-legacy';

Polymer({
  _template: html`<span>hello</span>`,
  is: 'hello-element',
});

Expected Results

There are no type errors.

Actual Results

The element renders fine but the following type error is shown:
Object literal may only specify known properties, but '_template' does not exist in type 'PolymerInit'. Did you mean to write 'template'?

Versions

• Polymer: v3.0.5
• TypeScript: v3.0.1

[a-xin]

Looks like changing the interface as follows solves the typing issues:

export interface PolymerInit {
  is: string;
  extends?: string;
  properties?: PolymerElementProperties;
  observers?: string[];
  _template?: string;
  hostAttributes?: {[key: string]: any};
  listeners?: {[key: string]: string};

  // TODO: What are the allowed keys for behaviors?
  behaviors?: Pick<PolymerInit, Exclude<keyof PolymerInit, 'is' | 'extends' | '_template'>>[];

  // Lifecycle methods
  registered?();
  created?();
  attached?();
  detached?();
  ready?();
  attributeChanged?(name: string, type);

  // Allow any other user-defined properties
  [others: string]: any;
}

Not sure if this is the right way to do it - happy to provide a PR for this.

Reference: https://github.com/Polymer/polymer/blob/master/interfaces.d.ts