[labs/ssr] Non-array iterables fail to render

[augustjk]

Which package(s) are affected?

SSR (@lit-labs/ssr)

Description

When template contains iterables besides an array, for instance like a Set, SSR does not actually render the values.

For example:

This

  render() {
    return html`<p>new Set([1, 2, 3])</p>`;
  }

results in a paragraph tag containing [object Set].

The same is true for other iterables that normally work client-side like Map or Generator, which means using the map() directive results in [object Generator].

This also breaks during hydration as there's a mismatch of the number of parts, displaying the following error:

Error: unexpected longer than expected iterable

Reproduction

https://stackblitz.com/edit/koa-starter-3x2kbs?file=index.js

Workaround

Use Array.from()

Is this a regression?

No or unsure. This never worked, or I haven't tried before.

Affected versions

2.2.1

Browser/OS/Node environment

Node version: v16.15.0

[nebarf]

Looking at https://github.com/lit/lit/blob/main/packages/labs/ssr/src/lib/render-lit-html.ts#L594, I can confirm what @augustjk reported since the check is on Array instead of Iterable objects