Rendering list items without a key is possible using text/comment nodes

[nolanlawson]

In principle, component authors are not supposed to be able to render lists without keys. However, it turns out that it's possible using text nodes or comment nodes:

export default class extends LightningElement {
  items = [1, 2, 3]
}

<template for:each={items} for:item="item">
    haha no key!
</template>

Renders: haha no key!haha no key!haha no key!

<template for:each={items} for:item="item">
    <!-- haha no key! -->
</template>

Renders: <!-- haha no key! --><!-- haha no key! --><!-- haha no key! -->

Whereas if you use elements (e.g. <div>) instead of text/comment nodes, you get a compile-time error:

Error: LWC1071: Missing key for element <div> inside of iterator. Elements within iterators 
must have a unique, computed key value.

At best, this is inconsistent, and at worst, it's a loophole in the intended behavior (lists must have keys).

[nolanlawson]

Fixing this would be a breaking change for sure. I'm sure someone is relying on this.