{#each} block destructuring can't have default values refer to destructured params

[tivac]

Describe the bug

There's a useful pattern you can use while destructuring in normal JS, which is that within the destructure you can reference previously-extracted properties.

Live Example

const o = { one : 1 };

// Here's where the exciting stuff happens
//           v-----------v
const { one, two = one + 1 } = o;

console.log(one, two);

I wanted to use (abuse?) this functionality to avoid having to create an entire array or modify an array that I was passed, because allocating a whole array for something that can be easily calculated on a per-element basis while iterating didn't seem worth it.

To Reproduce
https://svelte.dev/repl/0ee7227e1b45465b9b47d7a5ae2d1252?version=3.23.2

Here's the problematic generated code:

function get_each_context(ctx, list, i) {
	const child_ctx = ctx.slice();
	child_ctx[1] = list[i].one;
	// This reference to one isn't rewritten to be child_ctx[1], so one is undefined
	//                                                       v---------v
	child_ctx[2] = list[i].two !== undefined ? list[i].two : add(one, 1);
	return child_ctx;
}

Expected behavior
1 - 2 is logged

Information about your Svelte project:

• Firefox latest, but it's in the generated code so happens everywhere
• Windows 10
• [email protected]
• Rollup, can see in REPL

Severity
It mostly means that if you need to look up data to make decisions inside an {#each} you either need to modify the array elements or create a second, shadow array based on the elements containing the extra data. This is all doable but I'd much prefer not to if possible. Since templates can't declare new variables I haven't been able to come up with an alternative that lets it be handled entirely in template code.

[Conduitry]

This is fixed in 3.33.0 - https://svelte.dev/repl/0ee7227e1b45465b9b47d7a5ae2d1252?version=3.33.0