Not implemented undefined (List of Nodes in handle)

Describe the bug

Using svelte 3.42.4, building my project using snowpack fails with the following error:

[08:01:56] [snowpack] ! building files...
[08:02:01] [snowpack] Build Result Error: There was a problem with a file build result.
[08:02:01] [snowpack] Not implemented undefined
[08:02:01] [snowpack] Error: Not implemented undefined
    at handle (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6178:10)
    at /Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6414:19
    at Array.map (<anonymous>)
    at handle_body (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6413:22)
    at /Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6495:8
    at scoped_fn (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6382:11)
    at handle (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6181:18)
    at IfStatement (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6532:8)
    at handle (/Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6181:18)
    at /Users/cpennington/personal/dicedornot/node_modules/svelte/compiler.js:6414:19

The error is happening in this function in compiler.js

/**
	 * @param {Node} node
	 * @param {State} state
	 * @returns {Chunk[]}
	 */
	function handle(node, state) {
		const handler = handlers[node.type];

		if (!handler) {
			throw new Error(`Not implemented ${node.type}`);
		}

		const result = handler(node, state);

		if (node.leadingComments) {
			result.unshift(c(node.leadingComments.map(comment => comment.type === 'Block'
				? `/*${comment.value}*/${/** @type {any} */ (comment).has_trailing_newline ? `\n${state.indent}` : ` `}`
				: `//${comment.value}${/** @type {any} */ (comment).has_trailing_newline ? `\n${state.indent}` : ` `}`).join(``)));
		}

		if (node.trailingComments) {
			state.comments.push(node.trailingComments[0]); // there is only ever one
		}

		return result;
	}

Some debugging suggests that node being passed into this function is in fact a list of nodes, instead.

The same build completes with svelte 3.35.0.

Reproduction

Check out https://github.com/cpennington/dicedornot/tree/package-upgrade

Run

npm install
npm build

Logs

No response

System Info

npx: installed 1 in 1.857s

  System:
    OS: macOS 11.4
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 399.40 MB / 16.00 GB
    Shell: 3.2.1 - /usr/local/bin/fish
  Binaries:
    Node: 14.17.3 - ~/.nvm/versions/node/v14.17.3/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.13 - ~/.nvm/versions/node/v14.17.3/bin/npm
  Browsers:
    Chrome: 93.0.4577.63
    Firefox: 91.0
    Safari: 14.1.1
  npmPackages:
    snowpack: ^3.8.8 => 3.8.8 
    svelte: ^3.42.4 => 3.42.4

Severity

blocking an upgrade

[abextm]

This seems to be caused by this invalid cast from Node[] -> Node in Component.ts. estree-walker doesn’t provide a way to replace a single node with multiple as far as I can see.

[spiralmike]

Update- I just solved my issue. It may work for you.

Clued in by the code-red part that handled For loops, I’ve verified that the following code causes this error

for (var i of set)
{
}

whereas the following solves the problem.

for (let i of set)
{
}

The latter seems to be the appropriate form but ideally the for (var … ) case would be handled informatively by code-red.