add a test for #8459

[lovasoa]

See #8459

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

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[lovasoa]

@benmccann : I see you re-triggered the tests, but they are supposed to fail, in the current state of this PR. It just adds a test to the test suite that illustrates the bug reported in #8459 .

I am willing to help with solving the bug, but I would need some pointers. Looking at the code, I guess the issue is here:

svelte/src/runtime/internal/await_block.ts

Lines 82 to 101 in d42ca04

	if (is_promise(promise)) {
	const current_component = get_current_component();
	promise.then(value => {
	set_current_component(current_component);
	update(info.then, 1, info.value, value);
	set_current_component(null);
	}, error => {
	set_current_component(current_component);
	update(info.catch, 2, info.error, error);
	set_current_component(null);
	if (!info.hasCatch) {
	throw error;
	}
	});
	// if we previously had a then/catch block, destroy it
	if (info.current !== info.pending) {
	update(info.pending, 0);
	return true;
	}

I can see that that the block always immediately changes to a pending state when the function runs, even if the given promise is already resolved.

But I do not grasp what exactly is the role of each field in PromiseInfo and what invariants about it handle_promise is supposed to hold.

On a broader level, I am not sure I understand why a special AwaitBlockWrapper is needed. Couldn't the compiler just compile the {#await} syntax down to three if blocks and two variables storing the data and the error in the promise ?

[lovasoa]

@Rich-Harris @tanhauhau , I see you are the latest contributors to this part of the code, I would love if you could give me some pointers too :)

[dummdidumm]

The underlying issue is that promises that are resolved already are still treated as "need to render the await first and then resolve once the result is there". We want to look into changing this, but that won't happen before Svelte 5.

[lovasoa]

Ok, should I close this pr, then ?

[dummdidumm]

I added the "one day" label instead of closing so it has visibility, no further action needed from your side 👍

[Rich-Harris]

Closing Svelte 4 PRs as stale — thank you (this issue was fixed in Svelte 5)