Being able to omit the variable name when doing `{#await promise then value} {/await}`

[aradalvand]

Describe the problem

Suppose you have a promise that doesn't return anything (i.e. Promise<void> in TypeScript terms). If you want to use Svelte's await syntax for such a promise, you can do so with separate {#await} {:then} blocks, and you're perfectly allowed to omit the variable name in the {:then} block, like so:

{#await someVoidPromise}
  Waiting...
{:then}
  Done.
{/await}

Svelte also provides an alternative syntax for when you don't want to show anything until the promise is resolved. It looks like this:

{#await somePromise then value}
    Done: {value}
{/await}

Explained in the official tutorial here.

Unlike what I expected, you cannot use the above syntax with a void promise, in other words you can't omit the variable name, so you can't do:

{#await someVoidPromise then}
    Done.
{/await}

You'd get the following error:

Describe the proposed solution

I think the aforementioned syntax should be supported. You should be able to omit the variable name after then when you don't have a separate {#await} block, just like you can do so when you do have a separate {#await} block.

Alternatives considered

You'd have to do the following, and have an empty {#await} block:

{#await someVoidPromise}
{:then}
  Done.
{/await}

which isn't nice.

Importance

would make my life easier

[Conduitry]

Duplicate of #6270.