Describe the bug

I was a little silly today and somehow mixed up @render and @html.
Easy fix, but unfortunately the error message is incredibly unhelpful:

TypeError: Cannot read properties of undefined (reading 'out')
    at children (<snip>/src/routes/+page.svelte:13:4) [1]
    at Component (<snip>\src\lib\Component.svelte:7:8) [2]
    <snip>

The sourcemap's line numbers weren't particularly useful either, so I went digging in the generated _page.svelte.js.
After some fiddling I found that a component like this:

<script lang="ts">
    import type { Snippet } from 'svelte';

    let { children }: { children: Snippet } = $props();
</script>

<!-- Bad -->
{@html children()}

<!-- Good -->
{@render children()}

...compiles roughly to this:

function Component($$payload, $$props) {
    push();
    let { children } = $$props;
    
    // {@html children()}
    // Bad: children expects $$payload parameter
    $$payload.out += `${html(children())}`; // [2]

    // {@render children()}
    // Good: children gets $$payload parameter
    children($$payload);

    $$payload.out += `<!---->`;
    pop();
}

function _page($$payload) {
    Component($$payload, {
        children: ($$payload2) => {
            // Error: $$payload2 is undefined :(
            $$payload2.out += `<!---->Content`; // [1]
        }
    });
}

I know next to nothing about Svelte's internals, but I suppose invoking a Snippet inside @render gets special treatment from the compiler to insert the hidden $$payload parameter. This doesn't happen in other situations, therefore it blows up at runtime.

The JSDoc for Snippet says:

You can only call a snippet through the {@render ...} tag.

That makes sense to me, but it doesn't seem to be enforced in any way, nor is it mentioned on the docs page. I don't see a way to encode this into the type system, so I'd suggest adding an explicit error message. Preferably at compile time if possible, but a helpful runtime error would already be a great improvement :)

Reproduction

Here

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 15.74 GB / 31.81 GB
  Binaries:
    Node: 20.18.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.9.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.9.2 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (133.0.3065.69)
    Internet Explorer: 11.0.19041.4355
  npmPackages:
    svelte: ^5.0.0 => 5.23.0

Severity

annoyance