Slot `let` variables not accessible on slotted element root

[Crisfole]

In the following abbreviated components I'd expect to have access to foo in the h1 class attribute. But instead it's only accessible inside the slotted element content.

Parent

<Child {baz}>
  Content
  <h1 slot=foobar let:foo class='foobar-{foo.type}'>{foo.title}!</h1>
  <!--  Unexpected behaviour is here - - ^            ^- - - This works fine -->
</Child>

Child

<section class=child>
  <slot name=foobar foo={makeFoo(baz)} />
  {@html baz.content}
</Child>

I'd like the above example to work, with slot let variables in scope at the root slotted element

Alternatives to this include:

• Wrapping in a dummy element. (Yuck)
• bind:this and manually setting the attributes I want later. (Ulcer inducing)

This is pretty important for slotting tables: if you can't use data from the dataset to add attributes to slotted rows or cells you're missing something pretty darn important...

[Crisfole]

OK, apparently you can get variables exposed by named slots on the component itself:

Child

<script> export let items; </script>
<ul>
  {#each items as { id, name, type } (id)}
  <slot name=li {name} {type}/>
  {/each}
</ul>

Parent

<script>
  import Child from './Child.svelte';
  let items = [
    { id: 'abc', type: 'issue', text: "can't see well" },
    { id: 'def', type: 'propsal', text: "outsource the work" },
    { id: 'ghi', type: 'issue', text: "stuck in traffic" },
  ];
</script>

<Child {items} let:type>
  <li slot=li let:text class={type}>{text}</li>
</Child>

That's pretty counter intuitive to me...but it works. Is this deliberate? Can we document this better if so? Can we tighten up the constraints a bit?

[Conduitry]

Should be fixed in 3.17.2.