Slot prop types from generic components not propagating upwards

[AlbertMarashi]

Describe the bug

When using experimental generics, there is a bug happening where the let:prop type is not being passed up to the parent component, causing type issues within typescript only (the code works).

Reproduction

Main.svelte

<script>
import Foo from "./Foo.svelte"

</script>
<Foo xyz="foo" >
    <div slot="option" let:abc>
        {abc}
    </div>
</Foo>

The type of abc here is unknown when it should be string. This will appear as an error in VSCode

Foo.svelte

<script lang="ts">
import Bar from "./Bar.svelte"

type T = $$Generic
export let xyz: T

</script>
<Bar {xyz}>
    <svelte:fragment slot="option" let:bar>
        <slot name="option" abc={bar}/>
    </svelte:fragment>
</Bar>

Bar.svelte

<script lang="ts">
type T = $$Generic
export let xyz: T

</script>
<slot name="option" bar={xyz} />

---

Additionally, when you remove the name="option" and use the default to target the default slots the type becomes any instead of unknown. This does not appear as an error in VSCode, since it is any however all the type information is gone

Example using default slot:
Main.svelte

<script>
import Foo from "./Foo.svelte"

</script>
<Foo xyz="foo" let:abc>
    {abc}
</Foo>

Foo.svelte

<script lang="ts">
import Bar from "./Bar.svelte"

type T = $$Generic
export let xyz: T

</script>
<Bar {xyz} let:bar>
    <slot abc={bar}/>
</Bar>

Bar.svelte

<script lang="ts">
type T = $$Generic
export let xyz: T

</script>
<slot bar={xyz}/>

Logs

No response

System Info

System:
    OS: macOS 12.0.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 82.30 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.17.0 - /usr/local/bin/node
    npm: 8.15.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 104.0.5112.101
    Firefox Developer Edition: 105.0
    Safari: 15.1

Severity

blocking an upgrade

[AlbertMarashi]

A workaround I have discovered is to force Svelte to adopt a better interface for $$Slots

type T = $$Generic

interface $$Slots {
    default: { abc: T }
}

I still feel as if this should be automatic though

[dummdidumm]

Probably duplicate of #1344

[bztes]

In our case using the svelte-checker with '--use-new-transformation' has fixed the problem. No problems with VSCode.

The issue only occurred in some situations (some components worked, others didn't).

[AlbertMarashi]

In our case using the svelte-checker with '--use-new-transformation' has fixed the problem. No problems with VSCode.

The issue only occurred in some situations (some components worked, others didn't).

How does one apply this change?