`mount()` does not enforce generic component prop constraints

[janosh]

Describe the bug

mount() doesn't enforce generic component prop constraints. The Component<Props> type erases the component-level generic T, so Props resolves to the default/widened type and TypeScript can't enforce the extends constraint on props passed to mount().

Expected: type error when props violate the component's generic constraint.
Actual: invalid props silently accepted.

Note: using the component in a template does correctly catch the error.

Reproduction

GenericList.svelte

<script lang="ts" generics="T extends { id: number; label: string }">
  let { items = [] }: { items: T[] } = $props()
</script>

{#each items as item}
  <div>{item.label} (#{item.id})</div>
{/each}

App.svelte

<script lang="ts">
  import { mount } from 'svelte'
  import GenericList from './GenericList.svelte'

  // should error: { foo: 42 } doesn't satisfy { id: number; label: string }
  // but mount() accepts it without complaint
  mount(GenericList, {
    target: document.body,
    props: { items: [{ foo: 42 }] },
  })
</script>

Severity

annoyance