fix(types): fix types when using legacy components in Svelte 5 (#424)

src/__tests__/fixtures/Typed.svelte

@@ -1,9 +1,14 @@
 <script lang="ts">
+  import { createEventDispatcher } from 'svelte'
+
   export let name: string
   export let count: number
 
   export const hello: string = 'hello'
+
+  const dispatch = createEventDispatcher<{ greeting: string }>()
 </script>
 
 <h1>hello {name}</h1>
 <p>count: {count}</p>
+<button on:click={() => dispatch('greeting', 'hello')}>greet</button>

src/__tests__/render-runes.test-d.ts

@@ -1,7 +1,8 @@
 import { expectTypeOf } from 'expect-type'
-import { describe, test } from 'vitest'
+import { describe, test, vi } from 'vitest'
 
 import * as subject from '../index.js'
+import LegacyComponent from './fixtures/Typed.svelte'
 import Component from './fixtures/TypedRunes.svelte'
 
 describe('types', () => {
@@ -37,3 +38,32 @@ describe('types', () => {
     }>()
   })
 })
+
+describe('legacy component types', () => {
+  test('render accepts events', () => {
+    const onGreeting = vi.fn()
+    subject.render(LegacyComponent, {
+      props: { name: 'Alice', count: 42 },
+      events: { greeting: onGreeting },
+    })
+  })
+
+  test('component $set and $on are not allowed', () => {
+    const onGreeting = vi.fn()
+    const { component } = subject.render(LegacyComponent, {
+      name: 'Alice',
+      count: 42,
+    })
+
+    expectTypeOf(component).toMatchTypeOf<{ hello: string }>()
+
+    // @ts-expect-error: Svelte 5 mount does not return `$set`
+    component.$on('greeting', onGreeting)
+
+    // @ts-expect-error: Svelte 5 mount does not return `$set`
+    component.$set({ name: 'Bob' })
+
+    // @ts-expect-error: Svelte 5 mount does not return `$destroy`
+    component.$destroy()
+  })
+})

src/component-types.d.ts

@@ -45,17 +45,17 @@ export type Props<C extends Component> = ComponentProps<C>
  * In Svelte 5, this is the set of variables marked as `export`'d.
  * In Svelte 4, this is simply the instance of the component class.
  */
-export type Exports<C> = C extends LegacyComponent
-  ? C
-  : C extends ModernComponent<any, infer E>
+export type Exports<C> = IS_MODERN_SVELTE extends true
+  ? C extends ModernComponent<any, infer E>
     ? E
-    : never
+    : C & { $set: never; $on: never; $destroy: never }
+  : C
 
 /**
  * Options that may be passed to `mount` when rendering the component.
  *
  * In Svelte 4, these are the options passed to the component constructor.
  */
-export type MountOptions<C extends Component> = C extends LegacyComponent
-  ? LegacyConstructorOptions<Props<C>>
-  : Parameters<typeof mount<Props<C>, Exports<C>>>[1]
+export type MountOptions<C extends Component> = IS_MODERN_SVELTE extends true
+  ? Parameters<typeof mount<Props<C>, Exports<C>>>[1]
+  : LegacyConstructorOptions<Props<C>>