fix: type-level back-compat for default slot and children prop

If someone has an existing SvelteComponent type definition and using the slot generic to type the default slot, automatically add that slot to the prop type as "children"

Author: dummdidumm

.changeset/wet-games-fly.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: type-level back-compat for default slot and children prop

packages/svelte/src/main/public.d.ts

@@ -18,6 +18,14 @@ export interface ComponentConstructorOptions<
 	$$inline?: boolean;
 }
 
+// Utility type for ensuring backwards compatibility on a type level: If there's a default slot, add 'children' to the props if it doesn't exist there already
+type PropsWithChildren<Props, Slots> = Props &
+	(Props extends { children?: any }
+		? {}
+		: Slots extends { default: any }
+		? { children?: Snippet }
+		: {});
+
 /**
  * Can be used to create strongly typed Svelte components.
  *
@@ -57,13 +65,13 @@ export class SvelteComponent<
 	 * is a stop-gap solution. Migrate towards using `mount` or `createRoot` instead. See
 	 * https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more info.
 	 */
-	constructor(options: ComponentConstructorOptions<Props>);
+	constructor(options: ComponentConstructorOptions<PropsWithChildren<Props, Slots>>);
 	/**
 	 * For type checking capabilities only.
 	 * Does not exist at runtime.
 	 * ### DO NOT USE!
 	 * */
-	$$prop_def: Props;
+	$$prop_def: PropsWithChildren<Props, Slots>;
 	/**
 	 * For type checking capabilities only.
 	 * Does not exist at runtime.

packages/svelte/tests/types/component.ts

@@ -15,11 +15,11 @@ class LegacyComponent extends SvelteComponent<
 	{ slot: { slotProps: boolean } }
 > {}
 
-// @ts-expect-error
 const legacyComponent = new LegacyComponent({
 	target: null as any as Document | Element | ShadowRoot,
 	props: {
 		prop: 'foo',
+		// @ts-expect-error
 		x: ''
 	}
 });
@@ -56,14 +56,20 @@ class NewComponent extends SvelteComponent<
 	anExport: string = '';
 }
 
-// @ts-expect-error
 new NewComponent({
-	prop: 'foo',
-	x: ''
+	target: null as any,
+	props: {
+		prop: 'foo',
+		// @ts-expect-error
+		x: ''
+	}
 });
 
 const newComponent: NewComponent = new NewComponent({
-	prop: 'foo'
+	target: null as any,
+	props: {
+		prop: 'foo'
+	}
 });
 newComponent.$$events_def.event;
 // @ts-expect-error
@@ -123,11 +129,11 @@ instance.anExport === 1;
 // --------------------------------------------------------------------------- interop
 
 const AsLegacyComponent = asClassComponent(newComponent);
-// @ts-expect-error
 new AsLegacyComponent({
 	target: null as any,
 	props: {
 		prop: '',
+		// @ts-expect-error
 		x: ''
 	}
 });