warn if passing slot not defined into Component

src/compiler/compile/render_dom/index.ts

@@ -412,7 +412,8 @@ export default function dom(
 
 				${unknown_props_check}
 
-				${component.slots.size ? b`let { $$slots = {}, $$scope } = $$props;` : null}
+				${component.slots.size || component.compile_options.dev ? b`let { $$slots = {}, $$scope } = $$props;` : null}
+				${component.compile_options.dev && b`@validate_slot($$slots, [${Array.from(component.slots.keys()).map(key => `"${key}"`).join(',')}]);`}
 
 				${renderer.binding_groups.length > 0 && b`const $$binding_groups = [${renderer.binding_groups.map(_ => x`[]`)}];`}
 

src/runtime/internal/dev.ts

@@ -89,6 +89,14 @@ export function validate_each_argument(arg) {
 	}
 }
 
+export function validate_slot(slot, keys) {
+	keys = new Set(keys);
+	for (const slot_key of Object.keys(slot)) {
+		if (!keys.has(slot_key)) {
+			console.warn(`Received unexpected slot named "${slot_key}"`);
+		}
+	}
+}
 
 type Props = Record<string, any>;
 export interface SvelteComponentDev {

test/runtime/samples/component-slot-warning/Nested.svelte

@@ -0,0 +1 @@
+<slot name="slot2"></slot>

test/runtime/samples/component-slot-warning/_config.js

@@ -0,0 +1,9 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	warnings: [
+		'Received unexpected slot named "default"',
+		'Received unexpected slot named "slot1"'
+	]
+};

test/runtime/samples/component-slot-warning/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Nested from "./Nested.svelte";
+</script>
+<Nested>
+	<input slot="slot1">
+	<input slot="slot2">
+</Nested>