throw error if tag is void and it has content in dev mode

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -277,8 +277,11 @@ export default class ElementWrapper extends Wrapper {
 		const tag = this.node.tag_expr.manipulate(block);
 		block.add_variable(previous_tag, tag);
 
+		const has_children_of_dynamic_element = block.get_unique_name('has_children_of_dynamic_element');
+		block.add_variable(has_children_of_dynamic_element, x`${this.node.children.length > 0 ? 'true' : 'false'}`);
+
 		block.chunks.init.push(b`
-			${this.renderer.options.dev && b`@validate_dynamic_element(${tag});`}
+			${this.renderer.options.dev && b`@validate_dynamic_element(${tag}, ${has_children_of_dynamic_element});`}
 			let ${this.var} = ${tag} && ${this.child_dynamic_element_block.name}(#ctx);
 		`);
 
@@ -309,7 +312,7 @@ export default class ElementWrapper extends Wrapper {
 					${this.var}.m(${this.get_update_mount_node(anchor)}, ${anchor});
 				} else if (${not_equal}(${previous_tag}, ${tag})) {
 					${this.var}.d(1);
-					${this.renderer.options.dev && b`@validate_dynamic_element(${tag});`}
+					${this.renderer.options.dev && b`@validate_dynamic_element(${tag}, ${has_children_of_dynamic_element});`}
 					${this.var} = ${this.child_dynamic_element_block.name}(#ctx);
 					${this.var}.c();
 					${this.var}.m(${this.get_update_mount_node(anchor)}, ${anchor});

src/runtime/internal/dev.ts

@@ -1,5 +1,6 @@
 import { custom_event, append, append_hydration, insert, insert_hydration, detach, listen, attr } from './dom';
 import { SvelteComponent } from './Component';
+import { is_void } from '../../shared/utils/names';
 
 export function dispatch_dev<T=any>(type: string, detail?: T) {
 	document.dispatchEvent(custom_event(type, { version: '__VERSION__', ...detail }, { bubbles: true }));
@@ -107,10 +108,14 @@ export function validate_slots(name, slot, keys) {
 	}
 }
 
-export function validate_dynamic_element(tag: unknown) {
-	if (tag && typeof tag !== 'string') {
+export function validate_dynamic_element(tag: unknown, has_children: boolean) {
+	const is_string = typeof tag === 'string';
+	if (tag && !is_string) {
 		throw new Error('<svelte:element> expects "this" attribute to be a string.');
 	}
+	if (is_string && is_void(tag as string) && has_children) {
+		throw new Error(`<${tag}> element is self-closing and cannot have content.`);
+	}
 }
 
 type Props = Record<string, any>;

test/runtime/samples/dynamic-element-void-with-content1/_config.js

@@ -0,0 +1,9 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	props: {
+		tag: 'br'
+	},
+	error: '<br> element is self-closing and cannot have content.'
+};

test/runtime/samples/dynamic-element-void-with-content1/main.svelte

@@ -0,0 +1,5 @@
+<script>
+  export let tag;
+</script>
+
+<svelte:element this='{tag}'>foo</svelte:element>

test/runtime/samples/dynamic-element-void-with-content2/_config.js

@@ -0,0 +1,9 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	props: {
+		tag: 'br'
+	},
+	error: '<br> element is self-closing and cannot have content.'
+};

test/runtime/samples/dynamic-element-void-with-content2/main.svelte

@@ -0,0 +1,5 @@
+<script>
+  export let tag;
+</script>
+
+<svelte:element this='{tag}'><div>bar</div></svelte:element>

test/runtime/samples/dynamic-element-void-with-content3/Nested.svelte

@@ -0,0 +1 @@
+<div>This is nested</div>

test/runtime/samples/dynamic-element-void-with-content3/_config.js

@@ -0,0 +1,9 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	props: {
+		tag: 'br'
+	},
+	error: '<br> element is self-closing and cannot have content.'
+};

test/runtime/samples/dynamic-element-void-with-content3/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	import Nested from './Nested.svelte';
+  export let tag;
+</script>
+
+<svelte:element this='{tag}'><Nested/></svelte:element>