fix: prevent snippet children conflict (#10700)

closes #10385

Author: dummdidumm

.changeset/wet-wombats-repeat.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: prevent snippet children conflict

packages/svelte/src/compiler/errors.js

@@ -296,7 +296,9 @@ const slots = {
 	/** @param {string} name @param {string} component */
 	'duplicate-slot-name': (name, component) => `Duplicate slot name '${name}' in <${component}>`,
 	'invalid-default-slot-content': () =>
-		`Found default slot content alongside an explicit slot="default"`
+		`Found default slot content alongside an explicit slot="default"`,
+	'conflicting-children-snippet': () =>
+		`Cannot use explicit children snippet at the same time as implicit children content. Remove either the non-whitespace content or the children snippet block`
 };
 
 /** @satisfies {Errors} */

packages/svelte/src/compiler/phases/2-analyze/validation.js

@@ -521,6 +521,24 @@ const validation = {
 			);
 		}
 	},
+	SnippetBlock(node, { path }) {
+		if (node.expression.name !== 'children') return;
+		const parent = path.at(-2);
+		if (!parent) return;
+		if (
+			parent.type === 'Component' ||
+			parent.type === 'SvelteComponent' ||
+			parent.type === 'SvelteSelf'
+		) {
+			if (
+				parent.fragment.nodes.some(
+					(node) => node.type !== 'SnippetBlock' && (node.type !== 'Text' || node.data.trim())
+				)
+			) {
+				error(node, 'conflicting-children-snippet');
+			}
+		}
+	},
 	SvelteHead(node) {
 		const attribute = node.attributes[0];
 		if (attribute) {

packages/svelte/src/compiler/phases/3-transform/utils.js

@@ -1,8 +1,7 @@
 import {
 	regex_ends_with_whitespaces,
 	regex_not_whitespace,
-	regex_starts_with_whitespaces,
-	regex_whitespaces_strict
+	regex_starts_with_whitespaces
 } from '../patterns.js';
 import * as b from '../../utils/builders.js';
 import { walk } from 'zimmerframe';

packages/svelte/tests/compiler-errors/samples/snippet-children-conflict/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'conflicting-children-snippet',
+		message:
+			'Cannot use explicit children snippet at the same time as implicit children content. Remove either the non-whitespace content or the children snippet block',
+		position: [320, 353]
+	}
+});

packages/svelte/tests/compiler-errors/samples/snippet-children-conflict/main.svelte

@@ -0,0 +1,25 @@
+<!-- ok -->
+<Button>
+	hello
+</Button>
+<Button>
+	{#snippet children()}hi{/snippet}
+</Button>
+<Button>
+	hello
+	{#snippet foo()}x{/snippet}
+</Button>
+<Button>
+	{#snippet children()}hi{/snippet}
+	{#snippet foo()}x{/snippet}
+</Button>
+<div>
+	hello
+	{#snippet children()}hi{/snippet}
+</div>
+
+<!-- invalid -->
+<Button>
+	hello
+	{#snippet children()}hi{/snippet}
+</Button>