fix: suppress inner content warning when children prop is forwarded (#15269)

* fix: suppress inner content warning when children prop is forwarded

When a +layout.svelte passes children to a child component via
shorthand ({children}) or explicit prop (children={children}),
the "inner content will not be rendered" warning no longer fires.

Closes #13504

* refactor: extract has_children into utils.js to avoid logic duplication

Author: FrankFMY

.changeset/fix-layout-children-forwarding-warning.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: suppress false-positive inner content warning when children prop is forwarded to a child component

packages/kit/src/exports/vite/index.js

@@ -40,7 +40,7 @@ import {
 } from './module_ids.js';
 import { import_peer } from '../../utils/import.js';
 import { compact } from '../../utils/array.js';
-import { should_ignore } from './static_analysis/utils.js';
+import { should_ignore, has_children } from './static_analysis/utils.js';
 
 const cwd = posixify(process.cwd());
 
@@ -112,10 +112,8 @@ const warning_preprocessor = {
 		if (!filename) return;
 
 		const basename = path.basename(filename);
-		const has_children =
-			content.includes('<slot') || (isSvelte5Plus() && content.includes('{@render'));
 
-		if (basename.startsWith('+layout.') && !has_children) {
+		if (basename.startsWith('+layout.') && !has_children(content, isSvelte5Plus())) {
 			const message =
 				`\n${colors.bold().red(path.relative('.', filename))}\n` +
 				`\`<slot />\`${isSvelte5Plus() ? ' or `{@render ...}` tag' : ''}` +

packages/kit/src/exports/vite/static_analysis/utils.js

@@ -1,3 +1,20 @@
+/**
+ * Check if content has children rendering (slot, @render, or children prop forwarding)
+ * @param {string} content - The markup content
+ * @param {boolean} is_svelte_5_plus - Whether the project uses Svelte 5+
+ * @returns {boolean}
+ */
+export function has_children(content, is_svelte_5_plus) {
+	return (
+		content.includes('<slot') ||
+		(is_svelte_5_plus &&
+			(content.includes('{@render') ||
+				// children may be forwarded to a child component as a prop
+				content.includes('{children}') ||
+				content.includes('children={')))
+	);
+}
+
 /**
  * Check if a match position is within a comment or a string
  * @param {string} content - The full content

packages/kit/src/exports/vite/static_analysis/utils.spec.js

@@ -1,6 +1,6 @@
 import { expect, test, vi } from 'vitest';
 import path from 'node:path';
-import { should_ignore } from './utils.js';
+import { should_ignore, has_children } from './utils.js';
 
 // Mock the colors module to avoid issues in tests
 vi.mock('kleur', () => ({
@@ -153,3 +153,27 @@ test.each([
 	const result = should_warn_for_content(content, filename);
 	expect(result).toBe(should_warn);
 });
+
+test.each([
+	['layout with @render children()', '{@render children()}', true],
+	['layout with slot', '<slot />', true],
+	['layout with named slot', '<slot name="default" />', true],
+	[
+		'layout forwarding children as shorthand prop',
+		'<script>\n\tlet { children } = $props();\n</script>\n<Layout {children} />',
+		true
+	],
+	[
+		'layout forwarding children as explicit prop',
+		'<script>\n\tlet { children } = $props();\n</script>\n<Layout children={children} />',
+		true
+	],
+	[
+		'layout with no children handling',
+		'<script>\n\tlet { data } = $props();\n</script>\n<div>{data}</div>',
+		false
+	],
+	['empty layout', '', false]
+])('layout children detection: %s', (_description, content, expected) => {
+	expect(has_children(content, true)).toBe(expected);
+});