[feat] add fallback component for layouts without one

.changeset/purple-parrots-tell.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[feat] add fallback component for layouts without one

packages/kit/src/core/sync/create_manifest_data/index.js

@@ -258,7 +258,12 @@ function create_routes_and_nodes(cwd, config, fallback) {
 	// and smaller indexes take fewer bytes. also, this guarantees that
 	// the default error/layout are 0/1
 	for (const route of routes) {
-		if (route.layout) nodes.push(route.layout);
+		if (route.layout) {
+			if (!route.layout?.component) {
+				route.layout.component = posixify(path.relative(cwd, `${fallback}/layout.svelte`));
+			}
+			nodes.push(route.layout);
+		}
 		if (route.error) nodes.push(route.error);
 	}
 

packages/kit/src/core/sync/create_manifest_data/index.spec.js

@@ -635,6 +635,7 @@ test('handles pages without .svelte file', () => {
 		default_error,
 		{ component: 'samples/page-without-svelte-file/error/+error.svelte' },
 		{ component: 'samples/page-without-svelte-file/layout/+layout.svelte' },
+		{ ...default_layout, shared: 'samples/page-without-svelte-file/layout/exists/+layout.js' },
 		{ component: 'samples/page-without-svelte-file/+page.svelte' },
 		{ shared: 'samples/page-without-svelte-file/error/[...path]/+page.js' },
 		{ component: 'samples/page-without-svelte-file/layout/exists/+page.svelte' },
@@ -645,7 +646,7 @@ test('handles pages without .svelte file', () => {
 		{
 			id: '/',
 			pattern: '/^/$/',
-			page: { layouts: [0], errors: [1], leaf: 4 }
+			page: { layouts: [0], errors: [1], leaf: 5 }
 		},
 		{
 			id: '/error',
@@ -654,7 +655,7 @@ test('handles pages without .svelte file', () => {
 		{
 			id: '/error/[...path]',
 			pattern: '/^/error(?:/(.*))?/?$/',
-			page: { layouts: [0, undefined], errors: [1, 2], leaf: 5 }
+			page: { layouts: [0, undefined], errors: [1, 2], leaf: 6 }
 		},
 		{
 			id: '/layout',
@@ -663,12 +664,12 @@ test('handles pages without .svelte file', () => {
 		{
 			id: '/layout/exists',
 			pattern: '/^/layout/exists/?$/',
-			page: { layouts: [0, 3], errors: [1, undefined], leaf: 6 }
+			page: { layouts: [0, 3, 4], errors: [1, undefined, undefined], leaf: 7 }
 		},
 		{
 			id: '/layout/redirect',
 			pattern: '/^/layout/redirect/?$/',
-			page: { layouts: [0, 3], errors: [1, undefined], leaf: 7 }
+			page: { layouts: [0, 3], errors: [1, undefined], leaf: 8 }
 		}
 	]);
 });