fix: generate correct SSR code for the case where pending is an attribute (#16919)

Closes #16886

Author: paoloricciuti

.changeset/spicy-rabbits-drive.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: generate correct  SSR code for the case where `pending` is an attribute

packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js

@@ -43,11 +43,14 @@ export function SvelteBoundary(node, context) {
 			);
 			const pending = b.call(callee, b.id('$$renderer'));
 			const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
+			const statement = node.fragment.metadata.has_await
+				? create_async_block(b.block([block]))
+				: block;
 			context.state.template.push(
 				b.if(
 					callee,
-					b.block(build_template([block_open_else, pending, block_close])),
-					b.block(build_template([block_open, block, block_close]))
+					b.block([b.stmt(pending)]),
+					b.block(build_template([block_open, statement, block_close]))
 				)
 			);
 		} else {

packages/svelte/tests/server-side-rendering/samples/boundary-pending-prop-async/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/server-side-rendering/samples/boundary-pending-prop-async/_expected.html

@@ -0,0 +1 @@
+<!--[--><!---->Loading...<!--]-->

packages/svelte/tests/server-side-rendering/samples/boundary-pending-prop-async/main.svelte

@@ -0,0 +1,8 @@
+{#snippet pending()}
+	Loading...
+{/snippet}
+
+<svelte:boundary pending={pending}>
+	{@const data = await Promise.resolve('hello')}
+	<p>{data}</p>
+</svelte:boundary>