fix: add `then` to class component `render` output (#16783)

* fix: add `then` to class component `render` output

* drive-by tidy up

* fix types

.changeset/sixty-crabs-laugh.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add `then` to class component `render` output

packages/svelte/src/internal/server/index.js

@@ -1,5 +1,5 @@
 /** @import { ComponentType, SvelteComponent, Component } from 'svelte' */
-/** @import { RenderOutput, SSRContext } from '#server' */
+/** @import { RenderOutput } from '#server' */
 /** @import { Store } from '#shared' */
 /** @import { AccumulatedContent } from './payload.js' */
 export { FILENAME, HMR } from '../../constants.js';
@@ -14,14 +14,10 @@ import {
 } from '../../constants.js';
 import { escape_html } from '../../escaping.js';
 import { DEV } from 'esm-env';
-import { ssr_context, pop, push, set_ssr_context } from './context.js';
 import { EMPTY_COMMENT, BLOCK_CLOSE, BLOCK_OPEN, BLOCK_OPEN_ELSE } from './hydration.js';
 import { validate_store } from '../shared/validate.js';
 import { is_boolean_attribute, is_raw_text_element, is_void } from '../../utils.js';
-import { Payload, SSRState } from './payload.js';
-import { abort } from './abort-signal.js';
-import { async_mode_flag } from '../flags/index.js';
-import * as e from './errors.js';
+import { Payload } from './payload.js';
 
 // https://html.spec.whatwg.org/multipage/syntax.html#attributes-2
 // https://infra.spec.whatwg.org/#noncharacter

packages/svelte/src/legacy/legacy-server.js

@@ -1,11 +1,15 @@
 /** @import { SvelteComponent } from '../index.js' */
 import { asClassComponent as as_class_component, createClassComponent } from './legacy-client.js';
 import { render } from '../internal/server/index.js';
+import { async_mode_flag } from '../internal/flags/index.js';
+import * as w from '../internal/server/warnings.js';
 
 // By having this as a separate entry point for server environments, we save the client bundle from having to include the server runtime
 
 export { createClassComponent };
 
+/** @typedef {{ head: string, html: string, css: { code: string, map: null }}} LegacyRenderResult */
+
 /**
  * Takes a Svelte 5 component and returns a Svelte 4 compatible component constructor.
  *
@@ -21,15 +25,56 @@ export { createClassComponent };
  */
 export function asClassComponent(component) {
 	const component_constructor = as_class_component(component);
-	/** @type {(props?: {}, opts?: { $$slots?: {}; context?: Map<any, any>; }) => { html: any; css: { code: string; map: any; }; head: string; } } */
+	/** @type {(props?: {}, opts?: { $$slots?: {}; context?: Map<any, any>; }) => LegacyRenderResult & PromiseLike<LegacyRenderResult> } */
 	const _render = (props, { context } = {}) => {
 		// @ts-expect-error the typings are off, but this will work if the component is compiled in SSR mode
 		const result = render(component, { props, context });
-		return {
-			css: { code: '', map: null },
-			head: result.head,
-			html: result.body
-		};
+
+		const munged = Object.defineProperties(
+			/** @type {LegacyRenderResult & PromiseLike<LegacyRenderResult>} */ ({}),
+			{
+				css: {
+					value: { code: '', map: null }
+				},
+				head: {
+					get: () => result.head
+				},
+				html: {
+					get: () => result.body
+				},
+				then: {
+					/**
+					 * this is not type-safe, but honestly it's the best I can do right now, and it's a straightforward function.
+					 *
+					 * @template TResult1
+					 * @template [TResult2=never]
+					 * @param { (value: LegacyRenderResult) => TResult1 } onfulfilled
+					 * @param { (reason: unknown) => TResult2 } onrejected
+					 */
+					value: (onfulfilled, onrejected) => {
+						if (!async_mode_flag) {
+							w.experimental_async_ssr();
+							const user_result = onfulfilled({
+								css: munged.css,
+								head: munged.head,
+								html: munged.html
+							});
+							return Promise.resolve(user_result);
+						}
+
+						return result.then((result) => {
+							return onfulfilled({
+								css: munged.css,
+								head: result.head,
+								html: result.body
+							});
+						}, onrejected);
+					}
+				}
+			}
+		);
+
+		return munged;
 	};
 
 	// @ts-expect-error this is present for SSR