fix: use fallback if no generateMarkup

Author: wjhsf

packages/@lwc/ssr-compiler/src/__tests__/utils/expected-failures.ts

@@ -10,7 +10,6 @@
 export const expectedFailures = new Set([
     'attribute-global-html/as-component-prop/undeclared/index.js',
     'attribute-global-html/as-component-prop/without-@api/index.js',
-    'exports/component-as-default/index.js',
     'known-boolean-attributes/default-def-html-attributes/static-on-component/index.js',
     'render-dynamic-value/index.js',
     'slot-forwarding/slots/mixed/index.js',

packages/@lwc/ssr-compiler/src/compile-js/index.ts

@@ -20,13 +20,11 @@ import { catalogWireAdapters } from './wire';
 
 import { removeDecoratorImport } from './remove-decorator-import';
 import { generateError } from './errors';
-import type { NodePath } from 'estree-toolkit';
 import type { ComponentTransformOptions } from '../shared';
 import type {
     Identifier as EsIdentifier,
     Program as EsProgram,
     Decorator as EsDecorator,
-    ClassDeclaration as EsClassDeclaration,
 } from 'estree';
 import type { Visitors, ComponentMetaState } from './types';
 import type { CompilationMode } from '@lwc/shared';
@@ -74,7 +72,15 @@ const visitors: Visitors = {
     },
     ClassDeclaration(path, state) {
         const { node } = path;
-        if (123) {
+        if (
+            node?.superClass &&
+            // export default class extends LightningElement {}
+            (is.exportDefaultDeclaration(path.parentPath) ||
+                // class Cmp extends LightningElement {}; export default Cmp
+                path.scope
+                    ?.getBinding(node.id.name)
+                    ?.references.some((ref) => is.exportDefaultDeclaration(ref.parent)))
+        ) {
             // If it's a default-exported class with a superclass, then it's an LWC component!
             state.isLWC = true;
             if (node.id) {
@@ -212,43 +218,6 @@ const visitors: Visitors = {
     },
 };
 
-/**
- * Determines whether a class declaration is an LWC component. Returns true if the class has a
- * superclass and is a default export.
- */
-function isLwcComponent(path: NodePath<EsClassDeclaration>) {
-    const { node } = path;
-    if (!node?.superClass) {
-        // class Cmp {}
-        return false;
-    }
-    if (is.exportDefaultDeclaration(path.parentPath)) {
-        // export default class Cmp extends LightningElement {} => true
-        return true;
-    }
-    const binding = path.scope?.getBinding(node.id.name);
-    if (!binding) {
-        // Never exported
-        return false;
-    }
-    return binding.references.some((ref) => {
-        const { parent } = ref;
-        if (is.exportDefaultDeclaration(parent)) {
-            // class Cmp extends LightningElement {}; export default Cmp
-            return true;
-        }
-        if (is.exportSpecifier(parent)) {
-            // class Cmp extends LightningElement {}; export { Cmp as default }
-            // class Cmp extends LightningElement {}; export { Cmp as 'default' }
-            const exported = is.identifier(parent.exported)
-                ? parent.exported.name
-                : parent.exported.value;
-            return exported === 'default';
-        }
-        return false;
-    });
-}
-
 function validateUniqueDecorator(decorators: EsDecorator[]) {
     if (decorators.length < 2) {
         return;

packages/@lwc/ssr-runtime/src/render.ts

@@ -174,7 +174,7 @@ type GenerateMarkupFnVariants =
     | GenerateMarkupFnAsyncNoGen
     | GenerateMarkupFnSyncNoGen;
 
-interface ComponentWithGenerateMarkup {
+interface ComponentWithGenerateMarkup extends LightningElementConstructor {
     [SYMBOL__GENERATE_MARKUP]: GenerateMarkupFnVariants;
 }
 
@@ -195,6 +195,14 @@ export async function serverSideRenderComponent(
         markup += segment;
     };
 
+    if (!generateMarkup) {
+        // If a non-component is accidentally provided, render an empty template
+        emit(`<${tagName}>`);
+        fallbackTmplNoYield(emit, null, null, null, Component, null);
+        emit(`</${tagName}>`);
+        return markup;
+    }
+
     if (mode === 'asyncYield') {
         for await (const segment of (generateMarkup as GenerateMarkupFn)(
             tagName,