fix(ssr): handle export { Cmp as default } @W-17655297

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html

@@ -1,4 +1,19 @@
-<x-cmp>
+<x-parent>
   <template shadowrootmode="open">
+    <x-shadow>
+      <template shadowrootmode="open">
+      </template>
+    </x-shadow>
+    <x-shadow>
+      <template shadowrootmode="open">
+      </template>
+      <h1>
+        slotted content
+      </h1>
+    </x-shadow>
+    <x-light>
+    </x-light>
+    <x-light>
+    </x-light>
   </template>
-</x-cmp>
+</x-parent>

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/index.js

@@ -1,3 +1,3 @@
-export const tagName = 'x-cmp';
-export { default } from 'x/cmp';
-export * from 'x/cmp';
+export const tagName = 'x-parent';
+export { default } from 'x/parent';
+export * from 'x/parent';

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.html

@@ -0,0 +1,3 @@
+<template lwc:render-mode="light">
+  This template isn't actually used because `export {Component as default}` isn't recognized as an LWC component.
+</template>

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js

@@ -0,0 +1,7 @@
+import { LightningElement } from 'lwc';
+
+class Light extends LightningElement {
+    static renderMode = 'light';
+}
+
+export { Light as default };

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html

@@ -0,0 +1,6 @@
+<template>
+  <x-shadow></x-shadow>
+  <x-shadow><h1>slotted content</h1></x-shadow>
+  <x-light></x-light>
+  <x-light><h1>slotted content</h1></x-light>
+</template>

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js

@@ -0,0 +1,3 @@
+import { LightningElement } from 'lwc';
+
+export default class Parent extends LightningElement {}

packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js

@@ -0,0 +1,5 @@
+import { LightningElement } from 'lwc';
+
+class Shadow extends LightningElement {}
+
+export { Shadow as default };

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',
     'wire/errors/throws-when-colliding-prop-then-method/index.js',
 ]);

packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts

@@ -37,19 +37,22 @@ const bYieldFromChildGenerator = esTemplateWithYield`
         }
 
         const scopeToken = hasScopedStylesheets ? stylesheetScopeToken : undefined;
-        const Ctor = ${/* Component */ is.identifier};
-
-        yield* Ctor[__SYMBOL__GENERATE_MARKUP](
-            ${/* tag name */ is.literal}, 
-            childProps, 
-            childAttrs, 
-            shadowSlottedContent,
-            lightSlottedContentMap,
-            scopedSlottedContentMap,
-            instance,
-            scopeToken,
-            contextfulParent
-        );
+        const generateMarkup = ${/* Component */ is.identifier}[__SYMBOL__GENERATE_MARKUP];
+        if (!generateMarkup) {
+            yield* __unimplementedTmpl(${/* tag name */ is.literal}, instance, shadowSlottedContent, ${/* Component */ 3});
+        } else {
+            yield* generateMarkup(
+                ${/* tag name */ 4}, 
+                childProps, 
+                childAttrs, 
+                shadowSlottedContent,
+                lightSlottedContentMap,
+                scopedSlottedContentMap,
+                instance,
+                scopeToken,
+                contextfulParent
+            );
+        }
     }
 `<EsBlockStatement>;
 
@@ -60,6 +63,7 @@ export const Component: Transformer<IrComponent> = function Component(node, cxt)
     cxt.import({ default: childComponentLocalName }, importPath);
     cxt.import({
         SYMBOL__GENERATE_MARKUP: '__SYMBOL__GENERATE_MARKUP',
+        unimplementedTmpl: '__unimplementedTmpl',
     });
     const childTagName = node.name;
 

packages/@lwc/ssr-compiler/src/transmogrify.ts

@@ -114,6 +114,7 @@ const visitors: Visitors = {
         //
         // - renderAttrs vs renderAttrsNoYield
         // - fallbackTmpl vs fallbackTmplNoYield
+        // - unimplementedTmpl vs unimplementedTmplNoYield
         //
         // If this becomes too burdensome to maintain, we can officially deprecate the generator-based approach
         // and switch the @lwc/ssr-runtime implementation wholesale over to the no-generator paradigm.
@@ -136,6 +137,8 @@ const visitors: Visitors = {
             node.imported.name = 'fallbackTmplNoYield';
         } else if (node.imported.name === 'renderAttrs') {
             node.imported.name = 'renderAttrsNoYield';
+        } else if (node.imported.name === 'unimplementedTmpl') {
+            node.imported.name = 'unimplementedTmplNoYield';
         }
     },
 };

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

@@ -28,6 +28,8 @@ export {
     serverSideRenderComponent,
     // renderComponent is an alias for serverSideRenderComponent
     serverSideRenderComponent as renderComponent,
+    unimplementedTmpl,
+    unimplementedTmplNoYield,
 } from './render';
 export { normalizeTextContent, renderTextContent } from './render-text-content';
 export { hasScopedStaticStylesheets, renderStylesheets } from './styles';

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

@@ -105,7 +105,7 @@ export function* fallbackTmpl(
     _lightSlottedContent: unknown,
     _scopedSlottedContent: unknown,
     Cmp: LightningElementConstructor,
-    instance: unknown
+    instance: LightningElement
 ) {
     if (Cmp.renderMode !== 'light') {
         yield `<template shadowrootmode="open"></template>`;
@@ -117,11 +117,11 @@ export function* fallbackTmpl(
 
 export function fallbackTmplNoYield(
     emit: (segment: string) => void,
-    shadowSlottedContent: AsyncGeneratorFunction,
+    shadowSlottedContent: AsyncGeneratorFunction | null,
     _lightSlottedContent: unknown,
     _scopedSlottedContent: unknown,
     Cmp: LightningElementConstructor,
-    instance: unknown
+    instance: LightningElement | null
 ) {
     if (Cmp.renderMode !== 'light') {
         emit(`<template shadowrootmode="open"></template>`);
@@ -131,6 +131,49 @@ export function fallbackTmplNoYield(
     }
 }
 
+/**
+ * If a component is incorrectly implemented, and is missing a `generateMarkup` function,
+ * then use this template as a fallback so the world doesn't explode.
+ * @example export { Cmp as default }
+ */
+export function* unimplementedTmpl(
+    tagName: string,
+    instance: LightningElement,
+    shadowSlottedContent: AsyncGeneratorFunction,
+    Cmp?: LightningElementConstructor
+) {
+    yield `<${tagName}>`;
+    if (Cmp?.renderMode !== 'light') {
+        yield '<template shadowrootmode="open"></template>';
+        if (shadowSlottedContent) {
+            yield shadowSlottedContent(instance);
+        }
+    }
+    yield `</${tagName}>`;
+}
+
+/**
+ * If a component is incorrectly implemented, and is missing a `generateMarkup` function,
+ * then use this template as a fallback so the world doesn't explode.
+ * @example export { Cmp as default }
+ */
+export function unimplementedTmplNoYield(
+    emit: (segment: string) => void,
+    tagName: string,
+    instance: LightningElement,
+    shadowSlottedContent: AsyncGeneratorFunction,
+    Cmp?: LightningElementConstructor
+) {
+    emit(`<${tagName}>`);
+    if (Cmp?.renderMode !== 'light') {
+        emit('<template shadowrootmode="open"></template>');
+        if (shadowSlottedContent) {
+            shadowSlottedContent(emit, instance);
+        }
+    }
+    emit(`</${tagName}>`);
+}
+
 export type GenerateMarkupFn = (
     tagName: string,
     props: Properties | null,
@@ -180,7 +223,7 @@ type GenerateMarkupFnVariants =
     | GenerateMarkupFnAsyncNoGen
     | GenerateMarkupFnSyncNoGen;
 
-interface ComponentWithGenerateMarkup {
+interface ComponentWithGenerateMarkup extends LightningElementConstructor {
     [SYMBOL__GENERATE_MARKUP]: GenerateMarkupFnVariants;
 }
 
@@ -201,6 +244,14 @@ 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,