From 48d6fddf8e83dd9f093ff9401fd5123290875a99 Mon Sep 17 00:00:00 2001
From: Will Harney <wharney@salesforce.com>
Date: Thu, 23 Jan 2025 14:40:21 -0500
Subject: [PATCH 1/8] fix: allow export { Cmp as default }

---
 .../@lwc/ssr-compiler/src/compile-js/index.ts | 49 +++++++++++++++----
 1 file changed, 40 insertions(+), 9 deletions(-)

diff --git a/packages/@lwc/ssr-compiler/src/compile-js/index.ts b/packages/@lwc/ssr-compiler/src/compile-js/index.ts
index a6386dcadf..81ca7cb1e7 100644
--- a/packages/@lwc/ssr-compiler/src/compile-js/index.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-js/index.ts
@@ -20,11 +20,13 @@ 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';
@@ -72,15 +74,7 @@ const visitors: Visitors = {
     },
     ClassDeclaration(path, state) {
         const { node } = path;
-        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 (123) {
             // If it's a default-exported class with a superclass, then it's an LWC component!
             state.isLWC = true;
             if (node.id) {
@@ -218,6 +212,43 @@ 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;

From 1a8673c9ebe2f03c238acb26fcd0e8f49e02036f Mon Sep 17 00:00:00 2001
From: Will Harney <wharney@salesforce.com>
Date: Fri, 24 Jan 2025 12:31:13 -0500
Subject: [PATCH 2/8] test(ssr): update component-as-default to check light and
 shadow

---
 .../exports/component-as-default/expected.html         | 10 ++++++++--
 .../fixtures/exports/component-as-default/index.js     |  6 +++---
 .../component-as-default/modules/x/light/light.html    |  3 +++
 .../component-as-default/modules/x/light/light.js      |  7 +++++++
 .../component-as-default/modules/x/parent/parent.html  |  4 ++++
 .../component-as-default/modules/x/parent/parent.js    |  3 +++
 .../modules/x/{cmp/cmp.html => shadow/shadow.html}     |  0
 .../modules/x/{cmp/cmp.js => shadow/shadow.js}         |  1 +
 8 files changed, 29 insertions(+), 5 deletions(-)
 create mode 100644 packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.html
 create mode 100644 packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js
 create mode 100644 packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
 create mode 100644 packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js
 rename packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/{cmp/cmp.html => shadow/shadow.html} (100%)
 rename packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/{cmp/cmp.js => shadow/shadow.js} (99%)

diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
index ce996e78e1..d6a688f78a 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
@@ -1,4 +1,10 @@
-<x-cmp>
+<x-parent>
   <template shadowrootmode="open">
+    <x-shadow>
+      <template shadowrootmode="open">
+      </template>
+    </x-shadow>
+    <x-light>
+    </x-light>
   </template>
-</x-cmp>
\ No newline at end of file
+</x-parent>
\ No newline at end of file
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/index.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/index.js
index 55d4302e11..d5a55ceefa 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/index.js
+++ b/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';
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.html
new file mode 100644
index 0000000000..ef261dbb01
--- /dev/null
+++ b/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>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js
new file mode 100644
index 0000000000..3902ccb766
--- /dev/null
+++ b/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 Component extends LightningElement {
+    static renderMode = 'light';
+}
+
+export { Component as default };
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
new file mode 100644
index 0000000000..0c64ecaad1
--- /dev/null
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
@@ -0,0 +1,4 @@
+<template>
+  <x-shadow></x-shadow>
+  <x-light></x-light>
+</template>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js
new file mode 100644
index 0000000000..6d3542bb2f
--- /dev/null
+++ b/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 Component extends LightningElement {}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/cmp/cmp.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.html
similarity index 100%
rename from packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/cmp/cmp.html
rename to packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.html
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/cmp/cmp.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
similarity index 99%
rename from packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/cmp/cmp.js
rename to packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
index e0542c7a5d..3048e5440e 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/cmp/cmp.js
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
@@ -1,4 +1,5 @@
 import { LightningElement } from 'lwc';
 
 class Component extends LightningElement {}
+
 export { Component as default };

From c5432bfa8f987bb6bc158ab0604379d380ab9282 Mon Sep 17 00:00:00 2001
From: Will Harney <wharney@salesforce.com>
Date: Fri, 24 Jan 2025 13:24:26 -0500
Subject: [PATCH 3/8] fix: use fallback if no generateMarkup

---
 .../src/__tests__/utils/expected-failures.ts  |  1 -
 .../@lwc/ssr-compiler/src/compile-js/index.ts | 49 ++++---------------
 packages/@lwc/ssr-runtime/src/render.ts       | 10 +++-
 3 files changed, 18 insertions(+), 42 deletions(-)

diff --git a/packages/@lwc/ssr-compiler/src/__tests__/utils/expected-failures.ts b/packages/@lwc/ssr-compiler/src/__tests__/utils/expected-failures.ts
index b2f8518054..ffedb6f8f9 100644
--- a/packages/@lwc/ssr-compiler/src/__tests__/utils/expected-failures.ts
+++ b/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',
diff --git a/packages/@lwc/ssr-compiler/src/compile-js/index.ts b/packages/@lwc/ssr-compiler/src/compile-js/index.ts
index 81ca7cb1e7..a6386dcadf 100644
--- a/packages/@lwc/ssr-compiler/src/compile-js/index.ts
+++ b/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;
diff --git a/packages/@lwc/ssr-runtime/src/render.ts b/packages/@lwc/ssr-runtime/src/render.ts
index 188c169ba7..e9de71cfcf 100644
--- a/packages/@lwc/ssr-runtime/src/render.ts
+++ b/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,

From 17bb3424f4a8c8c2376c4426975aa2dfb95e00c6 Mon Sep 17 00:00:00 2001
From: Will Harney <wharney@salesforce.com>
Date: Wed, 29 Jan 2025 11:58:28 -0500
Subject: [PATCH 4/8] fix(ssr): support `export { Cmp as default }`

---
 .../modules/x/light/light.js                  |  4 +--
 .../modules/x/parent/parent.js                |  2 +-
 .../modules/x/shadow/shadow.js                |  4 +--
 .../transformers/component/component.ts       | 29 +++++++++++--------
 packages/@lwc/ssr-runtime/src/index.ts        |  1 +
 packages/@lwc/ssr-runtime/src/render.ts       | 14 +++++++++
 6 files changed, 37 insertions(+), 17 deletions(-)

diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js
index 3902ccb766..65a040de68 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/light/light.js
@@ -1,7 +1,7 @@
 import { LightningElement } from 'lwc';
 
-class Component extends LightningElement {
+class Light extends LightningElement {
     static renderMode = 'light';
 }
 
-export { Component as default };
+export { Light as default };
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js
index 6d3542bb2f..a5746a015a 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.js
@@ -1,3 +1,3 @@
 import { LightningElement } from 'lwc';
 
-export default class Component extends LightningElement {}
+export default class Parent extends LightningElement {}
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
index 3048e5440e..098f79650f 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/shadow/shadow.js
@@ -1,5 +1,5 @@
 import { LightningElement } from 'lwc';
 
-class Component extends LightningElement {}
+class Shadow extends LightningElement {}
 
-export { Component as default };
+export { Shadow as default };
diff --git a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
index c02ffe8bf8..935c35bd11 100644
--- a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
@@ -37,19 +37,23 @@ const bYieldFromChildGenerator = esTemplateWithYield`
         }
 
         const scopeToken = hasScopedStylesheets ? stylesheetScopeToken : undefined;
-        const Ctor = ${/* Component */ is.identifier};
+        const generateMarkup = ${/* Component */ is.identifier}[__SYMBOL__GENERATE_MARKUP];
 
-        yield* Ctor[__SYMBOL__GENERATE_MARKUP](
-            ${/* tag name */ is.literal}, 
-            childProps, 
-            childAttrs, 
-            shadowSlottedContent,
-            lightSlottedContentMap,
-            scopedSlottedContentMap,
-            instance,
-            scopeToken,
-            contextfulParent
-        );
+        if (!generateMarkup) {
+            yield __unimplementedTmpl(${/* tag name */ is.literal}, ${/* Component */ 3});
+        } else {
+            yield* generateMarkup(
+                ${/* tag name */ 4}, 
+                childProps, 
+                childAttrs, 
+                shadowSlottedContent,
+                lightSlottedContentMap,
+                scopedSlottedContentMap,
+                instance,
+                scopeToken,
+                contextfulParent
+            );
+        }
     }
 `<EsBlockStatement>;
 
@@ -60,6 +64,7 @@ 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;
 
diff --git a/packages/@lwc/ssr-runtime/src/index.ts b/packages/@lwc/ssr-runtime/src/index.ts
index cfa9b8dd53..3d89ed79e7 100644
--- a/packages/@lwc/ssr-runtime/src/index.ts
+++ b/packages/@lwc/ssr-runtime/src/index.ts
@@ -28,6 +28,7 @@ export {
     serverSideRenderComponent,
     // renderComponent is an alias for serverSideRenderComponent
     serverSideRenderComponent as renderComponent,
+    unimplementedTmpl,
 } from './render';
 export { normalizeTextContent, renderTextContent } from './render-text-content';
 export { hasScopedStaticStylesheets, renderStylesheets } from './styles';
diff --git a/packages/@lwc/ssr-runtime/src/render.ts b/packages/@lwc/ssr-runtime/src/render.ts
index e9de71cfcf..ca2ee8bd83 100644
--- a/packages/@lwc/ssr-runtime/src/render.ts
+++ b/packages/@lwc/ssr-runtime/src/render.ts
@@ -125,6 +125,20 @@ 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, Cmp?: LightningElementConstructor): string {
+    let html = `<${tagName}>`;
+    if (Cmp?.renderMode !== 'light') {
+        html += '<template shadowrootmode="open"></template>';
+    }
+    html += `</${tagName}>`;
+    return html;
+}
+
 export type GenerateMarkupFn = (
     tagName: string,
     props: Properties | null,

From b460d35b6a8b8b6674c9c5016079a0b4ff2eb00d Mon Sep 17 00:00:00 2001
From: John Hefferman <jhefferman@salesforce.com>
Date: Thu, 30 Jan 2025 14:41:35 -0700
Subject: [PATCH 5/8] fix: support slotted content for unimplemented templates

---
 .../component-as-default/expected.html        |  9 ++++
 .../modules/x/parent/parent.html              |  2 +
 .../transformers/component/component.ts       |  3 +-
 .../@lwc/ssr-compiler/src/transmogrify.ts     |  3 ++
 packages/@lwc/ssr-runtime/src/index.ts        |  1 +
 packages/@lwc/ssr-runtime/src/render.ts       | 45 +++++++++++++++----
 6 files changed, 53 insertions(+), 10 deletions(-)

diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
index d6a688f78a..1dd9a11014 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/expected.html
@@ -4,6 +4,15 @@
       <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>
diff --git a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
index 0c64ecaad1..7ace378547 100644
--- a/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
+++ b/packages/@lwc/engine-server/src/__tests__/fixtures/exports/component-as-default/modules/x/parent/parent.html
@@ -1,4 +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>
diff --git a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
index 935c35bd11..d773515d14 100644
--- a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
@@ -38,9 +38,8 @@ const bYieldFromChildGenerator = esTemplateWithYield`
 
         const scopeToken = hasScopedStylesheets ? stylesheetScopeToken : undefined;
         const generateMarkup = ${/* Component */ is.identifier}[__SYMBOL__GENERATE_MARKUP];
-
         if (!generateMarkup) {
-            yield __unimplementedTmpl(${/* tag name */ is.literal}, ${/* Component */ 3});
+            yield* __unimplementedTmpl(${/* tag name */ is.literal}, instance, shadowSlottedContent, ${/* Component */ 3});
         } else {
             yield* generateMarkup(
                 ${/* tag name */ 4}, 
diff --git a/packages/@lwc/ssr-compiler/src/transmogrify.ts b/packages/@lwc/ssr-compiler/src/transmogrify.ts
index 6a3a89a579..aeb1f58ff5 100644
--- a/packages/@lwc/ssr-compiler/src/transmogrify.ts
+++ b/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';
         }
     },
 };
diff --git a/packages/@lwc/ssr-runtime/src/index.ts b/packages/@lwc/ssr-runtime/src/index.ts
index 3d89ed79e7..51416bd6af 100644
--- a/packages/@lwc/ssr-runtime/src/index.ts
+++ b/packages/@lwc/ssr-runtime/src/index.ts
@@ -29,6 +29,7 @@ export {
     // 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';
diff --git a/packages/@lwc/ssr-runtime/src/render.ts b/packages/@lwc/ssr-runtime/src/render.ts
index 63cd8efcb8..6a8a03675a 100644
--- a/packages/@lwc/ssr-runtime/src/render.ts
+++ b/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>`);
@@ -136,13 +136,42 @@ export function fallbackTmplNoYield(
  * then use this template as a fallback so the world doesn't explode.
  * @example export { Cmp as default }
  */
-export function unimplementedTmpl(tagName: string, Cmp?: LightningElementConstructor): string {
-    let html = `<${tagName}>`;
+export function* unimplementedTmpl(
+    tagName: string,
+    instance: LightningElement,
+    shadowSlottedContent: AsyncGeneratorFunction,
+    Cmp?: LightningElementConstructor
+) {
+    yield `<${tagName}>`;
     if (Cmp?.renderMode !== 'light') {
-        html += '<template shadowrootmode="open"></template>';
+        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);
+        }
     }
-    html += `</${tagName}>`;
-    return html;
+    emit(`</${tagName}>`);
 }
 
 export type GenerateMarkupFn = (

From b5868f17abc35b6465653441f07581297d629c29 Mon Sep 17 00:00:00 2001
From: John Hefferman <jhefferman@salesforce.com>
Date: Fri, 31 Jan 2025 09:25:30 -0700
Subject: [PATCH 6/8] fix: use fallbackTmpl for all cases

---
 .../transformers/component/component.ts       | 12 ++++--
 .../@lwc/ssr-compiler/src/transmogrify.ts     |  3 --
 packages/@lwc/ssr-runtime/src/index.ts        |  2 -
 packages/@lwc/ssr-runtime/src/render.ts       | 43 -------------------
 4 files changed, 8 insertions(+), 52 deletions(-)

diff --git a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
index d773515d14..0d0232fb14 100644
--- a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
@@ -31,18 +31,22 @@ const bYieldFromChildGenerator = esTemplateWithYield`
                 Slotted content is inserted here.
                 Note that the slotted content will be stored in variables named 
                 `shadowSlottedContent`/`lightSlottedContentMap / scopedSlottedContentMap` which are used below 
-                when the child's generateMarkup function is invoked.
+            when the child's generateMarkup function is invoked.
             */
             is.statement
         }
 
         const scopeToken = hasScopedStylesheets ? stylesheetScopeToken : undefined;
         const generateMarkup = ${/* Component */ is.identifier}[__SYMBOL__GENERATE_MARKUP];
+        const tagName = ${/* tag name */ is.literal};
+
         if (!generateMarkup) {
-            yield* __unimplementedTmpl(${/* tag name */ is.literal}, instance, shadowSlottedContent, ${/* Component */ 3});
+            yield \`<\${tagName}>\`;
+            yield* __fallbackTmpl(shadowSlottedContent, lightSlottedContentMap, scopedSlottedContentMap, ${/* Component */ 3}, instance)
+            yield \`</\${tagName}>\`;
         } else {
             yield* generateMarkup(
-                ${/* tag name */ 4}, 
+                tagName, 
                 childProps, 
                 childAttrs, 
                 shadowSlottedContent,
@@ -63,7 +67,7 @@ export const Component: Transformer<IrComponent> = function Component(node, cxt)
     cxt.import({ default: childComponentLocalName }, importPath);
     cxt.import({
         SYMBOL__GENERATE_MARKUP: '__SYMBOL__GENERATE_MARKUP',
-        unimplementedTmpl: '__unimplementedTmpl',
+        fallbackTmpl: '__fallbackTmpl',
     });
     const childTagName = node.name;
 
diff --git a/packages/@lwc/ssr-compiler/src/transmogrify.ts b/packages/@lwc/ssr-compiler/src/transmogrify.ts
index aeb1f58ff5..6a3a89a579 100644
--- a/packages/@lwc/ssr-compiler/src/transmogrify.ts
+++ b/packages/@lwc/ssr-compiler/src/transmogrify.ts
@@ -114,7 +114,6 @@ 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.
@@ -137,8 +136,6 @@ 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';
         }
     },
 };
diff --git a/packages/@lwc/ssr-runtime/src/index.ts b/packages/@lwc/ssr-runtime/src/index.ts
index 51416bd6af..cfa9b8dd53 100644
--- a/packages/@lwc/ssr-runtime/src/index.ts
+++ b/packages/@lwc/ssr-runtime/src/index.ts
@@ -28,8 +28,6 @@ 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';
diff --git a/packages/@lwc/ssr-runtime/src/render.ts b/packages/@lwc/ssr-runtime/src/render.ts
index 6a8a03675a..98e15d4f25 100644
--- a/packages/@lwc/ssr-runtime/src/render.ts
+++ b/packages/@lwc/ssr-runtime/src/render.ts
@@ -131,49 +131,6 @@ 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,

From 01af84f859cf30aeca179e253a72145da5074e81 Mon Sep 17 00:00:00 2001
From: Will Harney <62956339+wjhsf@users.noreply.github.com>
Date: Fri, 31 Jan 2025 11:36:10 -0500
Subject: [PATCH 7/8] Update
 packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts

---
 .../transformers/component/component.ts              | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
index 0d0232fb14..6440a18db6 100644
--- a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
@@ -40,12 +40,8 @@ const bYieldFromChildGenerator = esTemplateWithYield`
         const generateMarkup = ${/* Component */ is.identifier}[__SYMBOL__GENERATE_MARKUP];
         const tagName = ${/* tag name */ is.literal};
 
-        if (!generateMarkup) {
-            yield \`<\${tagName}>\`;
-            yield* __fallbackTmpl(shadowSlottedContent, lightSlottedContentMap, scopedSlottedContentMap, ${/* Component */ 3}, instance)
-            yield \`</\${tagName}>\`;
-        } else {
-            yield* generateMarkup(
+        if (generateMarkup) {
+                yield* generateMarkup(
                 tagName, 
                 childProps, 
                 childAttrs, 
@@ -56,6 +52,10 @@ const bYieldFromChildGenerator = esTemplateWithYield`
                 scopeToken,
                 contextfulParent
             );
+        } else {
+            yield \`<\${tagName}>\`;
+            yield* __fallbackTmpl(shadowSlottedContent, lightSlottedContentMap, scopedSlottedContentMap, ${/* Component */ 3}, instance)
+            yield \`</\${tagName}>\`;
         }
     }
 `<EsBlockStatement>;

From e83a19e3e871e7d78e35c31054a6d132b7602e3f Mon Sep 17 00:00:00 2001
From: Will Harney <62956339+wjhsf@users.noreply.github.com>
Date: Fri, 31 Jan 2025 11:36:31 -0500
Subject: [PATCH 8/8] Update
 packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts

---
 .../src/compile-template/transformers/component/component.ts    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
index 6440a18db6..25540e1c51 100644
--- a/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
+++ b/packages/@lwc/ssr-compiler/src/compile-template/transformers/component/component.ts
@@ -41,7 +41,7 @@ const bYieldFromChildGenerator = esTemplateWithYield`
         const tagName = ${/* tag name */ is.literal};
 
         if (generateMarkup) {
-                yield* generateMarkup(
+            yield* generateMarkup(
                 tagName, 
                 childProps, 
                 childAttrs,