fix: address review feedback

packages/@lwc/engine-core/src/framework/api.ts

@@ -21,6 +21,7 @@ import {
     isUndefined,
     StringReplace,
     toString,
+    ArrayConcat,
 } from '@lwc/shared';
 
 import { logError } from '../shared/logger';
@@ -193,19 +194,19 @@ function s(
             if (vnode && isVScopedSlotContent(vnode)) {
                 const vmBeingRenderedInception = getVMBeingRendered();
                 let children: VNodes = [];
+                if (!isUndefined(slotset.owner)) {
+                    // Evaluate in the scope of the slot content's owner
+                    setVMBeingRendered(slotset.owner);
+                }
                 try {
-                    if (!isUndefined(slotset.owner)) {
-                        // Evaluate in the scope of the slot content's owner
-                        setVMBeingRendered(slotset.owner);
-                    }
                     children = vnode.factory(data.slotData);
                 } finally {
                     setVMBeingRendered(vmBeingRenderedInception);
                 }
-                return acc.concat(children);
+                return ArrayConcat.call(acc, children);
             } else {
                 // If the slot content is a static list of child nodes provided by the parent, nothing to do
-                return acc.concat(vnode);
+                return ArrayConcat.call(acc, vnode);
             }
         }, [] as VNodes);
     }

packages/@lwc/engine-core/src/framework/hydration.ts

@@ -247,7 +247,7 @@ function hydrateCustomElement(
     vnode.elm = elm;
     vnode.vm = vm;
 
-    allocateChildren(vnode, vm, owner);
+    allocateChildren(vnode, vm);
     patchElementPropsAndAttrs(vnode, renderer);
 
     // Insert hook section:

packages/@lwc/engine-core/src/framework/rendering.ts

@@ -310,7 +310,7 @@ function mountCustomElement(
     applyStyleScoping(elm, owner, renderer);
 
     if (vm) {
-        allocateChildren(vnode, vm, owner);
+        allocateChildren(vnode, vm);
     } else if (vnode.ctor !== UpgradableConstructor) {
         throw new TypeError(`Incorrect Component Constructor`);
     }
@@ -364,7 +364,7 @@ function patchCustomElement(
         if (!isUndefined(vm)) {
             // in fallback mode, the allocation will always set children to
             // empty and delegate the real allocation to the slot elements
-            allocateChildren(n2, vm, vm.owner!);
+            allocateChildren(n2, vm);
         }
 
         // in fallback mode, the children will be always empty, so, nothing
@@ -559,7 +559,7 @@ function applyElementRestrictions(elm: Element, vnode: VElement | VStatic) {
     }
 }
 
-export function allocateChildren(vnode: VCustomElement, vm: VM, owner: VM) {
+export function allocateChildren(vnode: VCustomElement, vm: VM) {
     // A component with slots will re-render because:
     // 1- There is a change of the internal state.
     // 2- There is a change on the external api (ex: slots)
@@ -577,7 +577,7 @@ export function allocateChildren(vnode: VCustomElement, vm: VM, owner: VM) {
     const { renderMode, shadowMode } = vm;
     if (shadowMode === ShadowMode.Synthetic || renderMode === RenderMode.Light) {
         // slow path
-        allocateInSlot(vm, children, owner);
+        allocateInSlot(vm, children, vnode.owner);
         // save the allocated children in case this vnode is reused.
         vnode.aChildren = children;
         // every child vnode is now allocated, and the host should receive none directly, it receives them via the shadow!
@@ -625,10 +625,10 @@ function allocateInSlot(vm: VM, children: VNodes, owner: VM) {
         }
 
         let slotName = '';
-        if (isVBaseElement(vnode) && !isUndefined(vnode.data.attrs?.slot)) {
-            slotName = vnode.data.attrs?.slot as string;
-        } else if (isVScopedSlotContent(vnode) && !isUndefined(vnode.slotName)) {
-            slotName = vnode.slotName;
+        if (isVBaseElement(vnode)) {
+            slotName = (vnode.data.attrs?.slot as string) ?? '';
+        } else if (isVScopedSlotContent(vnode)) {
+            slotName = vnode.slotName ?? '';
         }
 
         const vnodes: VNodes = (cmpSlotsMapping[slotName] = cmpSlotsMapping[slotName] || []);

packages/@lwc/shared/src/language.ts

@@ -23,6 +23,7 @@ const {
 const { isArray } = Array;
 
 const {
+    concat: ArrayConcat,
     copyWithin: ArrayCopyWithin,
     fill: ArrayFill,
     filter: ArrayFilter,
@@ -52,6 +53,7 @@ const {
 } = String.prototype;
 
 export {
+    ArrayConcat,
     ArrayFilter,
     ArrayFind,
     ArrayFill,

packages/@lwc/template-compiler/src/__tests__/fixtures/scoped-slots/invalid/slot-data/comment-direct-child/config.json

@@ -1,3 +1,4 @@
 {
-   "enableScopedSlots": true
+   "enableScopedSlots": true,
+   "preserveHtmlComments": true
 }

packages/@lwc/template-compiler/src/parser/index.ts

@@ -1070,7 +1070,7 @@ function parseScopedSlotContent(
     parent: ParentNode,
     parse5Elm: parse5.Element
 ): ScopedSlotContent | undefined {
-    const slotDataAttr = parsedAttr.pick('lwc:slot-data');
+    const slotDataAttr = parsedAttr.pick(ElementDirectiveName.SlotData);
     if (!slotDataAttr) {
         return;
     }
@@ -1437,7 +1437,7 @@ function validateChildren(ctx: ParserCtx, element?: BaseElement, directive?: Par
     // Note: An assumption here that ScopedSlotContent is the last processed directive in parseElementDirectives()
     if (directive && ast.isScopedSlotContent(directive)) {
         const commentOrTextChild = directive.children.find(
-            (child) => ast.isComment(child) || ast.isText(child)
+            (child) => (ctx.preserveComments && ast.isComment(child)) || ast.isText(child)
         );
         if (commentOrTextChild) {
             ctx.throwOnNode(ParserDiagnostics.NON_ELEMENT_SCOPED_SLOT_CONTENT, commentOrTextChild);