[clang] [Clang][C++26] Implement Pack Indexing (P2662R3). (PR #72644)

[Erich Keane via cfe-commits]

================
@@ -1665,6 +1665,43 @@ NonTypeTemplateParmDecl *SubstNonTypeTemplateParmExpr::getParameter() const {
       getReplacedTemplateParameterList(getAssociatedDecl())->asArray()[Index]);
 }
 
+PackIndexingExpr *PackIndexingExpr::Create(ASTContext &Context,
+                                           SourceLocation EllipsisLoc,
+                                           SourceLocation RSquareLoc,
+                                           Expr *PackIdExpr, Expr *IndexExpr,
+                                           std::optional<int64_t> Index,
+                                           ArrayRef<Expr *> SubstitutedExprs) {
+  QualType Type;
+  if (Index && !SubstitutedExprs.empty())
+    Type = SubstitutedExprs[*Index]->getType();
+  else
+    Type = Context.DependentTy;
+
+  void *Storage =
+      Context.Allocate(totalSizeToAlloc<Expr *>(SubstitutedExprs.size()));
+  return new (Storage)
+      PackIndexingExpr(Type, EllipsisLoc, RSquareLoc, PackIdExpr, IndexExpr,
+                       Index, SubstitutedExprs);
+}
+
+NamedDecl *PackIndexingExpr::getPackDecl() const {
+  if (auto *D = dyn_cast<DeclRefExpr>(getPackIdExpression()); D) {
+    NamedDecl *ND = dyn_cast<NamedDecl>(D->getDecl());
+    assert(ND && "exected a named decl");
+    return ND;
+  }
+  assert(false && "Non variables packs not supported");
----------------
erichkeane wrote:

```suggestion
  assert(false && "Non variable packs not supported");
```

https://github.com/llvm/llvm-project/pull/72644