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

[Erich Keane via cfe-commits]

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@@ -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())
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erichkeane wrote:

IF we are allowing passing the index here, this seems like it should be an assert.

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