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

[Bevin Hansson via cfe-commits]

================
@@ -4934,6 +4934,73 @@ class DependentDecltypeType : public DecltypeType, public llvm::FoldingSetNode {
                       Expr *E);
 };
 
+class PackIndexingType final
+    : public Type,
+      public llvm::FoldingSetNode,
+      private llvm::TrailingObjects<PackIndexingType, QualType> {
+  friend TrailingObjects;
+
+  const ASTContext &Context;
+  QualType Pattern;
+  Expr *IndexExpr;
+
+  unsigned Size;
+
+protected:
+  friend class ASTContext; // ASTContext creates these.
+  PackIndexingType(const ASTContext &Context, QualType Canonical,
+                   QualType Pattern, Expr *IndexExpr,
+                   ArrayRef<QualType> Expansions = {});
+
+public:
+  Expr *getIndexExpr() const { return IndexExpr; }
+  QualType getPattern() const { return Pattern; }
+
+  bool isSugared() const { return hasSelectedType(); }
+
+  QualType desugar() const {
+    if (hasSelectedType())
+      return getSelectedType();
+    return QualType(this, 0);
+  }
+
+  QualType getSelectedType() const {
+    assert(hasSelectedType() && "Type is dependant");
----------------
bevin-hansson wrote:

Trying to dump/print a broken pack indexing expression/type breaks on this assertion. Try adding -ast-dump to cxx2c-pack-indexing.cpp and see what happens.

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