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

[Erich Keane via cfe-commits]

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@@ -4884,6 +4884,72 @@ 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;
+  int Index = -1;
+
+protected:
+  friend class ASTContext; // ASTContext creates these.
+  PackIndexingType(const ASTContext &Context, QualType Canonical,
+                   QualType Pattern, Expr *IndexExpr,
+                   ArrayRef<QualType> Expansions = {}, int Index = -1);
+
+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");
+    return *(getExpansionsPtr() + Index);
+  }
+
+  bool hasSelectedType() const { return Index != -1 && !isDependentType(); }
----------------
erichkeane wrote:

Shouldn't JUST the second part of this be sufficient?  I would think that as soon as we're not dependent, the index should be 'set', correct?  Or am I misunderstanding?

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