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

[via cfe-commits]

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@@ -3767,6 +3767,57 @@ void DependentDecltypeType::Profile(llvm::FoldingSetNodeID &ID,
   E->Profile(ID, Context, true);
 }
 
+PackIndexingType::PackIndexingType(const ASTContext &Context,
+                                   QualType Canonical, QualType Pattern,
+                                   Expr *IndexExpr,
+                                   ArrayRef<QualType> Expansions)
+    : Type(PackIndexing, Canonical,
+           computeDependence(Pattern, IndexExpr, Expansions)),
+      Context(Context), Pattern(Pattern), IndexExpr(IndexExpr),
+      Size(Expansions.size()) {
+
+  std::uninitialized_copy(Expansions.begin(), Expansions.end(),
+                          getTrailingObjects<QualType>());
+}
+
+std::optional<unsigned> PackIndexingType::getSelectedIndex() const {
+  if (isInstantiationDependentType())
+    return std::nullopt;
+  // Should only be not a constant for error recovery.
+  ConstantExpr *CE = dyn_cast<ConstantExpr>(getIndexExpr());
+  if (!CE)
----------------
cor3ntin wrote:

```
int i = 0;
using foo = pack...[i];
```

We need to somehow parse that (if we fail to parse it it's going to at best not be classified as a type),
but i is not a constant expression. So it's error recovery in the general sense, not a RecoveryExpr



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