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

Implements https://isocpp.org/files/papers/P2662R3.pdf

The feature is exposed as an extension in older language modes.
Mangling is not yet supported and that is something we will have to do before release.

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/lib/AST/ComputeDependence.cpp b/clang/lib/AST/ComputeDependence.cpp
index 584b584..9d3856b 100644
--- a/clang/lib/AST/ComputeDependence.cpp
+++ b/clang/lib/AST/ComputeDependence.cpp
@@ -364,6 +364,21 @@ ExprDependence clang::computeDependence(PackExpansionExpr *E) {
          ExprDependence::TypeValueInstantiation;
 }
 
+ExprDependence clang::computeDependence(PackIndexingExpr *E) {
+  ExprDependence D = E->getIndexExpr()->getDependence();
+  ArrayRef<Expr *> Exprs = E->getExpressions();
+  if (Exprs.empty())
+    D |= (E->getPackIdExpression()->getDependence() |
+          ExprDependence::TypeValueInstantiation) &
+         ~ExprDependence::UnexpandedPack;
+  else if (!E->getIndexExpr()->isInstantiationDependent()) {
+    std::optional<unsigned> Index = E->getSelectedIndex();
+    assert(Index && *Index < Exprs.size() && "pack index out of bound");
+    D |= Exprs[*Index]->getDependence();
+  }
+  return D;
+}
+
 ExprDependence clang::computeDependence(SubstNonTypeTemplateParmExpr *E) {
   return E->getReplacement()->getDependence();
 }