[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, 70 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 4a7872b..903fbfd 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -184,6 +184,15 @@ namespace {
     bool TraversePackExpansionTypeLoc(PackExpansionTypeLoc TL) { return true; }
     bool TraversePackExpansionExpr(PackExpansionExpr *E) { return true; }
     bool TraverseCXXFoldExpr(CXXFoldExpr *E) { return true; }
+    bool TraversePackIndexingExpr(PackIndexingExpr *E) {
+      return inherited::TraverseStmt(E->getIndexExpr());
+    }
+    bool TraversePackIndexingType(PackIndexingType *E) {
+      return inherited::TraverseStmt(E->getIndexExpr());
+    }
+    bool TraversePackIndexingTypeLoc(PackIndexingTypeLoc TL) {
+      return inherited::TraverseStmt(TL.getIndexExpr());
+    }
 
     ///@}
 
@@ -865,6 +874,7 @@ std::optional<unsigned> Sema::getNumArgumentsInExpansion(
 bool Sema::containsUnexpandedParameterPacks(Declarator &D) {
   const DeclSpec &DS = D.getDeclSpec();
   switch (DS.getTypeSpecType()) {
+  case TST_typename_pack_indexing:
   case TST_typename:
   case TST_typeof_unqualType:
   case TST_typeofType:
@@ -1050,8 +1060,7 @@ ExprResult Sema::ActOnSizeofParameterPackExpr(Scope *S,
   }
 
   if (!ParameterPack || !ParameterPack->isParameterPack()) {
-    Diag(NameLoc, diag::err_sizeof_pack_no_pack_name)
-      << &Name;
+    Diag(NameLoc, diag::err_expected_name_of_pack) << &Name;
     return ExprError();
   }
 
@@ -1061,6 +1070,65 @@ ExprResult Sema::ActOnSizeofParameterPackExpr(Scope *S,
                                 RParenLoc);
 }
 
+static bool isParameterPack(Expr *PackExpression) {
+  if (auto *D = dyn_cast<DeclRefExpr>(PackExpression); D) {
+    ValueDecl *VD = D->getDecl();
+    return VD->isParameterPack();
+  }
+  return false;
+}
+
+ExprResult Sema::ActOnPackIndexingExpr(Scope *S, Expr *PackExpression,
+                                       SourceLocation EllipsisLoc,
+                                       SourceLocation LSquareLoc,
+                                       Expr *IndexExpr,
+                                       SourceLocation RSquareLoc) {
+  bool isParameterPack = ::isParameterPack(PackExpression);
+  if (!isParameterPack) {
+    CorrectDelayedTyposInExpr(IndexExpr);
+    Diag(PackExpression->getBeginLoc(), diag::err_expected_name_of_pack)
+        << PackExpression;
+    return ExprError();
+  }
+  ExprResult Res =
+      BuildPackIndexingExpr(PackExpression, EllipsisLoc, IndexExpr, RSquareLoc);
+  if (!Res.isInvalid())
+    Diag(Res.get()->getBeginLoc(), getLangOpts().CPlusPlus26
+                                       ? diag::warn_cxx23_pack_indexing
+                                       : diag::ext_pack_indexing);
+  return Res;
+}
+
+ExprResult
+Sema::BuildPackIndexingExpr(Expr *PackExpression, SourceLocation EllipsisLoc,
+                            Expr *IndexExpr, SourceLocation RSquareLoc,
+                            ArrayRef<Expr *> ExpandedExprs, bool EmptyPack) {
+
+  std::optional<int64_t> Index;
+  if (!IndexExpr->isInstantiationDependent()) {
+    llvm::APSInt Value(Context.getIntWidth(Context.getSizeType()));
+
+    ExprResult Res = CheckConvertedConstantExpression(
+        IndexExpr, Context.getSizeType(), Value, CCEK_ArrayBound);
+    if (!Res.isUsable())
+      return ExprError();
+    Index = Value.getExtValue();
+    IndexExpr = Res.get();
+  }
+
+  if (Index && (!ExpandedExprs.empty() || EmptyPack)) {
+    if (*Index < 0 || EmptyPack || *Index >= int64_t(ExpandedExprs.size())) {
+      Diag(PackExpression->getBeginLoc(), diag::err_pack_index_out_of_bound)
+          << *Index << PackExpression << ExpandedExprs.size();
+      return ExprError();
+    }
+  }
+
+  return PackIndexingExpr::Create(getASTContext(), EllipsisLoc, RSquareLoc,
+                                  PackExpression, IndexExpr, Index,
+                                  ExpandedExprs);
+}
+
 TemplateArgumentLoc Sema::getTemplateArgumentPackExpansionPattern(
     TemplateArgumentLoc OrigLoc, SourceLocation &Ellipsis,
     std::optional<unsigned> &NumExpansions) const {