[clang][Interp] Handle SizeOfPackExprs (#71929)

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -1624,6 +1624,11 @@ bool ByteCodeExprGen<Emitter>::VisitCXXScalarValueInitExpr(
   return this->visitZeroInitializer(classifyPrim(Ty), Ty, E);
 }
 
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::VisitSizeOfPackExpr(const SizeOfPackExpr *E) {
+  return this->emitConst(E->getPackLength(), E);
+}
+
 template <class Emitter> bool ByteCodeExprGen<Emitter>::discard(const Expr *E) {
   if (E->containsErrors())
     return false;

clang/lib/AST/Interp/ByteCodeExprGen.h

@@ -107,6 +107,7 @@ class ByteCodeExprGen : public ConstStmtVisitor<ByteCodeExprGen<Emitter>, bool>,
   bool VisitSourceLocExpr(const SourceLocExpr *E);
   bool VisitOffsetOfExpr(const OffsetOfExpr *E);
   bool VisitCXXScalarValueInitExpr(const CXXScalarValueInitExpr *E);
+  bool VisitSizeOfPackExpr(const SizeOfPackExpr *E);
 
 protected:
   bool visitExpr(const Expr *E) override;

clang/test/AST/Interp/functions.cpp

@@ -371,3 +371,10 @@ namespace Variadic {
   constexpr int (*VFP)(...) = variadic_function2;
   static_assert(VFP() == 12, "");
 }
+
+namespace Packs {
+  template<typename...T>
+  constexpr int foo() { return sizeof...(T); }
+  static_assert(foo<int, char>() == 2, "");
+  static_assert(foo<>() == 0, "");
+}