[clang][Interp] Handle DiscardResult for DeclRef- and ParenExprs

[tbaeder]

Summary

Fixes https://github.com/llvm/llvm-project/issues/62004

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

197

Next time this pattern shows up, I need to add a convenience function.

[tbaeder]

Ping

[shafik]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

bool ByteCodeExprGen<Emitter>::VisitFloatingLiteral(const FloatingLiteral *E) {
   if (DiscardResult)
     return true;
 
   return this->emitConstFloat(E->getValue(), E);
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitParenExpr(const ParenExpr *PE) {
-  return this->visit(PE->getSubExpr());
+  const Expr *SubExpr = PE->getSubExpr();
+
+  if (DiscardResult)
+    return this->discard(SubExpr);
+
+  return this->visit(SubExpr);
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitBinaryOperator(const BinaryOperator *BO) {
   // Need short-circuiting for these.
   if (BO->isLogicalOp())
     return this->VisitLogicalBinOp(BO);
 
   const Expr *LHS = BO->getLHS();
case UO_Coawait:
     assert(false && "Unhandled opcode");
   }
 
   return false;
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitDeclRefExpr(const DeclRefExpr *E) {
+  if (DiscardResult)
+    return true;
+
   const auto *D = E->getDecl();
 
   if (const auto *ECD = dyn_cast<EnumConstantDecl>(D)) {
     return this->emitConst(ECD->getInitVal(), E);
   } else if (const auto *BD = dyn_cast<BindingDecl>(D)) {
     return this->visit(BD->getBinding());
   } else if (const auto *FuncDecl = dyn_cast<FunctionDecl>(D)) {
     const Function *F = getFunction(FuncDecl);

clang/test/AST/Interp/literals.cpp

constexpr int ignoredDecls() {
   struct F { int a; };
   enum E { b };
   using A = int;
   typedef int Z;
 
   return F{12}.a;
 }
 static_assert(ignoredDecls() == 12, "");
+
+struct A{};
+constexpr int ignoredExprs() {
+  (void)(1 / 2);
+  A a;
+  a; // expected-warning {{unused}} \
+     // ref-warning {{unused}}
+  (void)a;
+  (a); // expected-warning {{unused}} \
+       // ref-warning {{unused}}
+
+  return 0;
+}
+
 #endif