[clang] [clang][bytecode] Fix discarding DerivedToBase casts (PR #123523)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr updated 
https://github.com/llvm/llvm-project/pull/123523

>From db874d225e0994587ca15384679d070651e0cc99 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbae...@redhat.com>
Date: Sun, 19 Jan 2025 19:48:31 +0100
Subject: [PATCH] [clang][bytecode] Fix discarding DerivedToBase casts

---
 clang/lib/AST/ByteCode/Compiler.cpp |  6 ++++++
 clang/test/AST/ByteCode/records.cpp | 15 +++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp 
b/clang/lib/AST/ByteCode/Compiler.cpp
index 7afae97f308ad5..750516efa1a3e1 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -253,6 +253,9 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
   case CK_UncheckedDerivedToBase:
   case CK_DerivedToBase: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
     if (!this->delegate(SubExpr))
       return false;
 
@@ -282,6 +285,9 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   }
 
   case CK_BaseToDerived: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
     if (!this->delegate(SubExpr))
       return false;
 
diff --git a/clang/test/AST/ByteCode/records.cpp 
b/clang/test/AST/ByteCode/records.cpp
index d329219264d893..9470e7d8e3dcbf 100644
--- a/clang/test/AST/ByteCode/records.cpp
+++ b/clang/test/AST/ByteCode/records.cpp
@@ -1684,3 +1684,18 @@ namespace ExplicitThisInTemporary {
   constexpr bool g(B b) { return &b == b.p; }
   static_assert(g({}), "");
 }
+
+namespace IgnoredMemberExpr {
+  class A {
+  public:
+    int a;
+  };
+  class B : public A {
+  public:
+    constexpr int foo() {
+      a; // both-warning {{expression result unused}}
+      return 0;
+    }
+  };
+  static_assert(B{}.foo() == 0, "");
+}

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