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

Author: tbaederr

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;
 

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, "");
+}