[clang][Interp] Fix discarded integral and floating casts (#77295)

We need to handle this at the CastExpr level.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -114,13 +114,17 @@ bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
   }
 
   case CK_FloatingCast: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
     if (!this->visit(SubExpr))
       return false;
     const auto *TargetSemantics = &Ctx.getFloatSemantics(CE->getType());
     return this->emitCastFP(TargetSemantics, getRoundingMode(CE), CE);
   }
 
   case CK_IntegralToFloating: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
     std::optional<PrimType> FromT = classify(SubExpr->getType());
     if (!FromT)
       return false;
@@ -135,6 +139,9 @@ bool ByteCodeExprGen<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
   case CK_FloatingToBoolean:
   case CK_FloatingToIntegral: {
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
     std::optional<PrimType> ToT = classify(CE->getType());
 
     if (!ToT)

clang/test/AST/Interp/literals.cpp

@@ -1024,6 +1024,10 @@ namespace DiscardExprs {
     __null;
     __builtin_offsetof(A, a);
     1,2;
+    (int)1.0;
+    (float)1;
+    (double)1.0f;
+    (signed)4u;
 
     return 0;
   }