[clang][bytecode] Fix discarded LValueToRValueBitCasts (#140034)

We handle discarding fine, but we used to ignore all discarded cast
expressions. Handle bitcasts differently.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -474,10 +474,6 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
       return false;
     return this->emitDecayPtr(*FromT, *ToT, CE);
   }
-
-  case CK_LValueToRValueBitCast:
-    return this->emitBuiltinBitCast(CE);
-
   case CK_IntegralToBoolean:
   case CK_FixedPointToBoolean: {
     // HLSL uses this to cast to one-element vectors.
@@ -735,6 +731,11 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   llvm_unreachable("Unhandled clang::CastKind enum");
 }
 
+template <class Emitter>
+bool Compiler<Emitter>::VisitBuiltinBitCastExpr(const BuiltinBitCastExpr *E) {
+  return this->emitBuiltinBitCast(E);
+}
+
 template <class Emitter>
 bool Compiler<Emitter>::VisitIntegerLiteral(const IntegerLiteral *LE) {
   if (DiscardResult)

clang/lib/AST/ByteCode/Compiler.h

@@ -126,6 +126,7 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
 
   // Expressions.
   bool VisitCastExpr(const CastExpr *E);
+  bool VisitBuiltinBitCastExpr(const BuiltinBitCastExpr *E);
   bool VisitIntegerLiteral(const IntegerLiteral *E);
   bool VisitFloatingLiteral(const FloatingLiteral *E);
   bool VisitImaginaryLiteral(const ImaginaryLiteral *E);

clang/test/AST/ByteCode/builtin-bit-cast.cpp

@@ -503,6 +503,16 @@ namespace OversizedBitField {
 #endif
 }
 
+namespace Discarded {
+  enum my_byte : unsigned char {};
+  struct pad {
+    char a;
+    int b;
+  };
+  constexpr int bad_my_byte = (__builtin_bit_cast(my_byte[8], pad{1, 2}), 0); // both-error {{must be initialized by a constant expression}} \
+                                                                              // both-note {{indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte';}}
+}
+
 typedef bool bool9 __attribute__((ext_vector_type(9)));
 // both-error@+2 {{constexpr variable 'bad_bool9_to_short' must be initialized by a constant expression}}
 // both-note@+1 {{bit_cast involving type 'bool __attribute__((ext_vector_type(9)))' (vector of 9 'bool' values) is not allowed in a constant expression; element size 1 * element count 9 is not a multiple of the byte size 8}}