[clang][bytecode] Check pointer data type for bitcast eligibility (#146552)

So we get the proper type for a heap-allocated value.

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -335,7 +335,8 @@ bool clang::interp::DoBitCast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
 
   BitcastBuffer Buffer(FullBitWidth);
   size_t BuffSize = FullBitWidth.roundToBytes();
-  if (!CheckBitcastType(S, OpPC, Ptr.getType(), /*IsToType=*/false))
+  QualType DataType = Ptr.getFieldDesc()->getDataType(S.getASTContext());
+  if (!CheckBitcastType(S, OpPC, DataType, /*IsToType=*/false))
     return false;
 
   bool Success = readPointerToBuffer(S.getContext(), Ptr, Buffer,
@@ -370,8 +371,8 @@ bool clang::interp::DoBitCastPtr(InterpState &S, CodePtr OpPC,
   assert(FromPtr.isBlockPointer());
   assert(ToPtr.isBlockPointer());
 
-  QualType FromType = FromPtr.getType();
-  QualType ToType = ToPtr.getType();
+  QualType FromType = FromPtr.getFieldDesc()->getDataType(S.getASTContext());
+  QualType ToType = ToPtr.getFieldDesc()->getDataType(S.getASTContext());
 
   if (!CheckBitcastType(S, OpPC, ToType, /*IsToType=*/true))
     return false;

clang/test/AST/ByteCode/placement-new.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -std=c++2c -fcxx-exceptions -fexperimental-new-constant-interpreter -verify=expected,both %s -DBYTECODE
 // RUN: %clang_cc1 -std=c++2c -fcxx-exceptions -verify=ref,both %s
 
+typedef __INT64_TYPE__ int64_t;
 namespace std {
   using size_t = decltype(sizeof(0));
   template<typename T> struct allocator {
@@ -465,3 +466,23 @@ namespace ArrayRoot {
 
   static_assert(foo() == 0);
 }
+
+namespace bitcast {
+  template <typename F, typename T>
+  constexpr T bit_cast(const F &f) {
+    return __builtin_bit_cast(T, f);
+  }
+  constexpr int foo() {
+    double *d = std::allocator<double>{}.allocate(2);
+    std::construct_at<double>(d, 0);
+
+    double &dd = *d;
+
+    int64_t i = bit_cast<double, int64_t>(*d);
+
+
+    std::allocator<double>{}.deallocate(d);
+    return i;
+  }
+  static_assert(foo() == 0);
+}