[clang][bytecode] Fix bitcasting from null pointers (#116999)

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -254,9 +254,13 @@ static bool readPointerToBuffer(const Context &Ctx, const Pointer &FromPtr,
         }
 
         assert(P.isInitialized());
-        // nullptr_t is a PT_Ptr for us, but it's still not std::is_pointer_v.
-        if (T == PT_Ptr)
-          assert(false && "Implement casting to pointer types");
+        if (T == PT_Ptr) {
+          assert(P.getType()->isNullPtrType());
+          // Clang treats nullptr_t has having NO bits in its value
+          // representation. So, we accept it here and leave its bits
+          // uninitialized.
+          return true;
+        }
 
         auto Buff =
             std::make_unique<std::byte[]>(ObjectReprChars.getQuantity());

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

@@ -133,8 +133,14 @@ namespace simple {
 
   /// This works in GCC and in the bytecode interpreter, but the current interpreter
   /// diagnoses it.
+  /// FIXME: Should also be rejected in the bytecode interpreter.
   static_assert(__builtin_bit_cast(intptr_t, nullptr) == 0); // ref-error {{not an integral constant expression}} \
                                                              // ref-note {{indeterminate value can only initialize an object}}
+
+  constexpr int test_from_nullptr_pass = (__builtin_bit_cast(unsigned char[sizeof(nullptr)], nullptr), 0);
+  constexpr unsigned char NPData[sizeof(nullptr)] = {1,2,3,4};
+  constexpr nullptr_t NP = __builtin_bit_cast(nullptr_t, NPData);
+  static_assert(NP == nullptr);
 }
 
 namespace Fail {