TClass and ROOT I/O mistreat bit-field members

[hahnjo]

Description

TClass / TDataMember do not carry enough information for bit-field members: The offset is the rounded-down byte offset, while the type is the underlying one. This leads to wrong behavior during I/O, so we should at least warn the user.

Reproducer

#include <TMemFile.h>
#include <TTree.h>

#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>

using ROOT::Experimental::RNTupleModel;
using ROOT::Experimental::RNTupleReader;
using ROOT::Experimental::RNTupleWriter;

#include <iostream>

struct BitField {
  int a : 16;
  int b : 16;
  int c; //!

  void Print() const {
    std::cout << "a = " << a << ", b = " << b << ", c = " << std::hex << c << "\n";
  }
};

void test_BitField() {
  TMemFile f("mem.root", "RECREATE");

  {
    std::cout << "Writing:\n";
    BitField obj{1, 2, 0x44444444};
    obj.Print();

    f.WriteObject(&obj, "obj");

    TTree tree;
    tree.Branch("obj", &obj);
    tree.Fill();
    f.WriteObject(&tree, "tree");

    auto model = RNTupleModel::Create();
    auto objPtr = model->MakeField<BitField>("obj");
    auto writer = RNTupleWriter::Append(std::move(model), "ntpl", f);
    *objPtr = obj;
    writer->Fill();
  }
  std::cout << "\n";

  {
    std::cout << "Reading:\n";
    BitField *objPtr = new BitField{0, 0, 0x55555555};
    objPtr->Print();
    std::cout << "(TDirectory::GetObject reallocates without freeing...)\n";
    f.GetObject("obj", objPtr);
    objPtr->Print();
    delete objPtr;
    std::cout << "\n";

    TTree *treePtr = nullptr;
    f.GetObject("tree", treePtr);
    objPtr = new BitField{0, 0, 0x55555555};
    objPtr->Print();
    treePtr->SetBranchAddress("obj", &objPtr);
    treePtr->GetEntry(0);
    objPtr->Print();
    delete objPtr;
    delete treePtr;
    std::cout << "\n";

    std::unique_ptr<ROOT::RNTuple> ntpl(f.Get<ROOT::RNTuple>("ntpl"));
    auto reader = RNTupleReader::Open(*ntpl);
    objPtr = reader->GetModel().GetDefaultEntry().GetPtr<BitField>("obj").get();
    *objPtr = BitField{0, 0, 0x55555555};
    objPtr->Print();
    reader->LoadEntry(0);
    objPtr->Print();
  }
}

Output:

Writing:
a = 1, b = 2, c = 44444444

Reading:
a = 0, b = 0, c = 55555555
(TDirectory::GetObject reallocates without freeing...)
a = 1, b = 2, c = 4444

a = 0, b = 0, c = 55555555
a = 1, b = 2, c = 55554444

a = 0, b = 0, c = 55555555
a = 1, b = 2, c = 55554444