Problem using type with default template parameters in the interpreter

[Nowakus]

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Description

The new cppyy produces an error when we try to call GetBranchAttr on one of our DataVector branches.

Reproducer

Reproducer using only the ATLAS environment, no files necessary:

export ATLAS_LOCAL_ROOT_BASE=/cvmfs/atlas.cern.ch/repo/ATLASLocalRootBase 
source $ATLAS_LOCAL_ROOT_BASE/user/atlasLocalSetup.sh
asetup main--dev4LCG,latest,Athena
python

    import ROOT, cppyy.ll
    ROOT.TClass.GetClass('DataVector<xAOD::PixelCluster_v1>').HasDictionary()

True

    cppyy.ll.cast['DataVector<xAOD::PixelCluster_v1>'](0)

TypeError: Could not find "cppyy_cast<DataVector<xAOD::PixelCluster_v1>>" (set cppyy.set_debug() for C++ errors):
  Failed to instantiate "cppyy_cast<DataVector<xAOD::PixelCluster_v1>>(int)"

but using a typedef to the DataVector works (note the real type of the returned object):

    cppyy.ll.cast['xAOD::PixelClusterContainer_v1>'](0)

<cppyy.gbl.DataVector<xAOD::PixelCluster_v1> object at 0x11cbd2c0>

and after the previous call autoloaded some libraries, the real type starts to work as well:

    cppyy.ll.cast['DataVector<xAOD::PixelCluster_v1>'](0)

<cppyy.gbl.DataVector<xAOD::PixelCluster_v1> object at 0x1f542770>

ROOT version

6.32+, possibly since 6.30

Installation method

SFT dev nightlies

Operating system

alma9

Additional context

No response

[amete]

Hi @guitargeek, has there been any progress on this? Many thanks.

[guitargeek]

As discussed offline, the underlying reason seems to be that the forward declaration of DataVector is inconsistent with the definition.

Here is the forward declaration:

template <class T, class BASE> class DataVector;

And here is the definition:

template <class T, class BASE = typename DataVectorBase<T>::Base>
class DataVector : public BASE
{

The definition has a default template argument, while the forward declaration has not. Depending on what is hit in the lookup, the instantiation with just one template parameter might therefore fail.

So, I think the issue needs to be fixed on the ATLAS Athena side, also adding the default value for the BASE template parameter to the forward declaration.

Let me know if this fixes it, so the issue can be closed!

[scott-snyder]

That's not actually a forward declaration of DataVector; it's a forward
declaration of ROOT::Meta::Selection::DataVector. That class does
not default its second argument.

The actual forward declaration that we're getting complaints about here
seems to be synthesized by cling. In one case we see an error message
pointing at the rootmap file:

Forward declarations from /cvmfs/atlas-nightlies.cern.ch/repo/sw/main_Athena_x86_64-el9-gcc13-opt/2024-11-12T2101/Athena/25.0.22/InstallArea/x86_64-el9-gcc13-opt/lib/Athena.rootmap:4:44: note: template is declared here
template <typename T, typename BASE> class DataVector;

In any case, adding a default to the selection class forward declaration
changes neither what's written to the rootmap file nor the error.

[guitargeek]

I see. Thanks for the follow up!

Then I guess this needs to be followed up be the experts for the rootmap files. I'll change the issue description to highlight that this is not a Python issue, since the exact same thing happens in the C++ ROOT interpreter.

Reproducer:

export ATLAS_LOCAL_ROOT_BASE=/cvmfs/atlas.cern.ch/repo/ATLASLocalRootBase 
source $ATLAS_LOCAL_ROOT_BASE/user/atlasLocalSetup.sh
asetup main--dev3LCG,latest,Athena
root

   ------------------------------------------------------------------
  | Welcome to ROOT 6.35.01                        https://root.cern |
  | (c) 1995-2024, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Nov 16 2024, 00:04:28                 |
  | From heads/master@v6-35-01-139-g2aa2426cbf                       |
  | With g++ (GCC) 14.2.0                                            |
  | Try '.help'/'.?', '.demo', '.license', '.credits', '.quit'/'.q'  |
   ------------------------------------------------------------------

Using the real type initially doesn't work:

root [0] DataVector<xAOD::PixelCluster_v1>{0}
ROOT_prompt_0:1:1: error: too few template arguments for class template 'DataVector'
DataVector<xAOD::PixelCluster_v1>{0}
^
Forward declarations from /cvmfs/atlas-nightlies.cern.ch/repo/sw/main--dev3LCG_Athena_x86_64-el9-gcc14-opt/2024-11-10T1100/Athena/25.0.22/InstallArea/x86_64-el9-gcc14-opt/lib/Athena.rootmap:5:44: note: template is declared here
template <typename T, typename BASE> class DataVector;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~       ^

But the typedef works:

root [1] xAOD::PixelClusterContainer_v1{0}
(xAOD::PixelClusterContainer_v1) { nullptr }

After using the typedef, the real type works too:

root [2] DataVector<xAOD::PixelCluster_v1>{0}
(DataVector<xAOD::PixelCluster_v1>) { nullptr }

The problem seems to be related to the fact that the the DataVector class has a second template parameter with a default.