[ntuple] Missing importer support for leaf count arrays in leaf list

[jblomer]

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Description

As discussed in #16478, the TTree feature of using a count leaf inside a leaf list is currently unsupported by the importer.

Reproducer

The following file cannot be imported:

std::unique_ptr<TFile> file(TFile::Open("tree.root", "RECREATE"));
auto tree = std::make_unique<TTree>("tree", "");
int count = 1;
struct {
   float foo = 2.0;
   Int_t values[20]; // is controled by [count]
} leafList;
leafList.values = new Int_t[1];
leafList.values[0] = 1;
tree->Branch("count", &count);
tree->Branch("leaflist", &leafList, "foo/F:values[count]/I");
tree->Fill();
tree->Write();

As well as having the count leaf inside the leaf list as well as "manually" encoding the leaf count arrays (see comments)

ROOT version

master

Installation method

n/a

Operating system

n/a

Additional context

No response