I think I understand (@eguiraud - the TTreeReaderArray currently sets up a TTreeReaderValue to read the branch / leaf count, and it got its underlying type wrong):
In principle, only int is supported as array size. But in reality we never really enforced that, so now TTreeReader needs to handle this case.
I’m (I was, actually) working on changing the way TTreeReaderArray reads its data; let me resume that - soon - and I’ll check this case. I have created https://sft.its.cern.ch/jira/browse/ROOT-8952 for me to remember this.
Would you be able to share a file with that tree, even if it’s just one tree entry? I need a test case to make sure that the new code (once I have it) is actually solving this!
Cheers, Axel.