I'm creating a TTree with a class that inherits from a base class, without a problem. However, in the same program a couple lines later,

TClass branchClass(c->GetBranch("event")->GetClassName());
if (branchClass.InheritsFrom("dicttest::EventDis") {....}

fails with

Warning in <TClass::TClass>: no dictionary for class dicttest::EventPythia is available
Error in <TObjArray::At>: index 0 out of bounds (size: 1, this: 0x127faed70)
Error in <TObjArray::At>: index 0 out of bounds (size: 1, this: 0x127faed70)

It works fine interactively but not when compiled. I have a demonstrator that's pretty minimal here:
https://github.com/kkauder/rootbreaker

This behavior is new to root 6.24, tested on Apple M1 and Ubuntu. The same code worked fine in root 6.20.04; I haven't been able to test other versions.