TClass::GetListOfDataMembers returns an empty list even-though the information is available.

[pcanal]

• Checked for duplicates

Describe the bug

As reported at cms-sw/cmsdist#6314 (comment)
we now are getting error:

An exception of category 'FatalRootError' occurred while
   [0] Constructing the EventProcessor
   [1] Constructing module: class=L2MuonProducer label='hltL2Muons'
   Additional Info:
      [a] Fatal Root Error: @SUB=TProtoClass::FindDataMember
data member with index 0 is not found in class tbb::internal::atomic_impl<unsigned long>

due to some inconsistencies.

Expected behavior

No error.

To Reproduce

scram p CMSSW_11_2_ROOT622_X_2020-10-15-1100
cd CMSSW_11_2_ROOT622_X_2020-10-15-1100/
cmsenv
runTheMatrix.py -l 11634.0
cd 11634.0_*
gdb --args cmsRun step2_DIGI_L1_DIGI2RAW_HLT.py

Setup

"the changes we are testing are d6156de...e4cd9d3 "

Additional context

[pcanal]

This is due to the introduction (in commit f3f0f13) of a fast path in TClass::GetListOfDataMember which did not take in consideration the case:

c = GetClass(someclassname)
c->GetState() == TClass::kForwardDeclared
c->GetListOfDataMember() -> list is now created but empty.
load and parse header file for `someclass`
c->GetState() == TClass::kInterpreted
c->GetListOfDataMember() -> list is still empty but should have been filled (loaded at this point).

The logic assumed incorrectly that if someclassname was indeed pointing to a class or struct and the list was created then it was loaded.