There is QualityControl build log (with error) included in the failing build logs, is it supposed to be there?

No, but it's there everywhere since yesterday. I could not yet understand what changed yet.

I see that the template instantiation errors in the QC log are definitely connected with Expressions: it picks up the variant from inside ArrowDatumSpec, which is variant<monostate,size_t,variant<int,bool.float,double>,string>, but I can not determine in which context.

The problem itself is due to the fact that the clang used by cling (5.0.2) has a known bug when using GCC headers < 8. The way to fix it is to hide whatever uses variant from cling. Why do you need to expose the ArrowDatum in Expression? Can you simply put an #ifdef __CLING__ around the variant and the header?

@ktf I moved the parsing helpers into a separate header that is not exposed in include, that should hide variant from cling.

Errors unrelated. This hides variant from ROOT.