the following plain tex file
\ifcsname T\ifincsname e\else \hbox{e}\fi X\endcsname\fi
\byeworks in etex/pdftex/xetex but generates an error in luatex (tl2017 pretest) ifincsname should be true in the implicit csname started by the outer ifincsname David It is perhaps related to this tracker issue about ifincsname in nested csname, but that one appears fixed, http://tracker.luatex.org/view.php?id=728
