error if harf mode specified running with luatex (vs. luahbtex)?

[kberry]

With this plain luatex input:

\input luaotfload.sty
\font\l="[lmroman10-regular.otf]:mode=harf;script=dflt;+tlig" at 20pt
\l --
\end

the output when running with luatex is two hyphens, since the +tlig feature evidently does not take effect when an unavailable mode is specified. With luahbtex, the output is an en-dash, as expected.

So, maybe it would be good to give an error, or at least a warning, when harf mode is specified but running under luatex instead of luahbtex? Instead of just giving incorrect/unexpected output. It took me a few minutes of stupidity before I realized what I was doing wrong.

I know almost everyone uses lualatex so can't make this mistake, but ... ? --thanks, karl.