[luatex] Font control sequence names: LuaTeX versus other engines

[Joseph Wright (CHE)]

Hello all,

Testing some LaTeX NFSS code, or rather tracing why the result comes out
different in LuaTeX from pdfTeX, we've come across the following:

    \tracingall
    \font\testa=cmr10
    \testa
    \font\testb\fontname\font\relax
    \testa
    T\bye

With all engines featuring e-TeX except LuaTeX you see

    {changing \testb=undefined}
    {into \testb=select font nullfont}
    {\fontname}
    {changing \testb=select font nullfont}
    {into \testb=select font cmr10}
    {\relax}
    {select font cmr10}
    {reassigning current font=\testb}

and in the output box

    ...\testb T

(Knuth's TeX also goes with \testb here, but the trace is different
without e-TeX.)

With LuaTeX, in contrast, we get

    {changing \testb=undefined}
    {into \testb=select font nullfont}
    {\fontname}
    {\relax}
    {select font cmr10}
    {reassigning current font=\testa}

and

    ...\testa T

Ignoring the difference in assignment detail for the font (reported
separately), the question for me here is 'Is the change of font control
sequence deliberate?'. I suspect it may be, but don't think I've seen it
documented. To be expected?
--
Joseph Wright