Am 28.02.19 um 11:45 schrieb Khaled Hosny:
They are implemented as ligatures in Computer Modern fonts, so it is not unreasonable for them to not work with any other font that does not have such ligatures. XeTeX and LuaTeX providing ways to emulate this with other fonts is already going out of their way, since even Knuth's TeX does not do this (if the font does not have a ligature it will do nothing).
true, but it is also correct that in the last 3 decades essentially any font usable with TeX (well LaTeX) had the ligatures, so the statement in the TeX book that --- is the way to get an emdash was/is true with virtually all fonts in use in pdftex. So it is not just a quirk of CM fonts, it is largely a feature of the ecosystem (or was in the days of Type 1 fonts) and as such it is a pity imho if it becomes a breaking change when processing a document with a unicode engine. frank