I have been learning in my Shiur the Chanukka Gemoro in Bameh Madlikin and "Agav" just found a small Heoroh which I invite you to make any comments.
Daf 23A Rashi D.H. Mearvin Bo why does Rashi only mention the Eruvei Chatzerois and Eiruvei Techumin would not the same Shaaleh be on Eruvei Tavshilin and if you are going to tell me it is Lav Davka that is not Rashi's style Lichoroh if he mentions 2 out of 3 perhaps he wants to be Meorer something any thoughts or have I missed something obvious which tends to happen!
Bernard Kahan, London, England
Not only Rashi, but all of the Mefarshim on Mishnayos and in the Yerushalmi mention only Eruvei Chatzeros and Techumin, and do not mention Tavshilin.
I would suggest that in the case of Tavshilin, Demai is not usable! In the case of the first two, leaving food in a place is considered as though the person himself is present there during Bein ha'Shemashos (Techumin) or that there is joint ownership (Chatzeros), and the Chachamim allowed Demai to be considered food for this Halachic requirement.
However, Eruv Tavshilin is to show how the person has started cooking for Shabbos (Rema OC 527:1), and if the food is Demai, he cannot eat it at all. (It does not help to say that if he was poor, then he could eat it, because he is not actually poor and there is no real food being prepared. In this case, "Halachic" food, which is not edible in practice, will not help.)
All the best,