Rif Rosh HaShana 5a {Rosh haShana 18b; 19b}

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5a

{Rosh haShana 18b}
It was stated {by Amoraim}:
Rav and Rabbi Chanina say: Megillat Taanit {which details joyous days during which the practice was not to fast} has been nullified.
And Rabbi Yochanan and Rabbi Yehoshua ben Levi say: It has not been nullified.

And the halacha is that it has been nullified.

{Rosh haShana 19b}
And the halacha is that it has not been nullified.

We have a contradiction between one halachic conclusion and another halachic conclusion!
This is not a question. Here is by Chanukka and Purim {where it has not been nullified and so we may not fast} and here is by all other days.

"And for Tishri on account of appointing the order of the (remaining) festivals {=Succot}":