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If one needs to bury a body on tisha bav but there is a heat wave, should the person refrain from burying the body until after tisha bav as to not have to break the fast? Or does the mitzvah of מת מצווה take precedence over the fast?
The mitzva to bury a מת מצווה is from the Torah.
As @DonielF says Tisha b'Av is not from the Torah.
The Torah mitzva takes precedence.
If the people assisting at the burial are weak, they should follow the normal rules for 9th Av.