Suppose you are an instructor and you have given a (midterm or final) exam. This is a substantial portion of the grade; a student who misses it has little chance of passing the class. (I have in mind a college course, but maybe this a question in high school as well.)
Suppose that a few students don't show up for the exam. Of course, if the students were ill or had personal emergencies, you would like to give them a chance to make up the exam. You send them an email after the exam, but hear nothing back for a few days.
After those few days, you grade the rest of the exams, and would like to return them to the class and post solutions online so that your students can learn from their mistakes
However, if any of the students who missed the exam will later ask for a makeup, then they will have the advantage of seeing the solutions. While you certainly wouldn't give them an identical exam, it seems like this would still be a substantial advantage.
What is the fair thing for you to do here?
I thank Peter K. for commenting. However, the referenced question is about a quite different situation; it has nothing to do with discussing or posting solutions for the rest of the class.
Don't give the same exam as a makeup exam. Rather, write a separate exam to use for makeups. Even if you haven't given out a set of solutions or returned graded papers, students who took the main exam are likely to have shared information about the exam with students who need to take a makeup.