It can't be either of those, because any given outcome is forcing a single value of y (or any variable in general).
By that logic, how is either (7,7) or (8,8) allowed?
translates (roughly) to:
(x|y) is a cartesian product--the set of all ordered pairs (X,Y) such that X is in x and Y is in Y, in order.
x has 2 elements: X=y and X=2.
y has 2 elements: Y=7 and Y=8.
So the set of all ordered pairs is (X=Y, Y=7), (X=Y, Y=8), (X=2, Y=7), (X=2,Y=8).
Since X=Y in the first two pairs, replace it with the value of Y from that pair: (X=7,Y=7), (X=8,Y=8).