Comment by skywhopper

Is it worse than having to create endless functions for each type pair?

    (b IntBox) MapToStringBox(f func(int) string) StringBox
    (b IntBox) MapToBoolBox(f func(int) bool) BoolBox
    (b StringBox) MapToIntBox(f func(string) int) IntBox

Etc etc etc?

The T, U, and f names are the cognitive load here, because they are meaningless variables. For a specific solution, those would have meaningful names that would make it easier to understand.