When is it feasible for function types to be instances of the Eq class?

[derekmahar]

The solution does not clearly explain when it is feasible for function types to be instances of the Eq class. Does the solution mean the types of the arguments and return value of the function?

https://github.com/evturn/programming-in-haskell/blob/0af2c48c8221b5bcd052492e2be2b79635f6994c/03-types-and-classes/03.11-exercises.hs#L61