It's rarely possible to future-proof a function by testing its entire range of possible inputs, so I'm not particularly concerned about trying to anticipate trivial cases that might become non-trivial in future—that just feels like a special case of future-proofing.
I think it's more important to ensure that, after each new commit, every non-trivial code path in that function is exercised. If someone adds new logic, they're responsible for adding new tests to cover that logic; similarly, if they turn a trivial code path into a non-trivial one, they're responsible for adding tests to cover that too.
I think it's more important to ensure that, after each new commit, every non-trivial code path in that function is exercised. If someone adds new logic, they're responsible for adding new tests to cover that logic; similarly, if they turn a trivial code path into a non-trivial one, they're responsible for adding tests to cover that too.