That argument applies to pretty much every single unit test ever written. A function running on a single long can take 2^64 possible values. Impossible to test

That argument applies to pretty much every single unit test ever written. A function running on a single long can take 2^64 possible values. Impossible to test by your logic. Yet they're tested without issues constantly. What you do is put together a long list of sample functions and sample arguments that covers the expected edge cases and then test those for equivalence. Hardly impossible. Just takes time. Not bulletproof but better than nothing.