It sounds to me like the author wants to see how their code behaves on certain inputs instead of predicting its correctness mentally. Current static type system

It sounds to me like the author wants to see how their code behaves on certain inputs instead of predicting its correctness mentally. Current static type systems don't solve this. There are proof assistants, but limited to academia. For a concrete (and possibly oversimplified) counterexample, try to use your favorite static type system to verify that a function returns the n-th prime.