I don't think it's possible to test a sufficiently complex system that isn't somewhat restricted in the states it goes through, which requires typing. Consider

I don't think it's possible to test a sufficiently complex system that isn't somewhat restricted in the states it goes through, which requires typing. Consider a finite state machine with N states. There are a priori NxN possible transitions. In many cases however most of them are impossible, and you really have only O(N) feasible transitions. Sure if you make O(N^2) tests you don't need typing... but typing could have made many of those transitions literally impossible. You don't need to test the impossible. For a sufficiently large codebase, you want to limit as much as possible the amount of tests you need. Also I have more than once identified issues in our testing framework because when I made stronger types I uncovered bugs that escaped our tests. Non-deterministic concurrency bugs are extremely hard to test for. So yes, testing can uncover type bugs, but typing can uncover untestable bugs.