[IggleSniggle]

I do think types and tests have a lot of overlap. In an ideal system, you would have tests that result in progressively more narrow types as each test case is added, providing the benefit of both.

This is basically the entire reason I am so excited about Zig.

Edit: it's also the reason I have a love/hate relationship with typescript. Typescript almost lets you do this if you try really hard, but at the end of the day it's not designed to truly test your code and requires you to "cheat" in order to make the types properly describe the situation; every time you "cheat" in order to get a more expressive type (whether that's looser or narrower), you would have been better off writing a test instead.