I think you could prove that you can't construct a sound & complete type system for Lua. But just saying "Your type system cannot be sound" by itself is definitely wrong. I don't understand why people are throwing out both soundness & completeness, instead of at least retaining one (and I think the choice here is pretty obvious, a sound but incomplete type system is much more useful than an unsound one).

From Flow's website[1] (a type checker for JavaScript):

> Flow tries to be as sound and complete as possible. But because JavaScript was not designed around a type system, Flow sometimes has to make a tradeoff. When this happens Flow tends to favor soundness over completeness, ensuring that code doesn't have any bugs.

I don't understand why other type systems for dynamically typed languages cannot strive for that, and in particular I am pretty salty at TypeScript for explicitly not caring about soundness.

[1]: https://flow.org/en/docs/lang/types-and-expressions/#toc-sou...