A tricky tradeoff is that, because Teal compiles to Lua source, adding those run-time type checks would incur some considerable overhead. I've seen other languages in this space easily run 2 or 3 times slower when run-time checks are turned on. It's an open research problem.

You might be interested in checking out Pallene. Like Teal it's also Lua with types but it does check types at run-time, in an efficient way. However, Pallene's type system is currently not as featureful and production ready as Teal.