When I say adequately tested, I mean much more than unit tests. Unit tests aren't going to give you bulletproof code. They don't detect bugs that arise from interactions at a higher level.

An arbitrarily complex type system, with dependent types, could detect any bug, since it's formally equivalent to requiring the program be proved correct. But using such a type system is so onerous that I don't know anyone who realistically does it in production.

If you wrote such types, you're basically giving a formal specification of what the program is supposed to do. That could then be used, and likely much more easily, for high volume property-based testing.