There is no way, in any language with any type system or any amount of tests (except maybe in a pure language with tests that cover the entire domain of the function, in which case you're just reproducing the entire function in the tests), to guarantee that calculateSalesTax produces the correct number in all cases. So the problem you describe exists equally for both dynamic and static type systems with tests. Dynamic type systems just rule out fewer possible errors (and fewer possible correct programs as well, to be fair) before the program runs.