[gruffle]

No, as I've pointed out - even in your trivial example you're not testing for all correct behaviour. What's the correct behaviour when the function is called with invalid argument types?

> But if the tests verify that the output is correct, then it implicitly follows that the types are correct, so you get type checking for free.

Your unit test checks the correct behaviour for one out of infinite possible inputs. You don't 'get type checking for free' , what you get is no type checking at all.

[goto11]

This is true - unit tests generally does not test all possible inputs. They test some representative examples and edge cases. In most cases there are infinitely many possible inputs, and you get diminishing returns the more cases you test.

If I understand you correctly, you a suggesting unit tests in a dynamic language should test all possible invalid inputs. Since statically typed languages eliminate a subset of invalid inputs (because they won't compile), statically typed languages saves you from writing some of these tests.

My argument is that you wouldn't write such tests in the first place since they provide little to no value.

If function `foo()` calls function `bar()` you want to ensure that foo passes the correct arguments. You don't do that by checking that bar() rejects all possible invalid types. You do it by checking that foo() actually works and provide the correct result. The types involved are an implementation detail.

[gruffle]

> You do it by checking that foo() actually works and provide the correct result.

But you're testing a tiny subset of possible scenarios of behaviour of the function. If you can anticipate all possible input types and value ranges, maybe unit tests are enough, but that's not realistic in a dynamic language/program where complex values are non-deterministically generated at runtime - based on user input, db, file, etc.