I don’t mean using testing as a poor man’s type system, I mean even the tests you would write in a statically typed language will quickly uncover any mistakes you could possibly make there.

I'm pretty sure I'd see multiple cases where the function would return different types depending on a code path.

Still, it's not like you're going to miss that a function doesn't return something expected. Even in statically typed languages you are going to have tests to validate that much.

> I'm pretty sure I'd see multiple cases where the function would return different types depending on a code path.

That's another problem entirely. Although even then it would be pretty hard for an errant type to not blow up your tests. Again, not tests specifically meant to check that your types or sane, just the same tests you would write in a statically typed language.

Quite commonly by design, or, “why union (and sum) types are a thing”.

Something returning type “Foo | nil” is very often going to return Foo on one branch and nil on another.