Clearly there are good examples either way. I'm more interested in the idea that this strong mismatch between the complexity of the code and the complexity of i

Clearly there are good examples either way. I'm more interested in the idea that this strong mismatch between the complexity of the code and the complexity of its tests is, in itself, a code smell that may point to an issue that is nothing to do with TDD. I note that the examples you give have trivial test cases - presuming you don't care about overflow, and if you do, then those methods now have basis paths, whether explicit or implicit depends on the language.