You're disagreeing with my 'I can't imagine'? So you're saying you've built non-toy systems with TDD where you've never tested anything but the user-facing behavior of the system?

I'd be interested in knowing how, and what kind of thing you build. I can almost imagine it for an API or framework, where almost every implementation choice is on the boundary. But even then, you have to test private functions to do TDD, surely.

I work on real time stuff, and in that stuff, the user-facing functionality is hard to test, and usually not tested in a TDD kind of way. The TDD stuff is all internal.