No, concepts interoperate with templates. I guess if you consider duck typing to be a feature, then using concepts can put constraints on that, but that is literally the purpose of them and nobody makes you use them.

If you aren't instantiating a template, then it isn't used, so who cares if it has theoretical errors to be figured out later? This behavior is in fact used to decide between alternative template specializations for the same template. Concepts do it better in some ways.

> but you lose out on much of the functionality of templates, right?

I don't think so? From my understanding what you can do with concepts isn't much different from what you can do with SFINAE. It (primarily?) just allows for friendlier diagnostics further up in the call chain.