I don't think there's one widely accepted definition of what constitutes "excellent" knowledge of C++.
It tends to be difficult to come up with a clean answer to what would constitute "good", "excellent", "superb", (etc.) knowledge of any language, framework, platform, or what have you. In many cases it's particularly difficult to draw a clean line between knowledge that's really useful, and basically knowing a lot of trivia.
This is particularly true in the case of something like C++ that's been around to decades, applied in many different ways and written in many different styles over that time. There's an almost frightening amount somebody could learn about the language that's almost entirely unrelated to using it to solve problems well. Knowing all the rules of things like overload resolution or partial ordering of function templates is only likely to be useful if you're trying to hire somebody to work on a compiler (or something very similar). I've used C++ since it first became reasonably widely available, and have yet to use (or even want) partial order of function templates in real code.
Though not technically responsive to the question asked, I hasten to advise anybody who includes such requirement in job ads to try to keep some balance in mind. I think excessive emphasis is often placed on programming languages, at the expense of things like problem solving ability and general algorithms. Knowing a programming language well helps one to express solutions well, but is usually of minimal help (at best) in designing the solution itself. A great solution poorly expressed will virtually always beat a mediocre solution, not matter how well it's expressed.
