How to write `is_complete` template?

Question

After answering this question I was trying to find is_complete template in Boost library and I realized that there is no such template in Boost.TypeTraits. Why

Answer 1

Just chiming in to signal that an answer (not given by me) to an unrelated question gives a solution for the is_complete template.

The answer is here. I'm not pasting it below in order to not mistakenly get credit for it.