Just to chime in, I have an implementation of the above (
https://github.com/avakar/container_of, without constexpr of course) and
also use it for intrusive containers (e.g.
https://github.com/avakar/intrusive_list). I was going to propose the
addition too, so this has +1 from me.

I'm not sure how discussion got this deep into implementation as it is way
compiler-specific (note that for msvc, the size of `T X::*` depends on
whether `X` has virtual bases or not). Constexpr-ness will indeed require
compiler magic.

-- 
Martin