This is clang
(and libclang
) issue, not Vim's plugin clang_complete
issue.
clang
is now in active development stage AFAIK, so, you can file bug-report at http://llvm.org/bugs . But, I myself filed several bugs related to code completion more than a month ago (e.g. this one) but they are still unresolved.
When I need to get available constructors list, I write code like yours Z b = Z
, select constructor, and then convert Z b = Z
to something I really need. Not very good, but better than nothing.