- Construct from elements is enabled iff each argument is_convertible to
corresponding element.
- Construct from sequence is enabled iff the single argument is a fusion
sequence.
- C++11 vector and tuple also disable construct from sequence that are
shorter than the destination. C++03 gives incorrect is_convertible
responses in this situation and fails to compile if that constructor is
used; C++11 can have instantation errors in
and_<is_convertible<U, T>...> without the additional check.
- C++11 tuple and vector support truncation conversion and assignment like
all other sequences.
