Holds if this class has a copy assignment operator that is either
explicitly declared (though possibly
= delete) or is auto-generated,
non-trivial and called from somewhere.
DEPRECATED: There is more than one reasonable definition of what it means to have a copy assignment operator, and we do not want to promote one particular definition by naming it with this predicate. Having a copy assignment operator could mean that such a member is declared or defined in the source or that it is callable by a particular caller. For C++11, there’s also a question of whether to include members that are defaulted or deleted.