http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60594
--- Comment #3 from Chandler Carruth <chandlerc at gmail dot com> --- Personally, I've no idea what the standard says. However, it seems extremely useful to be able to freely use std::function with return types (and argument types) within the definition those types which is how this actually started. It also seems reasonable to need to observe whether the std::function will be copyable (it will) prior to completing it. Without this, somewhat basic uses if std::function as a member with a type-erased bit of logic pertaining to the enclosing class with value semantic interfaces don't work.
