[Bug c++/57684] [c++11] Lambda is not convertible to std::function

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57684

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard says it is undefined to instantiate any std template with
incomplete types (with exceptions for shared_ptr and unique_ptr and maybe a few
others).  As an extension libstdc++ allows all containers to be instantiated
with incomplete types, except the unordered_xxx ones.