[c++11] Perfect forwarding an't that perfect.

Perfect forwarding isn’t really perfect. There are several kinds of arguments that cannot be perfectly forwarded, including (but not necessarily limited to):

• 0 as a null pointer constant.

• Names of function templates (e.g., std::endl and other manipulators).

• Braced initializer lists.

• In-class initialized const static data members lacking an out-of-class definition.

•Bit fields.