What can I “forward declare” in C++?

I know I can do

class Foo;

and probably

struct Bar;

and global functions

bool IsValid(int iVal);

What about a typed enum? What about a typed enum within an undeclared class? What about a function with an undeclared class? What about a static member within an undeclared class? What about these within an unknown namespace? Am I missing anything else that can be forward declared?