bf(C++) has added 
    emi(extended friend) em(declarations)
    hi(friend: extended declaration)
        to the language. When a class is declared as a friend, then the
tt(class) keyword no longer has to be provided. E.g.,
        verb(    class Friend;                   // declare a class
    using FriendClass = Friend;           // and maybe a using declaration

    class Class1
    {
        friend FriendClass;              // Friend: also OK
    };)

In the pre-C++11 standards the friend declaration required an explicit
tt(class); e.g., tt(friend class Friend).

The explicit use of tt(class) remains required if the compiler hasn't seen
the friend's name yet. E.g.,
        verb(    class Class1
    {
        // friend Unseen;           // fails to compile: Unseen unknown.
        friend class Unseen;        // OK
    };)

Section ref(TEMPFRIENDS) covers the use of extended friend declarations in
class templates.
