In the current standard, the specification regarding the friend specifier is too brief.

The friend specifier is used to specify access to class members; see [class.friend].

Unlike other specifiers, which are specified where they can appear. For example

The mutable specifier shall appear only in the declaration of a non-static data member ([class.mem]) whose type is neither const-qualified nor a reference type.

The virtual specifier shall be used only in the initial declaration of a non-static member function; see [class.virtual].

[class.friend] says nothing about this part. In order to make it clear where a friend specifier can appear, [dcl.friend] might be changed to

The friend specifier shall appear only in a member-declaration. It is used to specify access to class members; see [class.friend].