According to 14.3 [class.friend] paragraph 3,

A friend declaration that does not declare a function shall have one of the following forms:

friend elaborated-type-specifier ;
friend simple-type-specifier ;
friend typename-specifier ;

However, many implementations accept

  friend enum E;

even though that form is explicitly not allowed by 10.1.7.3 [dcl.type.elab] paragraph 1 (which only permits class-key and not enum-key in friend declarations). Some implementations also accept opaque enumeration declarations like

  friend enum E : int;

The latter form could plausibly be used in an example like:

  class C {
    constexpr static int priv = 15;
    friend enum class my_constants;
  };
  enum class my_constants {
    pub = C::priv;                // OK because of friend decl
  };

(See also issue 2131.)