Additional note (June, 2021):

The initial opinion of CWG (at the September, 2013 meeting) was that “member declarations” was intended to be the English equivalent of the syntactic nonterminal member-declaration, including a friend declaration/definition inside the member-specification of a class, making #3 well-formed. However, recent discussion has expressed concern over the different treatment of in-class and out-of-class definitions of friend functions and observed that there is still divergence among implementations.