Rationale (May, 2014):

The friend specifier can only appear in a member-declaration, which contains a member-declarator-list, not an init-declarator-list.

According to 9.1 [dcl.pre] paragraph 3,

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (Clause 11 [class]) or enumeration (9.7.1 [dcl.enum]), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (11.3 [class.name]), or an enum-specifier.

This does not allow for the new simplified friend declaration syntax (11.8.4 [class.friend] paragraph 3), which permits the forms

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