This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-04-18
_N4868_.9.8.2.3 [namespace.memdef] paragraph 2 says,
Members of a named namespace can also be defined outside that namespace by explicit qualification (6.5.5.3 [namespace.qual] ) of the name being defined, provided that the entity being defined was already declared in the namespace...It is not clear whether block-scope extern declarations and friend declarations are sufficient to permit the named entities to be defined outside their namespace. For example,
namespace NS {
struct A { friend struct B; };
void foo() { extern void bar(); }
}
struct NS::B { }; // 1) legal?
void NS::bar() { } // 2) legal?
Rationale (10/99): Entities whose names are "invisibly injected" into a namespace as a result of friend declarations are not "declared" in that namespace until an explicit declaration of the entity appears at namespace scope. Consequently, the definitions in the example are ill-formed.
(See also issues 95, 136, 138, 139, 143, and 166.)