[Bug c++/70522] New: Hidden friend functions block qualified name lookup into nested unnamed namespace

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70522

            Bug ID: 70522
           Summary: Hidden friend functions block qualified name lookup
                    into nested unnamed namespace
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

This program should compile without error:

namespace N {
    struct S {
        friend void f(S&) {}
    };
    namespace {
        int f;
    }
}

int main() {
    N::f = 42;
}

yet GCC 6 diagnoses:

prog.cc: In function 'int main()':
prog.cc:11:5: error: 'f' is not a member of 'N'
     N::f = 42;
     ^
prog.cc:11:5: note: suggested alternative:
prog.cc:6:13: note:   'N::{anonymous}::f'
         int f;
             ^

The program compiles correctly if either the friend declaration of f is
commented out, or the unnamed namespace is made an inline namespace.

Language lawyer justification:
The friend declaration of f is not visible to name lookup of N::f per
[namespace.memdef]/3, so the set of declarations S'(N,f) described in
[namespace.qual]/2 is empty. Name lookup should continue into namespaces
"nominated by using directives" in N; unnamed namespaces are such namespaces
per [namespace.unnamed]/1.