[Bug c++/59930] New: template friend declarations, namespaces, and explicit instantiations don't mix

["luto at mit dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59930

            Bug ID: 59930
           Summary: template friend declarations, namespaces, and explicit
                    instantiations don't mix
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luto at mit dot edu

namespace NS {

template<typename T>
class Holder
{
private:
    void func();

    template<typename>
    friend class User;
};

template class Holder<long>;

template<typename T>
class User
{
public:
    void method() const
    {
        Holder<T> x;
        x.func();
    }
};

} // namespace

// This one is okay, oddly.
// template class NS::Holder<long>;

void Foo()
{
    NS::User<long> decl;
    decl.method();
}

says (in trunk r207012):

$ g++-trunk -Wall -c template_friend.cc 
template_friend.cc: In instantiation of âvoid NS::User<T>::method() const [with
T = long int]â:
template_friend.cc:34:14:   required from here
template_friend.cc:7:7: error: âvoid NS::Holder<T>::func() [with T = long int]â
is private
  void func();
       ^
template_friend.cc:22:3: error: within this context
   x.func();
   ^

The 4.8 branch can't compile this either, but clang is okay with it.

Oddly, removing the namespace, removing the explicit instantiation, or moving
the explicit instantiation outside the namespace causes g++ to accept this
code.