[Bug c++/67632] New: explicit instantiation omits copy constructor and others

["jlink at drw dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67632

            Bug ID: 67632
           Summary: explicit instantiation omits copy constructor and
                    others
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlink at drw dot com
  Target Milestone: ---

When I compile the follow test case with g++ -std=c++11 -c, and view the output
with nm, it shows that the std::unordered_map copy constructor is never
emitted:

% cat t.cc 
#include <unordered_map>
template class std::unordered_map<int, int>;
% g++ -std=c++11 -c t.cc
% 

If I compile the following and link it with the above, I'll get the undefined
reference:

% cat x.cc 
#include <unordered_map>
extern template class std::unordered_map<int, int>;
std::unordered_map<int, int> copy(
   const std::unordered_map<int, int> & a) { return a; }

main() {}

% g++ -std=c++11 t.o x.cc     
/tmp/cc0dF94P.o: In function `copy(std::unordered_map<int, int, std::hash<int>,
std::equal_to<int>, std::allocator<std::pair<int const, int> > > const&)':
x.cc:(.text+0x1f): undefined reference to `std::unordered_map<int, int,
std::hash<int>, std::equal_to<int>, std::allocator<std::pair<int const, int> >
>::unordered_map(std::unordered_map<int, int, std::hash<int>,
std::equal_to<int>, std::allocator<std::pair<int const, int> > > const&)'
collect2: error: ld returned 1 exit status

This seems like a clear bug, right?  Either the explicit instantiation should
emit everything, or the use of extern template should know what still needs to
be emitted locally.