[Bug c++/58078] New: explicitly-defaulted template class methods are not instantiated by explicit template instantiation definition

["edward.hades at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58078

            Bug ID: 58078
           Summary: explicitly-defaulted template class methods are not
                    instantiated by explicit template instantiation
                    definition
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: edward.hades at gmail dot com

Created attachment 30606
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30606&action=edit
source code demonstrating the bug

The explicit instantiation of a class template with some explicitly-defaulted
class methods does not instantiate these methods, which leads to link errors.

Consider the code:

==> test.h <==
#include <vector>

template<class T>
class A
{
    std::vector<T> x;
public:
    A() = default;
    A(const A&) = default;
    A(A&&) = default;
    A& operator=(const A&) = default;
    A& operator=(A&&) = default;
};

extern template class A<double>;

==> test1.cpp <==
#include "test.h"

int main()
{
    A<double> a;
    A<double> b(a);
    A<double> c((A<double>()));
    a = b;
    a = A<double>();
    return 0;
}

==> test2.cpp <==
#include "test.h"

template class A<double>;

==> Makefile <==
CXX=g++
CXXFLAGS=-std=c++11 -Wall -Wextra -pedantic

test: test1.o test2.o
        $(CXX) -o $@ $+

test1.o: test1.cpp test.h
test2.o: test2.cpp test.h

==> (end) <==

This code compiles fine with, for example, clang, but fails with gcc:

g++-4.8.1 -std=c++11 -Wall -Wextra -pedantic   -c -o test1.o test1.cpp
g++-4.8.1 -std=c++11 -Wall -Wextra -pedantic   -c -o test2.o test2.cpp
g++-4.8.1 -o test test1.o test2.o
test1.o: In function `main':
test1.cpp:(.text+0x14): undefined reference to `A<double>::A()'
test1.cpp:(.text+0x27): undefined reference to `A<double>::A(A<double> const&)'
test1.cpp:(.text+0x54): undefined reference to `A<double>::A()'
test1.cpp:(.text+0x67): undefined reference to `A<double>::operator=(A<double>
const&)'
test1.cpp:(.text+0x8b): undefined reference to `A<double>::A()'
test1.cpp:(.text+0x9e): undefined reference to
`A<double>::operator=(A<double>&&)'
collect2: error: ld returned 1 exit status
make: *** [test] Error 1


I am attaching the source codes for your convenience.