[Bug c++/53954] New: [C++11] Undefined reference to non-odr-used static const member variable of variadic class template in DSO with -flto and -Wl,--export-dynamic

["ai.azuma at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53954

             Bug #: 53954
           Summary: [C++11] Undefined reference to non-odr-used static
                    const member variable of variadic class template in
                    DSO with -flto and -Wl,--export-dynamic
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ai.azuma@gmail.com


Created attachment 27785
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27785
Command-line log

The following code shows the problem with GCC 4.7.2 20120707 on
x86_64-unknown-linux-gnu.

//////////////////////////////////
// foo.cpp
template<typename...>
struct S
{
  static const bool value = false;
};

void f() {
  typedef decltype(S<>::value) B;
}
//////////////////////////////////

Obviously, `S<>::value' is not odr-used. Therefore, the program should not
require its definition. However, when a DSO is created from the above code with
GCC 4.7.2 20120707, `-flto' and `-Wl,--export-dynamic' options, it requires the
definition of `S<>::value'.

For example, the following command lines result in a link error.

$ g++ -std=c++11 -fPIC -flto -Wl,--export-dynamic -o libfoo.so -shared foo.cpp

$ g++ -std=c++11 -fPIC main.cpp -L. -lfoo
./libfoo.so: undefined reference to `S<>::value'
./libfoo.so: undefined reference to `S<>::value'
collect2: error: ld returned 1 exit status

where main.cpp is a trivial C++ source file like "int main(){}".

Note that neither GCC 4.6.4 20120706 nor 4.8.0 20120708 reproduces the problem.

Note also that the above reproducer originally comes from a likely C++ code
using `std::vector' and `-std=c++11' option, like the following;

/////////////////////
#include <vector>

void f()
{
  std::vector<int> v;
  v.push_back(0);
}
/////////////////////

When a DSO is created from this code with `-std=c++11', `-lfto' and
`-Wl,--export-dynamic' options, it requires the definition of
`std::allocator_traits<std::allocator<int> >::__construct_helper<int,
int>::value'.