[Bug c++/68070] New: Undefined reference to default constructor of member template class

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68070

            Bug ID: 68070
           Summary: Undefined reference to default constructor of member
                    template class
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frrrwww at gmail dot com
  Target Milestone: ---

Hello,

When compiling the following simple program with `g++ prog.cc -Wall -Wextra
-std=c++14`

-----
#include <vector>

template<typename Value>
struct A
{
    A() = default;
    std::vector<Value> m_content;
};

void func(A<int> a = {})
{
}

int main()
{
    func();
}
-----

We get the following error:

-----
/tmp/ccr6nbUo.o: In function `main':
prog.cc:(.text+0x1b): undefined reference to `std::vector<int,
std::allocator<int> >::vector()'
collect2: error: ld returned 1 exit status
-----

That error happens on gcc 4.9, 5.2 and 6.0, it does not happen on clang for
this specific code.

An alternative version of the code:

-----
#include <vector>

template<typename Value>
struct A
{
    A() = default;
    A(int) {}
    std::vector<Value> m_content;
};

struct B { A<int> a; };

void func(B b = {})
{
}

int main()
{
    func();
}
-----

gets this error:

-----
/tmp/ccvOFE5O.o: In function `main':
prog.cc:(.text+0x33): undefined reference to `A<int>::A()'
collect2: error: ld returned 1 exit status
-----