Default value of function parameter initialized by list initialization

Question

Could anyone help me with the following problem?

There is a simple code:

#include <vector>

struct A {
    std::vector<int> vec;
};

void func (A &&a = {}) {}

int main()
{
    func();
    return 0;
}

When I try to compile it by gcc 5.4.0 I get the error:

undefined reference to `std::vector<int, std::allocator<int> >::vector()'

Amazingly, but clang compiles it well. Also if to modify the code a little bit it is compiled without any problems:

#include <vector>

struct A {
    std::vector<int> vec;
};

void func (A &&a) {}

int main()
{
    func({});
    return 0;
}

I really cann't understand what's wrong with the first code.

Answer 1

This is a gcc bug. It can also be reproduced with

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

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

int main()
{
    func();
}

Currently though, there is no status on it.

I appears that the lack of an actual instance of the vector is causing the compiler to not stamp out the code for it which leads to the undefined reference.