Template specialization with constexpr non POD data initialization results in linker error when used in constructor as default value

Consider this:

struct TestStruct
{
    uint16_t m_a : 8;
    uint16_t m_b : 8;
};

template<typename T>
struct some_trait
{
    constexpr static const TestStruct value = {0,0};
};

template<>
struct some_trait<int>
{
    constexpr static const TestStruct value = {1,1};
};

template<class T>
class Obj
{
public:
    Obj(TestStruct t = some_trait<T>::value) : m_t(t)
    {

    }

    TestStruct m_t;
};

int main(int argc, const char * argv[])
{
// Linker error here -> Undefined symbol for some_trait<int>::value
    Obj<int> o; 

    TestStruct t = some_trait<int>::value;
    Obj<int> o1(t); // -> This works
}

The following produces a linker error, complaining that the some_trait is not defined. I have two questions:

1. 
   

   Why is this happening? I'm guessing it has to do with either the constexpr specifier or the non-POD type of TestStruct ?

   
   


2. 
   

   Is there a way to make it work, while still keeping the default value in the constructor?

   
   

Thanks!