I had a similar issue. In my case, I just had a situation where I wanted to initialize a variable of the template parameter type, not as a default return. I could not get any of the above methods to work. I used a variation on @jfMR's suggestion. (Posting as a separate answer for formatting). In my case, I also wanted to support both primitive types and object types.

I used a second template parameter to provide a callable struct producing the default value I need. Full sample:

#include <stdio.h>
#include <string>
#include <iostream>

struct DefaultInt
{
    int operator () () const { return 42; }
};
struct DefaultChar
{
    char operator () () const { return 'z'; }
};
struct DefaultString
{
    std::string operator () () const { return std::string("waldo"); }
};

template<class TP, class TPDefaultValue> class DemoWrapper
{
public:
    DemoWrapper()
    {
        TPDefaultValue valueFactory;
        _wrap = valueFactory();
    }
    DemoWrapper(TP wrap) : _wrap(wrap)
    {}

    void Print() { std::cout << _wrap << std::endl; }

private:
    TP _wrap;
};

int main(int argc, const char *argv[])
{
    std::string demoString("Thanks for all the fish");
    const int demoInt = 13;
    DemoWrapper<int, DefaultInt> dw1(demoInt);
    DemoWrapper<int, DefaultInt> dw2;
    DemoWrapper<std::string, DefaultString> dw3(demoString);
    DemoWrapper<std::string, DefaultString> dw4;
    
    dw1.Print();
    dw2.Print();
    dw3.Print();
    dw4.Print();
}

Output:

13
42
Thanks for all the fish
waldo