Undefined reference using inner function if I use separate .cpp and .hpp files

Question

Source:

test.hpp

template<class R>
class Base
{
public:
    virtual R copy() = 0;
    virtual ~Base() = default;
    R test();
    R testCallback();
};

class Derived: public Base<Derived>
{
public:
    Derived()
    {
    }

    Derived copy();
};

test.cpp

#include "test.hpp"

template<class R>
R Base<R>::testCallback()
{
    return R();
}

template<class R>
R Base<R>::test()
{
    return testCallback();
}

Derived Derived::copy()
{
    return Derived();
}

app.cpp

#include "test.hpp"

int main(int argc, char const *argv[])
{
    Derived a = Derived();
    Derived b = a.test();
    return 0;
}

For app compiling I use:

g++ source/test.cpp source/app.cpp -o test

And I get the error afterward:

Undefined reference to «Base::test()»

It works if I cut all code from test.cpp and put it to app.cpp. It also works if I mark R test() as a virtual function in the header file.

Why the linker doesn't see the test() implementation function? What have I done wrong?