Nested template classes: Parameter default value not accepted

Question

This compiles:

class A {
public:
  template <int, int> class B;
};

template <int y, int z = y>
class A::B {
};

int main() {}

This doesn’t:

template <int x>
class A {
public:
  template <int, int> class B;
};

template <int x>
template <int y, int z = y>
class A<x>::B {
};

int main() {}

g++ main.cpp says: (version 9.1.0)

main.cpp:24:13: error: default argument for template parameter for class enclosing ‘class A<x>::B<<anonymous>, <anonymous> >’
   24 | class A<x>::B {
      |             ^

What’s wrong?

Answer 1

Default parameter needs to be in the declaration:

template <int x>
class A {
public:
  template <int y, int = y> class B;
};

template <int x>
template <int y, int z>
class A<x>::B {
};

int main() {
    A<1>::B<2> b;
}

Default parameters are not allowed in the out-of-class definition of a member of a class template (they have to be provided in the declaration inside the class body). Note that member templates of non-template classes can use default parameters in their out-of-class definitions (see GCC bug 53856)

(https://en.cppreference.com/w/cpp/language/template_parameters)